210 



Mr. Abel on the Combustion of 



[April 21, 



atmospliere is sufficiently great this mixture, which has self-combustible 

 properties, retains sufficient heat as it escapes, to burn, more or less com- 

 pletely, according to the degree of rarefaction of the atmosphere. 



A series of experiments instituted with gun-cotton in highly rarefied 

 atmospheres of oxygen, showed that the additional proportion of this gas 

 thus introduced into the apparatus, beyond that which would have been 

 contained in it with the employment of air of the same rarefaction, affected 

 in a very important manner the behaviour of the explosion under the in- 

 fluence of heat. If eight or ten grains of gun-cotton are placed round the 

 platinum wire, and the pressure of the atmosphere of oxygen in the vessel 

 be reduced to four or three (in inches of mercury), the cotton explodes in- 

 stantaneously, with an intensely bright flash, when the wire is heated. In 

 a series of experiments made under gradually diminished pressures, oxygen 

 being used instead of air, it was found that the gun-cotton exploded instan- 

 taneously, with a bright flash, until the pressure was reduced to 1*2 inch; 

 from this pressure to that of 0*8 inch it still burned with a flash, but not 

 instantaneously ; and at pressures below 0*8 inch it no longer burned with 

 a bright flash, but exhibited the comparatively slow combustion, accom- 

 panied by the pale yellow flame, which has been spoken of as observed 

 when gun-cotton was ignited in air rarefied to pressures ranging from 1 inch 

 to 24 inches. 



The interesting phenomena exhibited by gun-cotton in highly rarefied 

 atmospheres, induced me to make some experiments of a corresponding 

 nature with gunpowder. The same apparatus was used as in the preceding 

 experiments, but a small glass cup was fixed immediately beneath the 

 platinum wire, so that, by bending the latter in the centre, it was made to 

 dip into the cup, and could be covered by grains of gunpowder. 



Two grains' weight of small grain gunpowder were heaped over the wire, 

 and the pressure of air in the apparatus was reduced to 0*65 inch. The 

 wire being heated to redness, three or four grains, in immediate proximity 

 to it, fused in a short time and appeared to boil, evolving yellowish vapours, 

 no doubt of sulphur. After the heat had been continued for eight or ten 

 seconds, those particular grains deflagrated, and the remainder of the 

 powder was scattered by the slight explosion, without being ignited. No 

 appreciable depression of the mercurial column occurred during the evolu- 

 tion of the yellowish vapours ; the permanent depression, after the defla- 

 gration, was only 0-15 inch. 



The experiment was repeated with small-grain gunpowder, amounting to 

 four grains, and the same phenomena were observed, with this difference, 

 that a second slight deflagration followed shortly after the first, probably in 

 consequence of a grain or two of the powder falling back into the cup. 



A single piece of gunpowder, weighing 14 grains, so shaped as to remain 

 in good contact with the wire, was placed over the latter, being supported 

 by the cup. The pressure of air in the apparatus was, as before, equal to 

 0'65 inch of mercury. There was no perceptible effect for a short time 



