TEMPERATURE OB IGNITION OF QASEOUS MIXTURES. 1003 T~XXXVI.--The Temperature of Ignition o j Gaseous Mixtures. By JAMES WALLACE MCDAVID. NUMEROUS methods for the determination of the ignition-tempra-ture of gaseous mixtures are t o be found in the literature and these have already been classed into three groups by Dixon and Coward (T. 1909 95 514) in their paper on this subject. The experimental conditions governing this division are as follows : I. A bulb containing the gaseous mixture was plunged into a bath which was maintained a t a constant temperature. 11. A mixture of the gases was passed through a tube in a bath which was gradually being heated. 111. The gaseous mixture was compressed adiabatically and from the pressure required to ignite it the temperature of the mix-ture a t the ignition-point was calculated.I n each of the above methods the gaseous mixture is retained in the state necessary f o r ignition for some considerable time and i t is quite possible therefore that before the ignition-temperature is reached a state of slow combustion ensues which generates sufficient heat t o raise the remainder of the gaseous mixture to its ignit ion-point With regard to the third method which was that used by Falk ( J . Amer. Chem. SOG. 1906 28 1517; 1907 29 1536) Dixon and Coward point out t h a t the temperature of ignition of a gaseous mixture under abnormal pressure may be totally different from that of the same mixture under normal pressure. I n the method adopted by Dixon and Coward the two gases were allowed t o flow through two concentric tubes which were heated in an electric furnace and allowed to mix a t the top of the shorter (inner) tube.A thermo-couple was placed near to the nozzle of the inner tube. I n these experiments in addition to the time factor the fact that the gases are flowing must also be taken into consideration and thO results obtained by these authors show that the rate of flow and the size of the nozzle of the inner tube affect the results; for example when the rate of flow of hydrogen was very dow namely 2.4 C.C. per minute and the rate of flow of oxygen was 50 C.C. per minute the temperature of ignition was 7 9 2 O whilst when the rate of flow of hydrogen was 19 C.C. per minute oxygen still flowing a t 50 C.C. per minute, the ignition-temperature waa found t o be 599O.These authors, VOL. CXT. R 1004 McDAVID THE TEMPERATURES OP however give in their paper a definition as t o what they have taken as the temperature of ignition It seems that' it is advisable to eliminate as far as possible the time factor in determining the ignition-temperature and it' is also preferable to have a stationary tolume of gas. In table I the results obtained by -various workers for several gaseous mixtures are given. TABLE I. I9 12.i t io 11- t e ) ) I p e m t ii I * es of 1) iff e re I I t G'cc .s e o 11 s Mi L t u r e s . Temperature of ignition Hydrogen and Meyer Hrause L4?aizale?~ 1891 264 85 ; 518-606" Gases. 0 bserl-er. Reference. found . Oxygen. Askenasy . ibicl. 1892,269 49 2421 Zeitsch.physikal. Chein., 1893 11 25. , , Rodenstein. Ibid. 1899 29 665. 653-710 , , Meyer and Freyer Ber. 1892 52 662 700 ) , GautierandHBlier G'ompt. rend. 1896 122 840 y y Helier Ann. chi^. Phys. 1897 845 y , Palk J . Ainer. Chem. Soc. 1906 514-540 556 [vii] 10 521. 28 1517; 1907 29, 1536. y 7 , DixoiiaiidCoward T. 1909 95 514 580-590 Rletliaiio and Illeyer and Freyer Zeitsch. physiknl. C17~on. 606-650 Oxygen. 1893,11 28. , , Meyer and Murich Ber. 1893 26 2421 656-678 , , Dixoii aiid Coward Zoc. cit. 5 5 6-7 00 Ethylene a.nd Meyer and Freyer ,) , , illeyer and Munch ,, , , DixonandCoward ,, Carbon mon- Meyer and Freyer ,, , , Bleyer and Munch ,, , , DixonttndCoward ,, Oxygen. oxide and oxygen 530-606 577-590 500-519 650-730 Combined quietly 637 658 The methods now described eliminate so far as is practically possible the time factor.The temperature of ignition in these experiments is taken t o be that temperature t o which the gaseous mixture must be heated by the npplicatioi? of a hot body so as to cause instantaneous ignition. The temperature of ignition 11x3 in the present woik only been determined for various gases when mixed with air. Dixon anti Coward have however showii t h a t there is very little difference in the temperature found when air or oxygen respectively is employed IGNITION OF GASEOUS MIXTURES. 1006 The first set of experiments deecribed below was originally undertaken with a view to obtain comparative values €or the temperatures of ignition of various gases but the results seemed of sufficient value to warrant further work being carried out.As a consequence numerous variations and improvements were intro-duced and finally a method was devised which was simple rapid, and capable of giving accurate iesults Details of all the variations of the method have however been included as several of them appear t o be useful for comparative purposes a t least although n o t sufficiently accurate for the deter-mination of absolute values. The results obtained by the method linally adopted for the ignition-temperatures of the various gases tested are given below: Coal gas air ....................................... 878" Ethylene-air ....................................... 1000 Hydrogen-air ....................................747 Carbon monoxide-air ........................... 931 Petrol (fraction O-SO")-air .................. 995 13enzene -air ....................................... 1062 Ether-~ir .......................................... 1033 E x P E R I M B N T A L. The essence of the method employed in the following experi-ments consists in the ignition of a small volume of the gaseous mixture contained in a soap bubble by means of an electrically heated wire or other red-hot body the temperature at which ignition just takes place being noted. A quantity of the gas to' be experimented on was mixed with air and made up t o the required concentration in a IO-litre gas holder. To the outlet' tube of the holder was attached a piece of capillary tubing terminating in a bulb tube shaped like the head of an ordinary tobacco pipe.This tube when not in use dipped into a solution of sodium oleate and glycerol in water. The apparatus employed for igniting the gas consisted of a platinum wire measuring about 0.025 cm. in diameter which was wound evenly round a small thin-walled silica tube. A thermo-couple made of platinum-rhodium which had previously been care-fully standardised against' a standard pyrometer was passed through the inside of the silica tube so that the thermo-junction was in the centre of the tube. The terminals of the thermo-couple were connected to a millivoltmeter from which the temperature could be read off. The experiments were carried out in a darkened roo~xi as in some cases the flame of the ignited gas was almost' invisible in daylight.The procedure was as follows When a11 the connexions had been made the heating current' which could be varied by means R R 1006 McDAVID THE TEMPERATURES OF of a rheostat was switched on. When the temperature as indicated by the millivoltmeter was constant the tap of the gas holder was opened and the mixture flowing through the pipe stem and soap solution produced a bubble. The t3ap was closed and the bubble still on the end of the pipe was placed in contact with the hot coil. Ignition was instantaneous and was usually accompanied by a slight explosion but in some cases t'his was so faint as to be detected only with difficulty. The temperature was varied over 60-80° until a sufficient number of readings had been obtained.The gases employed in the first instance were prepared as follows : Hydrogen.-From pure zinc and sulpiiuric acid. Methane.-By the action of a zinc-copper couple on methyl iodide. The gas was passed through a tube containing more of the zinc-copper couple in order t o free it from methyl iodide vapour. Et hyZene.-By passing alcohol vapour over heated aluminium oxide. The gas obtained contained 9 per cent. of methane. Coal Gas.-Samples were taken from time t o time from the local gas supply. Petrol.-Commercial petrol was fractionated and 2 C.C. of the fraction boiling between Oo and 80° were vaporised and made up t o 10 litres with air. Benzene and Et her.-Pure chemicals were used and vaporised as in the case of petrol. The results found are given in the following tables.TABLE 11. Coinprison of Coal Gas-Air and ,Wethane-Air Mixtures : 90.5 per cent. of Air in Mixture. Rotherham gas. Ardeer coal gas Methane. ,- \ 7'- I-ature. Result. ature. Result. ature. Result. Temper- Temper- Temper-760" No ignition 760" No ignition 840' No ignition 7 60 9 9 780 Ignition 860 9 9 765 9 9 780 No ignition 865 11 7 80 9 9 7 80 9 9 870 Y Y 7 90 9 9 7 90 Y Y 880 Y Y 790 Y Y 7 90 9 9 880 9 9 795 9 7 795 Ignition 880 1 9 800 Ignition 795 Y 9 880 9 9 805 9 9 800 No ignition 880 Y9 806 9 ) 800 Ignition 880 Ignition 810 Y9 800 9 9 880 9 > 810 9 Y 800 Y 9 8 80 Y 9 830 Y Y 810 9 9 890 9 ) 840 9 9 820 9 9 920 9 9 Temp. of ignition = 800'. Temp. of ignition= 800'. Temp. of ignition= 880' IGNITION OF GASEOUS MIXTURES.1007 TABLE 111. Comparison of Coal Gas Methane and Hydrogen-Air iNiztures : 86.5 per cent. of Air Present. Rotherham coal Tem- Tem- Tem- Tem-per- per - per - per -ature. Result. ature. Rosult. ature. Result. ature. Result. 780" No ignition 800" No ignition 6'70" No ignition 860' KO ignition gas. Ardeer coal gas. Hydrogen. Methane. /- I- -785 9 9 800 9 9 6 80 9 9 870 9 9 7 86 9 9 805 9 9 080 9 9 876 9 9 800 9 9 805 Ignition 690 9 9 880 No ignition 800 Ignition 810 No ignition 690 9 885 Ignition 800 9 9 810 ? ? 690 Ignition 885 ,, 805 Noignition 810 9 9 690 9 9 890 ? 9 815 , 700 9 9 710 9 9 720 ,) 9 9 9 9 805 690 875 Ignition 800 815 Ignition 810 Ignition 700 Noignition 900 ,, 815 9 9 810 , 700 Ignition 910 ,, 820 . 710 NO ignition 820 9 3 710 Ignition - -I__-Temp.of ignition Temp. of ignition Temp. of ignition Temp. of ignition = 806". = 810". = G95". = 885". TABLE IV. Cornprison of Petrol and Ethylene-Air Mixtures. Mixture of 9.1% C,H, O.9q0 CH,, and goo/ air. Tompsr- No. of ature. readings. Result. 845" 1 Ignition 840 2 9 ) 830 7 9 9 825 8 9 9 825 1 No ignition 820 4 Ignition 820 12 $0 ignition 815 3 9 9 s10 3 9 9 Temp. of ignition = 825". Mixture of 2.0 C.C. petrol (&SOo) vaporised and made up with air t o 10 litres. * r \ Temper- No. of ature. readings. Result. 900" 3 Ignition 890 3 9 9 8 80 6 9 , 8 80 7 No ignition 875 2 Ignition 875 5 No ignitioli 870 5 Y 9 865 2 9 9 Temp. of ignition = 885". A second series of experiments was carried out in which the heating coil was replaced by a semi-cylindrical piece of iron about 3.75 cm.long by 1.9 cm. in diameter covered round the circum-ference with thick asbestos cloth (No. 1 iron block). Running down the centre of the flat surface and about 0.08 cm. under the surface was a small hole to carry the thermo-couple. The block was hung by two platinum wires from a stand. It was then heated until about 50° above the required temperature when th 1008 McDAVID THE TEXPERATURES OF blowpipe was removed and bubbles of gas were brought into con-tact with the red-hot surface while the latter was cooling. When the temperature became too low for ignition t o take place the block was simply heated again and the procedure carried out as before until a sufficient number of readings had been obtained.An improvement was made in No. 2 block by boring a small hole in the centre of the flat face. By this means the thermo-couple became exposed to the air and since i t lay close to the surface i t ought to give a comparatively true figure for the temperature of the igniting surface. Tables V-VTII give com-parative results found by the above three methods. TABLE V. Contp-ison of Iroii Block and Electric Coil Mvtlzod with 13.5 per c e t t t . Coal Gas-Air illixtm-e. Electric coil method. Iron block (No. 1-Block) method. f - /-A Temper- No. of Temper- No. of atlure. rcdings. Result. atnre. ~.i.adings. Result'. 770" 775 7 SO 7 SO 785 786 790 790 795 800 :3 2 4 2 1 1 1 3 1 1 3 , Ignition KO ignition Ignition No ignition 1 gnit,ion 7 7 720" 730 710 750 760 765 7 70 790 800 810 1 2 2 1 1 1 2 2 1 > No ignit,ion Y 1 7 7 7 7 Ignition 9 7 Y 9 I 7 ? Y 810 1 7 7 -Temp.of ignition = 785" Temp. of ignit,ion == 760". TABLE VI. Cornprison of Electric Coil and Iron Block Method 15 per cent. of Hydrogen with Air. Electric coil. , Tern- NO. of per- read-ature ings. Result. 690" 2 Ignition 685 2 f ? 680 3 7 7 676 2 7 , 675 3 No ignition 070 4 ? ? 666 1 Y7 GOO 1 7 7 Temp. of ignition = 680". No. 1 iron block. No. 2 iron block. / -. 7-Tern- No. of per- read- per- read-atare ings. Result. ature ings. Result,. 750" 6 Ignition 705" 2 Ignition 710 9 7 7 700 4 7 Y 705 3 7 7 695 3 9 7 705 3 No igiiitioii 090 4 7 9 700 7 Y7 685 3 7 7 696 3 7 ) 680 2 3 , 090 3 7 7 680 0 No ignition Tern- No.of 676 2 ? ? A70 6 7 7 G G 6 1 7 7 Temp. of ignition= 705". Temp. of ignition= 085. IGNITION OF GASEOUS MIXTURES. 1009 TABLE V I I . Mixticre of Two Volumes of HydrogeIL wiih Five VolunLPs of Air: Two Series Carried Ozit with A'o. 2 Iron Block. Temper-ature. 730" 720 715 710 705 700 700 695 690 685 680 No. of readings. 3 3 1 2 1 1 1 3 1 1 I No igizitjioii Temper-ature. 730" 725 720 715 710 700 695 690 685 tj 80 670 No. of readings. 2 1 3 1 ti 2 3 4 1 2 1 Result. Ignition 9 9 9 1 9 9 ) No ignitioii Temp.of ignit.ion = 700". Temp. of ignition = 700". TABLE VIII. C'onapariso?z of Elect?-ic Coil arid Iron Block Methods 13.5 per cent. Mixtitre of Coal Gas with A i r . Electric coil. No.1 iron block. No. 2 iron block. h h r- > 7- c , Tern- No. of Tem- No. of Tern- No. of pera- read- pera- read- pera- read-ture. ings. Result. ture. ings. Result. ture. ings. Result. 786" 6 Ignition 810' 6 Ignition 800" 3 Jgnition 782 3 9 ) 805 1 9 9 795 1 Y Y 780 3 Y Y 800 1 Y > 790 3 7 Y 775 3 9 ) 795 3 Y Y 785 2 Y 9 775 1 KO ignition 795 2 No ignition 780 2 Y Y 772 2 Ignition 790 1 Ignition 775 2 > Y 773 5 No ignition 790 3 KO ignition 770 3 9 9 770 3 9 7 785 1 9 9 770 1 No ignition 768 3 9 780 4 9 765 3 Ignition 765 1 Ignition 775 3 Y 9 765 3 No ignition 765 3 No ignition 760 3 9 9 760 1 9 9 755 1 Y 9 750 2 9 9 Temp.of ignition= 775". Temp. of ignition= 795". Temp. of ignition= 765". .-From the results given i n tables 11-VIII it will be observed that the method gives a very sharp ignition-point and since the time taken to determine the temperature of ignition of any mix-ture was only about fifteen minutes i t is evident that f o r purposes of comparison a t least the method is very suitable. It will also be noted from the above results that the concentra-tion of the explosive gas has within the explosion limits no effect 011 the temperature of ignition. As however. the rate of cooling in the iron block method wa 1010 McDAVID THE TEMPERATURES OF very rapid the method was not very satisfactory for the purpose of obtaining absolute values.Accordingly it was discarded in favour of the method first described. Even in this method there are several factors that may affect t'he absolute value of the results whilst in no way affecting their comparat'ive value. These factors are the cooling effect of the bubble the presence of moisture in the gases the catalytic action of the platinum wire and the size of the silica tube. Dixon and Coward (Zoc. cit.) showed that the presence of moisture did not affect the ignition-temperature in the case of hydrogen and oxygen and in several other cases the difference was very small. The cooling effect of the bubble need not be taken into account, as its mass was very small compared with that of the heating source.Moreover although it was found that variations in the size of the bubble gave small differences in temperature this was probably due to a different cause which is explained later. The chief defect in this method lay in the fact t,hat the tempera-ture registered by the thermo-couple was probably slightly lower than that of the coil since the former was separated from the coil by the thickness of the silica and a volume of air. It is of course, obvious that the thermo-junction ought t o be near to the centre of the tube and that the coil must be as evenly wound as possible, so that there may be no unequal heating. I n order to study the effect of the size of the silica tube on the temperature recorded by the t'hermo-couple and if possible to obtain absolute values for the temperature of ignition of various gases a large number of determinations was carried out.For this purpose the gases employed were obtained in as pure a state as possible and on analysis gave the following result6 : Per cent. Hydrogen .............................. = 94.0 {zg = 6.0 ..................... Carbon monoxide Ethylene ................................. ............... C2H4 = 0.31 H = 1.43 N = 2.40 Methane (natural gas) I n making up the gaseous mixtures allowance was always made for the quantity of air present in the gas IGI.NI!CION OF GASEOUS MIXTURES. 1011 Table IX gives the dimensions of the various silica tubes employed in the determinatiolns. These were drawn out as evenly as possible from ordinary silica tubing. The experiments were carried out in the same manner as before, except that the ends of the silica tubes were in all cases stopped up with asbestos fibre so as t o diminish loss of heat within the tube by convection.Platinum and Eureka resistance wire were employed for making the ignition coils. The results found for hydrogen are given in table IX. TABLE IX. Showing No. of tube. 1 2 3 4 5 6 7 8 9 10 Bffect of the Size of the Silica Tube o n the Temperature of Igiiition of Hydrogen. Temperature of ignition of 20 per cent. of hydrogen in air using Thickness of wall of tube. 0.055 0.035 0.030 (em. 1 0.0425 0.030 0.035 0.035 0.0276 0.0225 0.025 Internal diameter . (cm.) 0.1575 0.1475 0.12 0,0775 0-075 0.075 0.095 0.0775 0.065 0.065 Length.(em. 1 6 6 6 6 3 6 5.5 6.0 4.0 4.0 Platjnum Eureka wire. wire. 682' 712" 688 712 { %} -{Z} -- 735 - 735 735 758 735 -It will be observed in the first place that for the same tube the figures found by using platinum are higher than those obtained when using Eureka wire indicat!ing a catalysing effect in the case of the latter. It is however probable that both substances exert a catalytic influence. It will also be noticed that as the thickness of the wall and the internal diameter of the silica tube decreased the temperature of ignition increased. It was not found possible to make tubes of smaller size than No. 9 and it was therefore impossible to say whether o r not the temperature given by No.9 were a maximum. It seemed prob-able however that the temperatures found when using this tube were very near t o the actual temperature of the heating coil. Accordingly this tube wound with platinum wire was used t o R R 1012 McDAVID THE TEMPERATURES OF determine the temperatures of ignition of other gases and the results found are summarised below : Hydrogen-air .................. 758" Carbon monoxide-air ......... 910 Ethylene-air ..................... 895-905 Coal gas-air ..................... 850 Petrol-air ........................ 960 Methane-air ..................... No ignition below 1000". In tables I and I1 the ignition-temperature of methane in air was found t o be 880-885O but. the gas probably contained a con-siderable quantity of hydrogen.I n order to verify this supposi-tion the effect of adding different proportions of hydrogen to methane-air mixtures was studied with the following results : TABLE X. Showing Effect of Hytlrogeu on the Temperature of Ignition of Methane and Air. Gaseous mixture. Temperature of ignition. 1000 ) hydrogen ............... 796" I 1 1000 C.C. methane 8000 , air 1500 C.C. methane 8000 , air 1850 C.C. methane 8000 ? air 500 ,) hydrogen ............... 836" 150 ,) hydrogen ............... No ignition up to 970". It is thus evident that pure methane when mixed with air does not ignite below 1000°. This fact is interesting and receives some confirmation from a statement in Brunswig's " Explosives " (English edition p. 55) in which it is stated that in the case of methane and air there is delayed ignition at 600° t o 7005 but that instantaneous ignition does not take place below 1000°.The foregoing experiments a1 t hough interesting were however, not quite conclusive but i t was hoped t l k t by making use of an instrument called the meldometer invented by Professor Joly, corroboration of the above results would be obtained. This instrument consists esseniially of an apparatus for measur-ing accurately the expansion of a platinum strip when subjected to heat. 'The strip which is held between two arms one fixed and the other movable is heated electrically and thus expands. Since the linear expansion in the case of platinum is almost directly proportional to the temperature i t is sufficient t o note the length of the strip at one or two fixed temperatures in orde IGNITION OF GASEOUS MIXTURES.1013 to find out by int)erpolation the temperature corresponding with any given length of the strip. Unfortunately the experiments carried out with this apparatus were unsuccessful. The strip was so very thin that when a bubble of the gaseous mixture was brought in contact with it slow com-bustion first ensued which raised the temperature of the strip until the latter glowed and thus exploded the residue of the gas. The method of standardising this instrument however indicated a method of standardising the apparatus used in the first series of experiments described herein. The apparatus finally employed, the method of standardisation and the results obtained are described below.The ignition apparatus in this final series of experiments con-sisted of a platinum coil wound regularly round two mica strips 3 cm. in length by about 0.3 cin. in width. These strips were notched along the edges so as to hold the wires in position whilst between them and thus insulated from the heating coil was placed a platinum-platinum-rhodium thermo-couple. The wire used for the heating coil was about 0*025 cin. in diameter and tlic pitch of the spiral was equal to about the diameter of the wire. Tho heating current was provided by storage batteries in order t o obtain no fluctuations in the temperature and the latter was varied by means of a sliding resistance. An ammeter was also placed in the circuit in order to measure the current.The thermo-couple which had previously been standardised against a standard thermo-couple was connected to a millivolt-meter. It will be noticed later however that i t was unnecessary to use a standardised thermo-couple; in fact it was ultimately found that no thermo-couple was necessary as the required results could be obtained by simply reading the ammeter. The apparatus was standardised in the following manner. Four salts the melting points of which were accurately known were selected and purified. A few crystals of each were ground to a powder and a small quantity of this powder was deposited on the platinum spiral. The current was then turned on and the coil heated the temperature being slowly raised until the salt just melted. A rough experiment was carried out first in each case to find out the approximate melting point the determination being then repeated several times with great care.It was found when the melting points of the salts were taken from time t o time during the experiments t h a t the readings on the millivoltmeter varied considerably but that the ammeter readings were practically constant for the same temperatura. R R" 1014 McDAVID THE TEMPERATURES OF The variation in the millivoltmeter readings may have been due to various causes for example the alteration in zero point of the instrument variation in room temperature or to the thermo-couple having been accidentally moved from the centre of the coil during the experiments. The ammeter readings theref ore, Temperature.Curve connecting ammeter readings with actual temperature. Standardised 1.5.16 and 2.5.16. only have been recorded. standardising the apparatus before and after use. Table XI gives the results obtained on TABLE XI. Ammeter readings. Before Before Potassium iodide . . . . . . 687' 3.74 3.76 A r -Salt employed. True m.p. experiments. experiments. Potassium bromide ... 723 3.90 3.90 Sodium chloride ...... 800 4.22 4-22 Potassium sulphate . . . 1072 5.45 5.47 The curve connecting the ammeter readings with the true Table XI1 gives the analysis of the gases employed. temperature is shown in the figure IGNITION O F GASEOUS MIXTURES. CO per cent. ...... 0, C,H . . . . . . . . co . . . . . . . . CH . . . . . . . . H . . . . . . . . 2 r . .. . . . . . . . . . . . . . . . . . . . . . TABLE XII. Analysis of Gases Used. Coal gas. Ethylene. Hydrogen. - - 0.26 0.47 4.53 94.0 9.84 50.30 33.00 - 95.8 1-60 - 6.0 4-2 - --- -- -- -1015 Carbon monoxide. ---97.5 ---2.5 I n table XIII the results found f o r the temperature of ignition of various mixtures of gases with air are given. I n cases where the gas contained air as impurity this was allowed for in making up the mixture. TABLE XIII. Zgnitiol2rtemperntures of Different Inflammable Gas-Air Mixtures. True temperature Mixture. Ammeter reading. of ignition. 15% coal gas-air .................. 4.59 S78" 10% ethylene-air .................. 5.12 1000 10 yo hydrogen-air ............... 4.00 747 Petrol (b.p. O-SO")-air ......... 5-10 985 Carbon monoxide-air ............ 4.82 931 Benzene-air ........................ 5.41 1062 Ether-air ........................... 6-27 1033 It was found that by enlarging the size of the bubble the mix-ture could be made to ignite a t an apparently lower temperature. This however was probably due to the fact that slow combustion of part of the gas took place a t the lower temperature and by heating the surrounding gas caused it to ignite without showing a corresponding rise on the ammeter. This ignition was as a rule quite perceptibly delayed. By reducing the size of the bubble t o about 3.7 cm. diameter instantaneous ignition occurred. The results given in table XIII are the mean oE a large number of experiments and experimental error can be t.aken as being less than + 3 O . The author wishes t o express his thanks t o Messrs. Nobel's Explosives Co. Ltd. for granting permission t o publish these results. NOBEL RESEARCH LABORATORIES, ARDEER STEVENSTON N.B. [Received September 6th 1917.