206 



Mr. Abel on the Combustion of 



[April 23, 



the two pieces, falling to the bottom of the vessel, ceased to burn almost 

 immediately. Of a piece of gun-cotton weighing 2*17 grains, there re- 

 mained unchanged I'SO gr. ; the quantity burned amounted therefore to 

 0-37 gr., and corresponded closely to the quantity which was completely 

 burned in the preceding experiments. (The depression of the mercurial 

 column in this experiment, by the gases generated from the gun-cotton, 

 amounted to 0*2 inch.) 



A piece of the twist, 1 J inch long, was placed across the wire, and 

 supported by a plate of plaster of Paris, fixed immediately beneath it. The 

 current was established to an extent just sufficient to heat the wire to the 

 point of ignition of the gun-cotton, and then interrupted. The twist 

 burned slowly in both directions until about a quarter of an inch was con- 

 sumed on either side of the wire, when the combustion ceased. The 

 same result was obtained in repetitions of the experiment, the wire being at 

 once raised to a red heat, and thus maintained until the gun-cotton ceased 

 to burn. But upon increasing the battery-power, doubling the thickness 

 of the wire, and maintaining the heat, while a similar piece of twist was 

 burning in both directions, the slow combustion continued until the entire 

 quantity was transformed into gas. The same result was obtained by re- 

 peating this experiment with similar and larger quantities of gun-cotton, 

 placed in the same position as before with reference to the wire. 



In the next experiment, the mass of cotton exposed at one time to heat 

 was increased by doubling a piece of the twist (4 inches long) and laying 

 it thus doubled across the wire, as before. The current was allowed to 

 pass until the wire was heated just sufficiently to ignite the gun-cotton, 

 and then interrupted. In this case the slow combustion proceeded through- 

 out the entire mass of the cotton. The permanent depression of mercury in 

 this experiment was 0*6 inch. It was particularly noticed on this occasion, 

 that, as the decomposition of the gun-cotton crept slowly along the mass, 

 the burning portions or extremities of twist were surrounded by a beautiful 

 green light, more like a phosphorescence than a flame, and in form some- 

 thing similar to the brush of an electric discharge. 



Eight inches of the twist were laid fourfold over the wire, which was 

 heated just sufficiently to ignite the cotton. The decomposition proceeded, 

 as before, gradually throughout the mass of the gun-cotton, but became 

 somewhat more rapid towards the end, when the green glow, observed at 

 first, was superseded by a pale yellowish lambent flame, very different in 

 appearance from the flame which accompanies the combustion of gun-cot- 

 ton under ordinary conditions. The permanent depression of the column 

 of mercury in this experiment was 1*2 inch. 



The various modifications in the nature and extent of combustion which 

 gun-cotton may be made to undergo, as demonstrated by the above experi- 

 ments, when exposed to heat in highly rarefied atmospheres under variously 

 modified conditions, are evidently due to the same causes which affect the 

 rate of combustion of fuses under different atmospheric pressures, and which 



