125 IX.-Cjn Catlmrism or the ihJuence of Chemically Clean Surfaces. By CHARLESTOMLINSON, F.R.S. F.C.S. I PROPOSE to commence this lecture with an account of Borne experiments and remarks By three distinguished observers whose names. cannot fail to be received with respect in the Chemical Society. I refer to Oersted Schonbein and Liebig. As long ago as the year 1805,O erst ed published an account of an experiment* in which a solution of a carbonated alkali was filtered into a cylindrical glass not too narrow and upon this with the Leak of the funnel just touching the alkaline solution wa8 delivered drop by drop some dilute hydrochloric acid. At the moment when the acid drops touched the alkaline solution there was a slight effervescence but the gas was given off from the sidea of the vessel at the boundary line of the two liquids.Dilute sulphuric nitric or acetic acid might be used and the alkaline solution might be that of carbonate of potash of soda or of ammonia care of course being taken that the acid layer was of less density than the alkaline. Or a dense acid solution might form the bottom layer and a less dense alkaline solution might be filtered upon this until the volumes of the two liquids were about equal. The two strata remained quite distinct as was seen by colouring the acid red and the alkali green. The acid aiid the alkaline solutions could also be separated by means of a layer of water in which case there was little or no effervescence on filtering the acid upon the alkali or the alkali upon the acid.The liquids were ta be filtered into the centre or along the axis of the jar. If the funnel were so inclined as to make the drops trickle down the side of the jar there was a brisk effervescence. But if the experiment were properly conducted there was no escape of gas after the &-st slight effervescence. The glass might be left undisturbed during many hours without any ap- pearance of gas but the moment a solid was introduced such as a platinum wire a glass rod a bit of' shell lac the finger or any minute speck of solid matter such a solid not only be-# Ciehlen's Journal i 276 VOL. XXII. L TOMLINSON ON CATBARISM cttme instantly covered with gas but discharged gas briskly fiom its surface. Oersted put hia tongue into the jar and on examining it before a looking-glass saw that it was covered with gas-bubbles.He also perceived the taste of champagne and he argues that the tongue is efficient in separating gas fiom creamy wines. He also drew a much wider conclusion from his experiments namely that gas in solution is never given off except in contact with a solid. In order to see whether any rise of' temperature accompanied the escape of the gas 0erst ed put a thermometer into the jar; the bulb and the whole of the immersed portion of the stem were instantly covered with gas. There was no rise in temperature but thinking the thermometer would be more sensitive if' covered with silver wire he was surprised to find that the wire liberated gas even more abundantly than did the thermometer.There was however no rise in temperature. 0erst e d confesges that he cannot account for the connection between a solid and the liberation of gas from solution but he calls to mind many examples of such an action as when water is put under the receiver of an air-pump and the air exhausted the sides of the vessel become covered with globules of air and if a platinum wire be put into the water this too is simi- larly covered. So also if a little dilute spirits of wine be poured into fuming nitric acid and a glass rod be inserted gas is given off fiom its surface and still more abundaiitly from a platinum wire. Or if a little dilute hydrochloric acid be gently delivered to a solution of potassium sulphide there is scarcely any escape of gas or smell of hydro-sulphuric acid until a platinum wire is inserted and then the gas is given off abun- dantly.He further remarks that every one knows the influence of a solid on the crystallisation of saline solutions and also that water cooled below its freezing point instantly becomes solid when touched. Bla gden found that perfectly clean water was not readily frozen but water containing particles that diminished its clearness passed intv the solid state with ease. Liquids that are set aside to ferment do not begin to give off gas until they become clouded with solid particles; if these be removed ths fermentation ceases. In the effervescent drink made with carbonate of potash and citric acid the two solutions mixed in a large glass give off but little gas until stirred with a solid.If not stirred the mixture even after standing 24 houm will OR THE INFLUENCE OF CHEMICALLY CLEAN SURFACES. 127 enter into bi-isk effervescence if poured upon the surface of a linen filter. Thus far Oersted. In 1837 Schonbein published a number of facts pointing in the same direction as those contributed by 0erst ed." He says that in the formation of gas or npour in liquids certain physical circumstances have an influence and may even produce decomposition ; as for example when a solid is put into a solution of nitrous acid or into nitric acid con- taining some of the lower oxides of nitrogen. If such acid be covered with a layer of water and a platinum.wire be passed down to the boundary of the two liquids a lively effer- vescence takes place nitric oxide being liberated. Gas is also given off abundantly when copper brass iron or silver wire is put into so dilute a solution as one part of nitrous acid to ten parts of water. The escape of gas is far too copious to be ac- counted for by chemical action ; indeed there is no such action for there is not only no rise in temperature during the strongest effervescence but actually a slight fall. Moreover a bit of pine wood liberates gas with nearly as much energy as a brass wire but if the wood be deprived of air by long boiliug it becomes inactive. It is he says of importance to the interests both of physics and of chemistry to be able to explain these phenomena.It seems likely that the solids act by carrying down air into which the gas expands and that when deprivedof air they are inoperative. In 1839 Liebig while giving the details of an analysis of a mineral water,? noticed some phenomena connected with the liberationof gas from solution by the contact of solids as when sugar is thrown into a mineral water containing much gas it produces a lively effervescence; or if a stoneware bottle filled with such water be well shaken so much gas may be set free as to drive out the cork; 80 also if the mouth of a glass containing an effervescent wine be suddenly struck with the palm of the hand a foam appears on the surface. In all these cases air plays an important part. There is air in the pores of the sugar there is air in the stoneware jar which by shaking,mingles with the water and air also is driven into the wine by the sudden blow of the hand.Now taking into account the relative solubilities of air and of carbonic acid gas it * Poggendorffs Annalen xl 382. .t. Ann. Ch. Pharm. xxx 13. L2 TOMLINSON ON CATHBRISM follow that for every cubic inch of air driven into the water twenty cubic inches of carbonic acid will be set free. Such are the facts and remarks contributed by three first rate observers in science. Similar facts may be multiplied iiot only as it relates to the liberation of gases from solution but also of vapour from boiliug liquids and the crystallisation of saline solutions. Thus in the Scotch distilleries in converting the ‘‘wash ” into ‘‘low wines,” it is customary to throw a lump of soap into the still every time it is charged.The effect is to cause the slteam to rise quietly. The sugar-boilers also are accustomed to throw a lump of butter or of paraffin into the vacuum pan for the purpose of preventing that irregular kind of boiling which displays itself in furious bursts separated by almost passive intervals. It is also a usual practice in the laboratory during distillations to introduce into the retort solid matter such as eharp or angular pieces of metal glass &c the points being supposed to favour the generation of steam and prevent soubresauts or jumping ebullition. So also in crystallisation on a large scale strings stretched acrogs the vessel or a cinder thrown in favour the formation of crystals.But whether gas or vapour or salt escape fiom solution the influence of Bome mysterious function in the air is asserted by most writers. Thus Le Grand.,* who paid great attention to the phenomena of boiling says as De Luc had done half a century before that as Boon as the liquid water for instance has discharged all its air the boiling becomes difficult and irregular; indeed De Luc denies that water can boil at all if completely purged of air. Le Grand referring to the use of platinum for preventing bumping on account of its unalterable character says it is a mistake to employ that metal for as soon as the air introduced by it is expelled by the heat the bumping is renewed.He recommends zinc or iron-metals that decom- pose water with the greatest facility-and of the two zinc is to be preferred. Writers also frequently refer to changes in the molecular condition of the vessels containing gaseous or saline solutions or boiling liquids to account for unexplained phenomena. The idea originated wit8h Gay-Lussac in his paper published in * Ann. Chim. Phys. Iix 426. OR THE INFLUENCE OF CHE~CALLY CLEAN SURFACES. 129 1817,” on the shifting of the boiling point of liquids under a constant pressure in vessels of different materials. He imagines the boiling point to vary with the nature of the vessel and in the same vessel with the condition of its surface its degree of polish and also its conducting power for heat.The cohesion or viscosity of the liquid and its adhesion to the sides of the vessel must also exert an influence on the boiling point. So also in 18197 he explained the state of supersaturation of sodic sulphate on similar grounds such as the nature of the con- taining vessel its state of polish its conducting power the influence of electricity and a particular disposition of the saline molecules by virtue of which they resist more or less their change of state. Gay Lussac exerted so powerful an influence on the scien- tific mind of France that any suggestion thrown out by him took deeper rout than he perhaps intended. Moreover he was a man of lively imagination and inventive power and would readily throw off half a dozen suggestions to account for phenomena that were not clear to him without always stopping to test these suggestions in the crucible of experiment.But later observers with less invention regard these suggestions thrown off so carelessly by Gay-Lusgac as pearls of great price and estimate and cherish them as the talismans which are to explain some of nature’s mysteries. One of the greatest authorities on the subject of super-saturated saline solutions is the French chemist Liiwel who devoted eight or nine years of conscientious labour to the study of this subject the result of which is given in six elaborate memoirs,$ in all of which he relies upon a theory of molecular changes to account not only for the leading phenomena of supersaturation but also to explain many anomalous cases and this theory was applied as well to the solutions themselves as to the sides of the vessels containing them.Thus to select two or three passage out of many he says- ‘‘I think that at these low temperatures the inner surfaces of the tubes and flasks usually reassume their active or dynamic property of determining crystallisation which heat had de-prived them of.” Again he says with reference to the formation of the modsed salt :-* Ana Chim. Phys. vii SOT. tIbid. xia296. t Ibid. 1850 to 1857. TOMLINSON ON CATHARISM LLThereis every reason to believe that the walls of the vessel do not perfmm a merely pamive part.” And again- ‘‘Such solutions remain supersxtumted until in consequence of lapse of time and abovc! all of low temperature the inner surface of the phials have recovered that particiilar property of determining crystallisation of which heat had deprived them the cause of which is unknown to us.” Lowel also regards the functions of nuclei in determining crystallisation.“as the effect of one of those mysterious contact actions known a8 catalytic of which science has not yet been able to give a satisfkctory explanation.” I cannot help thinking that of iate years the terms C‘mole- cular change,” 66 molecular force,” ‘(molecularity,” &c. have been applied to phenomena in physico-chemistry without always sufficient cliscrimination. As used with respect to n large class of phenomena such terms do not advance our knowledge at all and every time we displace the word “molecular” by a simple common sense explanation of the phenomena in hand we per- form in my opinion a real service to science.Of the varied examples which I have thus selected from the labours of others Oersted gives no explanation at all; Schonbein accounts for his facts by reference to the action of the air which however he thinks insufficient; Liebig en-deavours to explain the phenomena brought forward by him by meam of the displacing power of common air over carbonic acid but for this he gives no experimental proof; Gay-Lussac offers a number of suggestions by way of explanation of his facts and from him Lowel and others have derived the idea of 6cmolecular change,” and from Berzelius t.hat of i4 catalysis.” In appearing before you to-night with a new scientific term I may seem to be guilty myself of the fault which I condemn ill others.But I venture to submit that if the term b6molecular change” is according lo my view a vague one in physico- chemistry the term “catharized condition ” has at least the merit of clearness. The Greek word lcaeupos signifies ‘6 pure ” or LL clean,” bnt the cleanliness of matter implied bF “catharizedn is very different from what is usually understood in the ordinary application of the word ‘‘clean.” 0erst ed found his finger act as a nucleus in liberating gas from solution but no amount of cleaning would have made it otherwise than uclean OR THE INFLUENCE OP CHEM~CALLYCLEAN SURFACES. 131 in the sense now intended.But if Oersted had cleaned his glass rod or platinum wire in a caustic alkaline solution or in sulphuric or nitric acid and rinsed it well in clean water he would have catharized it or made it chemically clean and it would then have no longer liberated gas from his solutions or salt from supersaturated or other solutions or vayour from a liquid at or near the boiling point. When bodies act as nuclei in separating gas or salt 01' vapour from solution it is because the gas or the salt or the vapour has a stronger adhesion to or attraction for the surface of the nucleus than the liquid portion of the solution has for such surface. If the nucleus be chemically clean there is no appreciable difference between the adhesion of the gas a'nd such surface and the liquid that holds the gas in solution.Hence there will be no separation of gas because the solution adheres perfectly and as a whole to a catharized surface. Bodies that are exposed to the air and to the products of respiration and of ordinary combustion that are handled or wiped with a cloth contract more or less of a greasy film which lessens the attraction between the liquid portion of a solution and such surface while the attraction between the gas &c. of such solution remains the same as before. Hence there is a separation of gas &c. from solution since gas or salt or vapour will adhere perfectly to a greasy surface and the attraction between such a surface and a gas is so strong as in some cases to produce chemical decomposition as when chloride of nitrogen is touched with an oily or greasy surface.The dust of a room which is constantly floating in the air is more or less contaminated with greasy or organic matter and acts as a nucleus. If such dust be collected on a filter and washed with a solution of caustic potash rinsed with water and dried out of contact with air it ceases to act as a nucleus. A nucleus then may be defined as a body that has a stronger adhesion for the gas or the salt or the vapour of a solution than for the liquid which holds it in solution. I believe this new principle of catharism is sufficient to gene-ralize and account for in a scientific manner the numerous facts already introduced to your notice. When Liebig shook a bottle half full of a carbonated mineral water the gas was liberated by coming into contact with the unclean sides of the vessel.When he struck his hand on a glass containing sparkling TOMLINSON ON CATHARISM Moselle or other gaseous wine he not only precipitated Borne of the unclean dust of the air upon it bnt he also shook the wine against the unclean sides of the glass. If a bottle of soda water be gently poured into a cutharized glass not a sinyle bubble will become attached to the sides. If water in n similarly clean glass containing a clean glass rod or wire be put under the receiver of an air-pump and the air be exhausted not a single bubhle of air will attach itself to the sides of the vessel or to the glass rod or to the wire. When Schijnbein found a bit of pine wood by long boiling inactive in liberating gas from solution it was not that the boiling had driven the air out I of the wood but that the boiling had catharized the wood.There is not a greater mistake than to suppose the air to have an influence in setting gas or salt or vapour fiee from solution. When air appears to act it is merely as a carrier of some unclean mote or speck of dust that ifi floating in it. And this explains the fact noticed by Low el and others that supersaturated saline solutions can be kept longest without crystallising in narrow- necked vessels the time being long in proportion to the narrow- ness 80 that if the mouth of a vessel be contracted to a capillary bore the solution can be kept as in a close vessel.I have found that highly charged supersaturated saline solutions in wide-mouthed flasks can be opened in a garden or a field in the country where the ah is free from the dust and motes of a room and be kept open for a long time without crystallising and when crystallisation does take place a nucleus is to be found in the shape of a small fly or other unclean objecf. By attending to this principle ofcatharism we may be able to account for and to eliminate those anomalous cases of crystal-lisation which have given rise to such terms as ‘‘ the mysterious action of the air,” “ the altered molecular condition of the sides of the vessel,” ‘‘ the molecular change which takes place in the solution,” &c. Previous obscrvers are I believe agreed as to the sensitiveness to cold of supersaturated saline solutions.According to them there is no separation of salt from solution during the summer months but the first touch of autumn’s or of winter’s cold immediately produces a deposit of salt. Hence according to Lowel whose view is generally adopted a super- saturated solution of the ordinary ten-atom sodic sulphate undergoes no change so long as the temperature is about 60”F. ; but below this it undergoes a molecular change and assumes the OR TRE INFLUENCE OF CEEMICALLY CLEAN SURFACES. 133 constitution of the more soluble seven-atom hydrate. Hence he ingeniously argues that supersaturation is a phenomenon in ap- pearance only and not in fact for the solution is but an ordinary saturated not supersaturated solution of the rnore soluble seven-atom hydrate.Why I have had highly charged super- saturated solutions of sodic sulphate (five salt to one water) in catharized tubes in freezing mixtures at 10' F. for hours to- gether without any separatioh of the salt ! And this observation applies not to sodic sulphate merely but to a number of other salts that form supersaturated solutions. Highly saturated boiling solutions of sodic acetate of sodio -potassic tartrate of sodic arseniate of potash alum &c. may be cooled down to near the zero of Fahrenheit wit'hout showing any gign of crystallisation. A salt of greater solubility is not formed and the state of supersaturation so far from being one in appearance only and not of fact is one both of fact and appearance.Take the case of potash alum. This is one of those salts which according to some observers throws down crystals in abundance when a hot saturated solution is left to cool in a close vessel. This is a true observation if the experiment be conducted in a vessel that is not chemically clean but if a strong solution of alum (3$ salt to 1 water) be boiled in a chemically clean flask and then filtered into a similar flask boiled again and a plug of cotton wool be fitted into the neck not only will such a solution cool down to the temperature of the air without any separation of the salt but the flask may be kept in a freezing mixture of ice and water for hours without any sign of crystallisation although the solution contains upwards of sixty times more salt than the water can take up at the reduced temperature.It was not known that the inner surface of a chemically un- clean flask acts as a nucleus on the solution as it cools down. L6w el often noticed the embarrassing fact that at low tempera- tures the sides of his flasks appeared to regain the active con- dition that heat had deprived them of and seeing that the glass rod and other solid nuclei which had been rendered inactive by means of flame or by long immersion in water and cooling or drying out of free contact with air became active by exposure to the air he naturally attached some mysterious property to the air yet to be discovered. All these anomalies disappear by making use of chemically cleau flasks with carefully filtered TOMLINSON ON CATHARISM solutions.The state of supersaturation then resolves itself in the majority of cases not into the formation of a more soluble salt but into a case of no nucleus in fact into the absence of any predisposing cause that influences crystallisation. We must even modify our ideas as to the influence of low temperature in inducing crystallisation. I think it a remarkable fact that alum for example of which 3.9 parts dissolve in 100 of water at 32' F. and 357.58 parts in the same quantity of water at 212O should allow a solution containing upwards of 60 parts more salt than it can dissolve at 32" to be cooled down to that point and even below it without any separation of the salt.And this is not a state of unstable equilibrium which it is difficult to maintain for such a solution in a bottle half full of air can be violently shaken or kept for a length of time without change or put into freezing mixtures or be exposed to various vicissitudes of temperature. It had not been hitherto supposed that highly saturated solutions could thus be made independent of low temperature. Since LOwel's time the course of inquiry has been chiefly directed to ascertaining the real function of nuclei in inducing cry stallisation. Solid bodies have been ex- amined by hundreds in the hope that in this multiplicity of examples a few might be got to speak the truth. My own labours in this direction conducted during a long period with a great respect for the authority of Lowel and others led to very contradictory results.By slow degrees it began to dawn upon me that in the simplicity of nature's dealings a slight difference in the force of adhesion between the nucleus the solvent and the thing dissolved was sufficient to explain the matter. A great many salts form supersaturated solutions. Only a few produce the modified salt of a lower degree of hydration and of greater solubility than the normal salt. My explanation ofthe formation of the modified salt when it does occur has already been published in full elsewhere,* but I shall have occasion to describe it in brief presently. In the mean time I may be allowed to remark that if Lowel's molecular theory were true,it ought to apply to all salts that form supersaturated solutions.Indeed it is so applied by L Owe 1's followers and they explain supersatnration by supposing the molecular character of every solution to change on a certain depression of * Phil. Trans. for 1868 page 666. OR THE INFLUENCE OF CHERIICALLY CLEAN SURFACES. 135 temperature so as to assume the condition of greater solubility of the more modified salt. Lowel in the course of his long inquiry extending as it did over eight or nine years never succeeded in showing that more than about three salts formed the more sohzble modified salt and his followers have done nothing to add to the nnmber. Anomalous cases occur which cannot be explained except in a way which seems to me to be fatal to the tliemy.For example to notice only one such case. Lowel poured a strong boiling solution of sodic sulphate into two bottles of the same size wliich were then tightly corked and set aside to cool under apparently the same conditions. The solution in one of the phials deposited the modified seven-atom salt while the solution in the other became solid throughout as the normal ten-atom hydrate. Lowel .supposes that in one case the solution did undergo the molecular change which led to the formation of the seven-atom hydrate while in the other case from some cauBe or other the molecular change did not take place. My explanation is this :-The one bottle was cleaner than the other or was made so by the hot solution. As the solution cooled down it deposited the excess of anhydrous salt which it could no longer hold.The heat thus liberated and also a rise in atmospheric temperature after sunrise caused the anhydrous salt to re-enter into solution and to form a dense lower stratum from which the seven-atom hydrate in small quantity crpstallised out there not being sufficient water in this dense stratum to form the ten-atom hydrate. In the other case the solution during the cooling met with a nncleus high up in the bottle to which the salt could attach itself and the action once begun in the presence of a nucleus was propagated rapidly throughout the needles having suflicient or carrying down sufficient water to form in every part of the solution the ordinary ten-atom salt. Cases of this sort have repeatedly happened to me in the course of my investigations.If we open a flask containing a supersaturated solution crystallisation sets in at the surface consequent on the entrance of some speck of dust fkom the air and the needles proceed from the surface to the bottom solidifying the whole contents of the flask ; whereas the modified salt when it forms at all invariably does so in small quantity at the bottom of the flask while the superposed mother-liquor is still a supersaturated solution. If the flask be now opened the normal salt forms on the surface crystallisation TOMLINSCJN ON CATHARISM proceeds rapidly downwards and the needles seem to inter- penetrate the transparent modified salt giving it in the case of sodic sulphate three additional equivalents of water and rendering it opaque in the process.The action of nuclei explained according to my view with reference to gaseous and saline solutions applies equally well to the liberation of vapow from a liquid at or near the boiling point. Such a liquid may be regarded as a supersaturated solution of its own vapour. If the vessel in which the liquid is boiled be chemically clean or becomes so by the action of the liquid the vaporous solution adheres perfectly and completely to the sides but as the heat is continually acting the liquid becomes more and more saturated with the vapour until not being able to dissolve any mare it relieves itself by a sudden burst a,s from a safety valve. This sudden burst of vapour occasions the jumping or rebound of the vessel by a mechanical reaction.The burst of vapour follows the direction of the line of least resistance or upwards along the axis of the vessel; this produces an equal reaction in a downward direction which tends to force the vessel further into the ring of the retort stand and it is the rebound from this that constitutes the soubresaut or the phenomena of bunqhg or jurnpivig ebullition. This explanation which I venture to give may be tested by Buspending a flask or a tube by means of a spiral spring or a thread of elastic against a board with a line drawn horizontally from the mouth. If the vessel be chemically clean or contain one of those solutions which according to Le Grand favour the production of sobresauts it will be found on the application of a spirit-lamp that every burst of vapour is accompanied by a downward kick of the vessel.If with the view of prei-enting soubresauts we throw into the vessel a solid of any kind that has been handled or exposed to the air the violent action is got rid of because the vapour attaches itself to such solid with greater force than the liquid does so that under the action of the heat there is a constant liberation of vapour from the surface of the solid. But should the liquid be of such a nature as to bring the solid into a catharized condition the bumpings return with even more violence than before because we have now increased the adhesion surfaces instead of the vapour-giving surfaces. If instead of an unclean body we introduce a catharized body into a vessel in which a liquid is boiling with OR THE INFLUENCE OF CHEMICALLY CLEAN SURFACES.137 little or no bumping we may at once bring about such violent bumping as to endanger the safety of the vessel. A little sand steeped in strong sulphuric acid and well rinsed in water or in a weak alkali is sufficient for the purpose in a flask in which distilled water is boiling. Even the iron on the zinc so much insisted an by Le Grand as favouring tranquil boiling produce soubresauts if made chemically clean. Zinc however often contains minute specks of poroiis matter which act in another way as will be noticed immediately. It is said that pointed or rough bodies are better “promotere of vaporization” than smooth ones.Make them all alike chemically clean and they act alike in impeding rather than promoting the liberation of vapour. It is true that rough bodies are apt to store up between their furrows or teeth that unclean matter which acts so well as a nucleus. But if a rat’s tail file for example which is a very active nucleus in its ordinary state be catharized either by the action of flame or of acidand alkaline solutions it becomes perfectly inactive in a boiling liquid so far as the liberation of vapour is concerned. It will be seen that I take no account of the influence of the dissolved air upon the phenomena of boiling and so far from assigning to it the exalted function of being absolute17 neces- sary to boiling I doubt whether it has any influence at all except in some cases slightly to diminish the cohesive force of the liquid molecules but this effect must soon disappear.Water for example when cold dissolves only about &th of its volume of oxygen gas and about d,th of nitrogen and these small quantities must be expelled by the heat and the absorptive power of the boiling liquid for air reduced to a minimum. Can it be supposed that the minute speck of air still left if it be still left in the boiling liquid is absolutely necessary for the vapour to expand into and produce the phenomena of boiling 3 Can it further be maintained that when bodies act in preventing soubresauts it is by the air they carry down and that such bodies cease to act as promoters of vaporization when they have discharged all their air? I cannot admit that there is experi- mental proof for either assertion.A lump of flint that has been exposed to the air when thrown into a boiling liquid is instantly covered with bubbles of vapour. Break it into two and throw in the fragments not a single bubble of vapour will be seen on the freshly fiactured surfaces but the old surfaces TOMLINSOX ON CATHARISM will be covered as before. If air had anything to do with the matter the freshly fkactured surfaces must have carried down air as well as the old ones. Why then do the new surfaces apparently refuse to act while the old ones are as active as before ? We can explain nothing on this theory but if we say that the freshly fi-actured surfaces are chemically clean we can understand why no vapour is given off fiom them because there is perfect adhesion between them and the solution as a whole.There is thus far an identity of action in nuclei whether aa applied to gaseous or saline solutions or to liquids at or near the boiling point. I have now to iiivite attention to an exten-aion of the principle of adhesion consequent on the extension of surface presented by porous bodies. The same force which according to Saussure enables 1volume of box-wood charcoal to absorb 90 volumes of ammoniacal gas 85 of hydrochloric acid gas 65 of sulphurous acid gas and so on enables charcoal and some other porous bodies to absorb vapour from boiling liquids and under the continued action of the heat to give it out in never-ceasing jets relieving the vessel of all tendency to bumping making the boiling soft gentle and regular and increasing the quantity of the distillate.The most remarkable circmmstance about this property of charcoal is its untiring activity. No amount of heating or washing in acid or alkaline solutions suffices to cathaiize it. These processes seem rather to improve its strong attraction for gas or vapour in solution. When once in action it will continue for days and weeks together to give off vapour from the boiling liquid and unlike the soap paraffir, &c. used by distillers on the large scale for the purpose of facilitating the discharge of vapoui- the charcoal or other porous body 80 far as my experience goes does not require to be renewed.In order to show the value of charcoal and other porus bodies in preventing bumping and promoting vaporization methylated spirit was distilled in a glass retort at a fixed boiling point of 171' F. The distillate collected in five minutes was weighed and found to be 244 grains. Three or four fragments of char- coal part.ly from box-wood and partly from cocoa-nut shell altogether weighing 20 grains were now added to the retort and when the spirit wak again fairly boiling the distillate duiing five minutes was again collected and weighed. It was now found to amount to 325 grains. The ratio of the results OR THE INFLUENCE OF CHEMICALLY CLEAN SURFACES. 139 may be thus stated:-As 244 325 : 100 133.2. Instead of charcoal 20 grains of pumice-stone in four fragments were next used in the retort and the ratio of the result was as 100 121.7; with 20 grains of meerschaum-as 100 112 ; and with 20 grains of coke as 100 107.46.These numerical results are however very much under-stated if compared with those obtained in a retort that is structurally free from nuclei which was by no means the case with the retort actually used. Indeed is is seldom that we get a retort or a flask or even a test-tube that is free from porous specks of ferric oxide or of carbon. These become attached to the glass while it is still soft in the process of nianufacture and they act as small but powerful nuclei in promoting vaporization and preventing bumping. Indeed were it not for the presence of these accidental impurities many a chemical operation must fail fiom the fracture of the vessel by excessive bumping and chemists would have only too close an acquaintance with the phenomena of soubresauts.Several writers have noticed that when a liquid is boiled in a glass vessel the boiling seems to proceed from certain points in preference to othera. Donny calls them ‘‘ foci of vaporization,” and Magnus noticed in the case of an old platinum vessel containing cracks and scratches that the vapour was more readily formed at certain points than at others. These phenomena are perfectly explicable on the principle of adhesion on the part of unclean or porous bodies which having a stronger attraction for the gases or vapours than for the liquids which hold them in solution thus effect their separation.During the last seven years I have applied the principle I am now advocating to the phenomena of the cohesion figures of liquids ; to the production of camphor-currents and camphor pulsations ; to the motions of creosote on water and its dis- placing power with respect to various films ; to the adhesion of liquids to liquids ; to the curious attractions and repulsions of eugenic acid on water ; to the production of tears in the wine- glass and in solutions ; to the production of lightning-figures which illustrate some new points respecting the disruptive dis-charge and the formation of fulgurites ; to the effect of clean and unclean surfaces in vegetable nature such as the different action of the leaves compared with the roots of plants; why dew is formed in globules and not in sheets of water ; to certain 140 TOMLINSON ON CATHARISM phenomena connected with animal and vegetable secretions and so on.I will now with your permission glance at a few other phenomena which do not seem to me to have been hitherto explained. I think it was Sir Humphry Davy who found that in at-tempting to make a voltaic circuit with pairs of plates of metal of the same name there was no current while the metals were in the mme physical condition but that if one metal of the pair had its surface covered with a film of fatty matter a feeble current was produced. In such a cage the urrclean plate aeparated the gas &om the solution and was thus enabled to perform the part of the conducting plate.So also it was found that zinc tin iron and copper heated in the air until they had become tarnished were negative towards the bright metals of the same name in acid alkaline or saline liquids. In these cases also the plates covered with a film had a stronger attraction for the gas of the cell than for the liquid and thus relieving the cell of gas were in the condition to act as con-ducting plates. A plate of pure zinc is scarcely acted on by dilute sulphuric acid on account of the perfect adhesion of the acid to the clean mrface. Or if gas be liberated when the plate is first immersed it is fiom unclean patches or points but these soon becoming cleansed or catharizedby the action of the acid there is perfect adhesion between the acid and the p€ate and chemical action is arrested.The same remark applies to amalgamated zinc or to impure zinc covered with a film of mercury. The mercury makes the plate chemically clean and the adhesion of the acid is too close to allow of the escape of gas. Or if gaB form when the plate and the acid first meet the gas goes into solu- tion and this solution adheres with such force to the plate as to prevent the formation of more gas. So also in what is called the ‘‘passive ” condition of iron and a few other metals in nitric acid of the specific gravity 1.4 to 1.5 the principle of catharism coupled with the fact that the iiitrates of such metals require at least six proportionals of water of crystallisation which are wanting in the strong acid Beems to me to explain phenomena that have excited an unusual degree of discussion.Gmelin in his Handbook of Chemistry (Cavendish Society’s Translation) devotes twenty closely printed pages to Ihe mere analysis of what has been done on OR THE INFLUENUE OF CHEMICALLY CLEAN SURFACES. 141 the subject. He says :-“ Many metals when immersed in coil-centrated nitric acid undergo a change; they become more electro-negative less oxidizable and lose either wholly or for the most part their tendency to decompose acids and metallic saline solutions ; they pass from their ordinary active state into a passive state.” Passive iron for example will not separate copper from a solution of cupric sulphate or nitrate nor mer- cury from mercurous nitrate nor silver from argentic nitrate and so on.Strong nitric acid is so powerful a catharizer that it renders chemically clean an iron wire however dirty it may be when immersed in it and the acid adheres to it with such force that it is not readily displaced. I have transferred a bright iron wire from iiitrk acid of 1.410 sp. gr. to ah acid of 1.200 sp. gr. and it has remained for hours in a perfectly passive condition; but if shaken or rubbed beneath the surface of the acid it instantly starts into activity it changes colour and a brisk effervescence sets in with the abundant evolution of red fumes. In what is called a ‘‘ pulsating wire” gas is liberated in bursts separated by intervals of repose.This is similar to what takes place when a glass rod covered with an oily film is put into a strong solution of carbonic acid as in Oersted’s experiment. The gas that is liberated from the surface of the wire imme- diately enters into solution with the adhering acid (which must be diluted to a certain strength determined by experiment for the wire used). As more gas forins on the surface of the wire and the acid can hold no more that first dissolved is driven off with a burst ; it accumulates again again goes into solution and once more escapes with a burst and this intermittent action produces the phenomena called pulsation. The most common case of nucleus is when a body is contami- nated with oily fatty or greasy matter; but this does not render an iron wire active in strong nitric acid for such acid immediately acts upon and displaces the impurity and catharizes the wire.Iron wire covered with a chemically clean film of oxide which would be quite inactive in separating a salt or a gas from an aqueous solution is powerfully active in iiitric acid of a certain strength. So also if the inactive wire be put into chlorine or held in the vapour of bromine or in hydro- chloric acid gas it may be active for a time at least if returned to the nitric acid in which it was previously pasaive. The same VOL. XXII. M TOMLINSON ON CATHARISM result may be produced by rubbing the passive wire against a softer metal. I fear I have already exceeded the time allotted to a lecture in this place but perhaps a few worh may be allowed by way of conclusion.I trust I shall not be assuming too much if I claim for catharisrn the properties of a principle of nature namely generality and breadth of application. The principle is as yet new to science. The work that lies before us in connection with it seems to me to be of importance. I believe that the proper action of nuclei which this principle teacheb will explain the formation of agates arid other siliceouls minerals which is at present obscure. I believe that catharism will account for many if not for all the cases of increased activity consequent on the nascent state of matter. In that state matter is chemi- cally clean and hence is endowed with a wonderful activity and power of combiiiation; but no sooner does it emerge from the nascent state and come into contact with unclean vessels and unclean solutions than it sinks to the level of activity of ordinary matter.I believe that catharism is competent to explain many of the anomalies connected with ozone and the increased activity of oxygen at the sea coast or still better on the wide ocean. I believe that catharism explains why sulphur phosphorus and aorrie other bodies sonietimes remain liquid during a great length of time at ordinary temperatures; why water may be cooled below the freezing point as well as many facts in vegetable and animal physiol!gy that are at present not well understood. If health and eyesight are spared to me €or some time longer I hope to cortinue my explorations in this rich field of inquiry; but I shall require time for my meane are very limited and I have no assistant or assistance from any one.But if what I have done am doing and propose fo do is likely to be in any way conducive to the interests of science rriy reward has been already earned. DISCUSSION. The Pre sident observed that the genera1 conclusion deriv-able fi-om Mr. Tomlinson’s experiments appears to be this :-that there are certain bodies which have the power of causing the liberation of gase8 or salts &om solution and others which OR THE INFLUZNCE OF CHE3IICALLY CLEAN SURFACES. 143 have not that power ; a perfectly clean glass vessel for example does not appear to possess this power but there are certain substances which may overlie its surface and induce the par- ticular action in question ; but we have not yet discovered what it is that causes the separation of one body from another which holds it in solution.Charcoal even when made perfectly clean appears to possess this peculiar power in a very high degree. Dr. W. A. Miller said that Mr. Tomlinson’s observations seem to show that the principal agent in producing these singular phenomena is a film of grease which very generally adheres to the surface of glass vessels arid other bodies and disturbs the balance of adhesion between the solvent and that which it holds in solution. The experiments in question may not have brought us any nearer to the knowledge of the cir- cumstances which cause adhesion between two substances but they have at least enabled us to advance one step viz.that of tracing a comiderable number of widely scattered phenomena to one very generally operating cause. Professor Williamson with reference to the presence of air in liquids during their vaporization directed attention to the very remarkable observations made Borne years ago by Mr. Grove,” showing that water if carefully freed fiom air and excluded from access of air may be heated to the boiling point for a very long time without showing any signs of ebul- lition. We know that eveiy vapour is formed much more readily in preseme of a permanent vapour or gas than when no such gas is present and though it would be going too far to say that no ebullition can take place without air we have as yet no evidence that ebullition can take place if permanent gases are completely excluded.The effect of charcoal in facilitating the escape of gases from solutions is decidedly in favour of the view which regards the presence of a permanent gas as necessary to ebullition ;for charcoal retains gases within its pore@with great tenacity and even after it has been to a great extent exhausted of air it probably still retains quantities considerably greater than Mr. Grove found to be contained in the water with which he experimented. With regard to the term ‘‘chemically pure,” which Mr. Tomlinson uses to dis- tinguish bodies which do not exert this action from those which do Dr. Williamson said that he felt Borne difliculty in * Chem.SOC.J. [2] i 263. TOMLINSON ON CATHARISM this application of it since to take one example among others the crystallisation of R supersaturatt?d solution of sodic sulphate is brought about by contact with a crystal of the same salt more certainly and rapidly than by any other substance; now such a crystal cannot well be called chemically impure or nn- clean. Dr. Gladstone thought that Mr. Tornlinson’s researches had done good service in pointing outc the incorrectness of maiiy of the explanations that have bceii given of the phenomena under consideration particularly in showing that the Bmooth-ness or roughness of a surface has nothing to do with its effect in determining the separation of a gas or of a solid fiom a solution.This separation appears according to Mi*. T omlins011 to depend upon the fact that the nucleus or solid substance immersed in the liquid has a stronger adhesion to the dissolved gas or solid than to the solvent itself. This however is not a matter of cleanliness or uncleanliness and therefore in speaking of such matters it wonld be well to get rid altogether of the words “clea~ii” and “unclean.” A piece of paraffin may be made perfectly clean and yet it would certainly produce the effect of removing a salt froin solution. Dr. Gladstone also referred to the well known instance of t’he ci-ystallisation of potassium bitartrate when a potassium salt is mixed with tartaric acid and the sides of the vessel are rubbed with a glass stirrer the crystals then forming chiefly on the rubbed portions of the surface; this again is a case with which cleauliness or iincleanliiiess has nothing to do.The sudden crystallisation of a supersaturated solution on dropping iiito it a crystal of the same salt appears to depend on the strong tendency which a crystal has to draw to itself other particles of the same substance iu fact of the tendency of all crystals to grow aiid may therefore be regarded as affording a strong argument 111 favour of the general expla- nation above suggested. Professor A. Vernon Harcourt objected to the principle of the paper that it is purely a iiegative princ;ple namely that certain results do not occur in the case of the absence of a number of different substances that a minute quantity of these substances will produce the results and that these substances being diffused through the air and in the vessels we employ are very liable to pass into the eolution.The only positive result we could have on the subject would be h the OR THE INFLUENCE OF CEE~OALLY CLEAN SURFACES. 145 way of a determination as suggested by the President of what kind these substances are which do produce this result. We want to know for instance whether charcoal is a substance which produces it in the greatest degree or whether it is a general property of porous substances whether it depends upon the evolution of a gas or what the substances are to which these results are due. Professor Harcourt here referred to a recent observation of his own which appears to have a bearing upon this subject although the case is rather an exception to than an example of the general fact brought forward by Mr.Tomlinson. In determining the rate of the decomposition of peroxide of hydrogen he had introduced a meamred quan- tity of dilute solution into glass bulbs at a certain tempera- ture and then determined what amount of peroxide of hydrogen remained hoping so to ascertain what relation the rate of decomposition of the substance bore to the temperature to which it was heated and the concentration of the solution; but he found some difficulty in making these experiments and obtaining concordant results. Taking exactly the same quan- tities being very careful as to the temperature of the solution and the concentration and as far as could be observed having everything in the same way the quantity of peroxide decom- posed after the Bame length of time was found to differ very much indeed according to the glass bulbs employed; and the more pains he took in cleaning these bulbs by heating acids in them and by heating alkalies in them the greater and not the less was the amount of decompoaition.On examining the matter closely he observed the very greatest differences bet ween the amount of decomposition according as he took glass as thoroughly clean as possible or glass which was not clean. On taking a little oil and noticing what happened when the bulbs were made dirty in that way he found that the rate of decom-position was very much reduced indeed; and when the bulbs were varnished over inside so as to prevent the contact of the glass with the liquid the decomposition was reduced to a minimum.A dilute solution of peroxide of hydrogen in a perfectly clean glass bulb at 10" or SO" centigrade would be nearly decomposedin half an hour but would be hardly decom-posed in any degree if it was placed in a dirty bulb or in a bulb varnished inside. Decomposition of t3he peroxide of hydro-gen is very much like the solution of oxygen in water and the 3x2 3 46 TOAILINSON ON CATHARZSM rate at which the gas is given off in this caBe seems to be increased by the cleanliness of the glass and to be diminished when the glass is dirty. Dr. 0dl in g said that he felt some little difficulty in seeiiig his way clearly to the conclusion to which Mr.Tomlinson haB arrived.This conclusion appears to be that when a liquid containing a gas or crydallisable solid is in contact with a body of a particular kind the adhesion is not 80 perfect a8 in others and in that case either the gas is evolved or the crystals are thrown down. In the cases where the adhesion is perfect no gas is evolved and no crystal is thrown down and if this be the case it becomes a question with which minute division has very little to do. Taking the car;re for instance of the retoi-t to which Mr. Tomlinson has alluded we have there two different materials. We have the material which constitutes the great body of the glaaa and we have the material which constitutes the nucleus and under thoae circumstances it woiild perhaps hardly be a sufficient explanation to say that the adhesion of the glass is simply leas perfect to the nucleiis than elsewhere.Supposing we have a retort which is all nucleus under those circumstances the adhesion will be equal in every direction; nevertheless there would probably not be any points from which this evolution of gas could take place. Take the case of paraffin which Dr. Gladstone has alluded to. Now it is a common thing as Mr. Tomlinson said to throw grease into water in order to promote ebullition. Sup-pose we make a retort entirely of paraffin or one such as Mr. Harcour t haa jurrt described under those circumstancea the adhesion of the liquid however imperfect will be uniform and according to Mr.Tomlinson’s view there aeema no reaaon why the ready ebullition of a liquid of low boiling point due to the non-adhesion to the walls of the paraffin retort should ever Come to an end-why that is to say the efficacy of the paraffi or pure grease in promoting ebullition should be temporary only. With regard to the observations of Dr. WilliamBon respecting the great difficulty of removing the last traces of air fiom water Dr.Odling said that if he remembered Mr. Grove’~3 experiment accurately the amount of water given off gradually got less and less up to a certain point and then there was an almost constant ratio between the amount of vapour given off and the amount of gab and that ratio rtontinued no OR THE INFLUENCB OF CLEAN SURFACES.147 matter how long the ebullition was maintained. Related to this subject ie the fa& of many liquids which nevertheless there is reitsou to believe are not very absorptive of gaa boiling with very great facility auch bodiea as the hydrocarbons for instance ; but it is by no mean8 to be taken asa matter of proof that the quantities o€ gas retained by a boiling hydrocarbon may not be at least equal to those minute quantities of garr which are capable of being retained by water which has been boiled for a long tims With reference also to the question of charcoal if it be really the charcoal itself that has this ac~on the densest and most compact kin& of charcoal ought to amwer almost ag well as those which are more porous.Profwor Foster said that he was unable to we in any of the facts brought forward by Mr. Tomlinson a proof of the incorrectness of De Luc’s theory which supposers that the presence of air or gas of some kind or other is a necessary condition of ebullition. The different effect of a glaas rod when it hast been exposed to the air andbeen carefully cleaned and the different effects of other bodies under like conditiona Beem to resolve themselves simply into different degreea of facility with which they are wetted by the solution into which they are immersed. Now water cannot wet a body thoroughly unless it is able to &place the air which adheres to the body. If it does displace it then it wets it but the greasiness or whatever may be the condition of the surface acts in a great many cases at any rate by increasing the adhesion of the air or diminishing the adhesion of the water or liquid whatever it may be.It is very difficult indeed to form any clear idea of vaporization taking place in the midst of a liquid. If we try to form a definite idea of what the mechanism of vaporization is we must mmmo that it consists of such a motion of the particlesof the substance as drives them for an instant out of the spbere of attraction of the aeighbouring particles and when they once get to a certain distance from them the attraction of the remaining particles k unable to bring them back again. But if we take Clausius’s view of vaporization or any other equiva-lent to that it is impossible to gee how any such process could go on in the middle of a mass ofliquid; but if we take the amallerrt quantity of vapour already foimed or gas or the smallest possible quantity of vacuous space (of course a perfect vac~m would do ahl well if we could get it) then it ia easy to see how 148 TOXLINSON ON UATHARISM molecules should be driven by the increased action of their neighbours into thi8 space and liberated once for all from the attraction of their neighbours and thus appear as gas.Dufour found that liquids of very various character could be heated far above their boiling point if suspended out of contact either with solid wadls to which the gases might adhere or out of contact with the atmosphere but that an unfailing way of causing liquids in such a state when above their boiling-point to enter into ebullition was to send an electric current through them.If he had the wires of a battery projecting into the liquid a globule of liquid heated above its boiling-point and suspended in some liquid which did not dissolve it might come into contact with the wire with perfect impunity but if the battery contact was made so as to make an incipient decomposition and form a globule of gas rapid ebullition took place. Professor Foster further stated that in distilling liquids which one may suppose do not readily dissolve air in very large quantities for instance the iodides of methyl and ethyl he had noticed more than once that ebullition in the distillation of such a liquid ceases com- pletely without the evolution being much slower evaporation appearing to take place from the surface.All these facts seem to support De Luc’s theor?- and he copld not see that the facts brought forward by Mr. Tomlinson are sufficient to overthrow it. Mr. Heisch referring to Mr. Tomlinson’s attempt to explain the passive state of iron by supposing the iron had been rendered perfectly clean by immersion in strong nitric acid said we must remember that if we immerse but half an inch of a wire in strong nitric acid it renders any length of the wire perfectly passive to dilute acid. You may take 50 or 60 feet and provided you immerse the end in the strong acid the whole is rendered passive in the dilute acid and it is passive only to that same sample.But if you take another vessel and dip the wire into it the wire immediately becomes active in that vessel but not in the original nitric acid. More-over if you take pieces of iron wire bend them into a U shape and place one end in very strong and then in dilute acid that wire is passive. If you take another vessel and bring the other end of the U into it that part is active and dissolves completely. Then take another U and place one end of it first of all in the OR THE INFLUENCE 0%’CHEMICALLY CLEAN SURFACES. 149 glass in which the iron is being dissolved and then put the other end of the U into the glass in which it is not being dis- aolved and you will find that it is passive at oue end and active at the other.Rut if you reverse the experiment you make the whole perfectly active. NOW,it appears scarcely possible that the fact of the wire being cleaned by the nitric acid can explain these phenomena at all. Dr. F. Crace Calvert mentioned some remarkable facts relating to the sudden crystallisation of carbolic acid and of glacial acetic acid. Carbolic acid presents in a higher degree than acetic acid the curious property that it may be agitated with a stirrer or anything else or with dirt-with a solid or not-without undergoing crystallisation. In fact carbolic acid has been known to go all the way from Manchester to the south of France and reach there perfectly liquid; but on droppifig just one crystal of carbolic acid into the liquid the whole will become a solid mass within the space of perhaps three or four minutes.That is the method which Dr. Calvert constantly adopts in his works and it affords an instance in which there is not a mere question of a nucleus or of a solid body determining the formation of crystals. The vessel is far from being chemically clean the rods which the men use are far from being chemically clean and yet all this is of no use in causing the liquid to become solid; but just take the most minute quantity the size of a pin’s head and put it into a vessel where there is 5 cwt. of carbolic acid and in a moment it will be perfectly solid. The same may be done with acetic acid. You may take a jar of glacial acetic acid stir it move it shake it; but just put a crystal of glacial acetic acid into it and the whole vesselful whatever niay be its size will in five minutes be a solid mass.The President in calling upon Mr. Tomlinson to reply to the preceding observations wished to direct his attention par- ticularly-1. To the effect of different conditions of surface in the same body as in the platinized platinum of Smee’s battery this being a case in which porosity may be supposed to be con- cerned. 2. To the facilitation of the escape of vapour from boiling water by contact with paraffin or other greasy matter which appears to be similar to the case mentioned by Dr. Williamson inasmuch as we have there a vapour of a different kind into which the vapour of water can diffuse 150 TOiMLINSON ON CATHARISM itself incre readily than into its own vapaur.3. To the question whether these bodies which are called “dirty,” have really certain peculiar properties in themselves and whether if ae Dr. 0dling has supposed they were the containing vessel they would produce this evolution of gas or induce crystallisa- tion or whether it is not their presencein contact with another body-whether it is not the dm1 action-whether it is not the sides of the vessel haT-ing a certain amount of attraction for one part of the liquid and the unclean body for the other which produces a kind of rending action. Mr. Tomlinson replied in substance as follows :-With regard to the President’s observations upon the effect of char- coal in facilitating the evolution of gas this he could not attribute to the presence of gas in the pores of the char- coal because after charcoal has been made red-hot and then suddenly quenched under mercury or plunged into a boiling liquid it cannot be supposed to retain much air; but by its powerful capillarity it is always re-absorbing the liquid and under the continued action of heat giving it out again.The denser kinds of charcoal act most quickly hecause they sink to the bottom of the liquid and are thus brought nearer to the soiirce of heat. The platinized plates of Smee’s battery Mr. Tomlinson regards as very much in the condition of the plates employed by Davy exposing them to the air and gett-ing them oxidized. They collect particles of dust and get into an unclean condition and immediately there is a different adhesion between the gas of the cell and the liquid in the cell and thus there is an easy separation.Interior sides of vessela also are often in a condition of chemical impurity and it is this difference in adhesive force of a clean and an unclean surface which appears to constitute the whole matter. If t~ glass rod be exposed to the air or drawn through the hand or wiped with a cloth and then put into a solution,-say 9f soda-water,-it is instantly covered with gas bubbles; the reason of this appears to be that gas will adhere to ail oily greasy 01-fatty surface and water will not. But if the rod be freed from grease and then put into the soda-water the solution adheres to it as a whole and there is no separation because the adheBion is the same in both cases.With reference to the phenomena of ebullition alluded to by Dr. Williamson Mr. Foster and others Mr. Tomlinaon OR THE INFLUENCE OF CHEMICALLY CLEAN SURFACES. 151 expressed his opinion that the phenomenon of ebullition or generally the escape of gas or vapour from a liquid does not depend on the presence of air in the liquid. In connection with this subject he referred to two papers which he had published one in the 6‘Philosophical Magazine,” about two years ago the other in a recent number of the “Proceedings of the Royal Society,” in which the following experiments are described :-A wire cage made chemically clean was lowered int’o a glass of soda-water where there was a mass of air surrounded by the solution of carbonic acid and not a single bubble of gas escaped.But the moment the cage was rubbed between the hands and then put in it was covered with carbonic acid. So in the case of a boiling liquid. Several boiling liquids were tried. A wire-gauze cage was lowered so as to secure a mas8 of air in the centre or in the midst of a boiling liquid ; and one would suppose that if the air had anything to do with the liberation of vapour the vapour would have disdiarged itself through the meshes of the wire gauze into the cage; but so long a8 the cage was tho- roughly clean there was no such discharge of steam into the air. But a bit of paper or any solid let fall upon the cage instantly disengaged large quantities of vapour.From this and a large number of experiments of a similar nature Mr. Tomlinson infers that nuclei do not act by carrying tiown air and that air is not necessary to the ebullition of liquids. In Dufour’e experiment referred to by Professor Forjter Mr. Tomlinson considers that the water was in the spheroidal state. With regard to the sudden crystallisation of a salt or other substance from solution on dropping into the liquid a particle of the dissolved salt Xlr. Tomlinson concludes from his own observations that if the crystal be perfectly clean no such effect takes place ; it must be exposed to the air handled or otherwise rendered unclean before it will induce the crystal- lisation. NOTE. -In the discussion above reported several distin- guished chemists have objected to the term catharked or chernicaZ7y clean as not embracing all the phenomena.Now although by far the most commoii form of’ impurity is some kind of greasy matter which renders bodies unclean and there- fore active as nuclei while the absence of such fatty matter renders them cZem2 and therefore passive I am nevertheless TOMLINSON ON CATHARISM ETC. bound to admit that cases may arise i~iwhich the distinction between clean and unclean may not apply; aB when a solution of a solid in an oil is touched with a bit of ice both the s01u-tion and the ice may be cliemically cZean and yet the ice may act as a nucleus. So also a supersaturated aqueous solution cannot be kept as such in a vessel of stearin although such vessel be chemically clean.Stearin would act as a nucleus in mch a case. I have already (Phil. Trans. 1868) defined a nucleus as a body that has a stronger adhesion for the thing dissolved than for the liquid which dissolves it. This differential kind of action is not always expressed I admit by such terms as unclean and dean unless by an ex- tension of the meaning of such terms as are expressed in the words a-catl~arized and catharized:; but admitting the defkiition of a nucleus as just given to be correct the terms nucleixed and de-nucleizad would perhaps be sufficiently comprehensive to include all cases. Thus a glass rod which had been exposed to the air or drawn through the hand would be nzicleixed or simply a nucleus as respects aqueous and probably alcoholic solutions while a glass rod that had been washed in caustic alkali &c.would be de-nzccleixed or inactive as respects such solut'ions. The terms active and pawive do not quite accord with my view since the solutions exert a much stronger adhesive force on a so-called passive (clean) as compared with an active (unclean) rod. The principle I advocate is now on it'strial. If it be admitted a correct term will be found for it in the course of examination and further inquiry. I am indifferent as to the name because I am certain of the reality of the principle. The other objections that several gentlemen were so good as to bring forward will be of great assistance to me in enabling me to apply new tests to my work.I propose shortly to reply to them in a separate paper. C. T. Highgate N. 24th March 1869.