ARSENIATES OF LIME BARYTA AND MAGNESIA. 111.-On the existence of a Second Grystallizabb muorescent Substance (Paviin) in the Bark of the Horse-Chestnut. BY G. G. STOKES, MA. SEC.R. S. &c. ON examining a good while ago infusions of the barks of various species of Bsculus and the closely allied genus Pavia I found that the remarkably- strong fluorescence shown by the horse- chestnut ran through the whole family. The tint of the fluores-cent light was however different in different cases being as a general rule blue throughout the genus Bsculus and a blue-green throughout Pavia. This alone rendered it evident either that there were at least two fluorescent substances present one in one bark and another in another or which appeared more probable that VOL.XJ. C STOKES ON A SECOND CRYSTALLIZABLE there were two (or possibly more) fluorescent substances present in different proportions in different barks. On examining under a deep violet glass a freshly cut section of a young shoot of at least two years’ growth of these various trees the sap which oozed out from different parts of the bark or pith was found to emit a differently coloured fluorescent light. Hence even the same bark must have contained more than one fluorescent substance; and as the existence of two would account for the fluorescent tints of the whole family a family so closely allied botanically the second of the suppositions mentioned above appeared by far the more probable. I happened to put some small pieces of horse-chestnnt bark with a little ether into a bottle which was laid aside imperfectly corked.On examining the bottle after some time the ether was found to have evaporated and had left behind a substance crystal- lized in delicate radiating crystals. This substance which I will call paviin when dissolved in water yields like aesculin a highly fluorescent solution and the fluorescence is in both cases destroyed (comparatively speaking) by acids and restored by alkalies. The tint however of the fluorescent light is decidedly different from that given by pure mculin for a specimen of which I am indebted to the kindness of the Prince of Salm-Horstmrtr being a blue-green in place of a sky-blue. The fluorescent tint of an infusion of horse-chestnut bark is intermediate between the two but much nearer to xwulin than to paviin.In all probability the fluorescence of the infusions of barks from the closely allied genera Bsculus and Pavia is due to mculin and paviin present in different proportions xsculin predominating generally in the genus Bsculus and paviin in Pavia. Bsculin and paviin are extremely similar in their properties so far as they have yet been observed. They are most easily distin- guished by the different colour of the fluorescent light of their solutions a character which is especially trustworthy as it does not iequire for its observation that the solutions should be pure. Paviin as appears from the way in which it was first obtained must be much more soluble than aesculin in ether.Bsculin is indeed described as insoluble in ether but it is siifficiently soluble to render the ether fluorescent Paviin like aesculin is withdrawn from its ethereal solution by agitation with water. Though of feeble affinities it is rather more &posed than zsculin to combine with oxide of lead. If a decoction of horse-chestnut bark be SUBSTANCE IN THE HORSE-CHESTNUT. purified by adding a sufficient quantity of a salt of peroxide of iron or of alumina precipitating by ammonia and filtering and the ammoniacal filtrate be partially precipitated by very dilute acetate of lead the whole redissolved by acetic acid reprecipitated by ammonia and filtered the fluorescent tint of the filtrate will be found to be a deeper blue than that of the original solution; while if the fluorescent substances combined with oxide of lead (the compound itself is not fluorescent) be again obtained in alkaline solution the tint as compared with the original will be found to verge towards green.The required solution is most easily obtained from the lead-compounds by means of an alkaline bicarbonate which plays the double part of an acid and an alkali yielding carbonic acid to the oxide of lead and ensuring the alkalinity of the filtrate from carbonate of lead. It is very easy in this way by repeating the process if necessary on the filtrate from the first precipitate to obtain a solution which will serve as a standard for the fluoreecent tint of pure aesculin. A solution serving nearly enough as a standard of comparison in this respect for pure paviin may be had by making a decoction of a little ash bark adding a considerable quantity of a salt of alumina precipitating by ammonia and filtering.By partial precipitation in the manner explained it is very easy to prove a mixture of zsculin and paviin to be a mixture even when operating on extremely small quantities. It must be carefully borne in mind that the characteristic fluor- escent tint of a solution is that of the fluorescent light coming from the solution directly to the eye. Even should a solution of the pure substance be nearly colourless by transmitted light though strong enough to develope the fluorescence to perfection if the solution be impure it is liable to be coloured most com- monly yellow of some kind which would make a blue seen through it appear green.To depend upon the fluorescent tint as seen through and modified by a coloured solution would be like depending on the analysis not of the substance to be investigated but of a mixture containing it. Yet in solutions obtained from the horse-chestnut and in similar cases the true fluorescent tint can be observed very well in spite of considerable colour in the solution. The best method of observing the true fluorescent tint is to dilute the fluid greatly and to pass into it a beam of sunlight condensed by a lens fixed in a board in such a manner that as c2 STOKES ON A SECOND CRYSTALLIZABLE small a thickness of the fluid as may be shall intervene between the fluorescent beam and the eye.If a stratum of this thickness of the dilute eolution be sensibly colourless the tint of the fluor-escent light will not be sensibly modified by subsequent absorp- tion. This however requires sunlight which is not always to be had. Another excellent method requiring only daylight and capable of practically superseding the former in the examination of horse-chestnut bark is the following in using which it is best that the solutions should he pretty strong or at least not extremely dilute. A glass vessel with water is placed at a window the vessel being blackened internally at the bottom by sinking a piece of black cloth or velvet in the water or otherwise. The solutions to be compared as to their fluorescent tint are placed in two test tubes which are held nearly vertically in the water their tops slightly inclining from the window and the observer regards the fluores- cent light from above looking outside the test tubes.Since by far the greater part of the fluorescent light comes from a very thin stratum of fluid next the surface by which the light enters the fluorescent rays have mostly to traverse only a very small thickness of the coloured fluid before reaching the eye; the water permits the escape of those fluorescent rays which would otherwise be internally reflected at the external surface of the test tubes; and the intensity of the light of which the tint is to be observed is increased by foreshortening. The observer would do well to practice with a fluorescent fluid purposely made yellow by intro- ducing some non-fluorescent indifferent substance ;thus a portion of the standard solution of esculin mentioned above may be ren- dered yellow by ferrid-cyanide of potassium.The more com-pletely the fluorescent tints of the yellow and the nearly colourless solution agree the more nearly perfect is the method of observa-tion. If ferro-cyanide of potassium be used in the experiment suggested instead of ferridcyanide the most marked effect is a diminution in the intensity of the fluorescent light the cause of which is that the absorption by this salt takes places more upon the active or fluorogenic than upon the fluorescent rays. Since sub- stauces of a similar eharacter may be present in an impure solution the observer must not always infer poverty with regard to fluor- escent substances from a want of brilliancy in the fluorescent light.The existence of paviin may perhaps account for the discrepan- cies between the analyses of ssculin given by different chemists. SUBSTANCE IN THE HOltSE-CHESTNUT. I should mention however that I have met with three specimens of zesculin and they all appeared to be free from paviin. The reason why asculin was obtained pure from a decoction containing paviin also is probably that the former greatly preponderates over the latter in the bark of the horse-chestnut. A decoction of this bark yielded to me a copious crop of crystals of mculin while the paviin together with a quantity of mculin still apparently in excess remained in the mother liquor.I map perhaps on some future occasion communicate to the Society the metliod employed when I have leisure to examine it further; I will merely state for the present that it enabled me to obtain crystallized zesculin in a few hours without employing any other solvent than water. In the method commonly employed the first crystallization of aesculin is described as requiring some fourteen days. On account of the small quantity apparently of paviin as com- pared with mculin present in the bark of the horse-chestnut a chemist who wished to obtain the substance for analysis would probably do well to examine a bark from the genuspavia if such could be procured. The richness of the bark in paviin as compared with mculin may be judged of by boiling a small portion with water in a test tube; those barks in which the substance presumed to be paviin abounds yield a decoction having almost exactly the same fluorescent tint as that of a decoction of ash bark.A crystallizable substance giving a highly fluorescent solution has been discovered in the bark of the ash by the Prince of Salm-Horstmar,* who Elas favoured me with a specimen. This sub-stance which has been named fraxin by its discoverer is so similar in its optical characters to paviin that the two can hardly if at all be distinguished thereby; but as fraxin is stated to be insoluble in ether it can hardly be identical with paviin which was left in a crystallized state by that solvent. I find however that fraxin is sufficiently soluble irr ether to render the fluid fluor-escent so that after all it is only a question of degree which cannot be satisfactorily settled till paviin shall have been prepared in greater quantity. *Poggendorffs Annalen vol. 100 (1857) p. 607.