550 PHILOSOPHICAL TRANSACTIONS. [anNO 1779. 



flammation ; so that the difference of the explosive force in the inflammation of 

 the 2 compounds can be no less than as 4 is to 2000. 



It must be here remembered, that air being a very compressible body, a mode- 

 rate resistance acting against its rarefaction easily overcomes the force of its ex- 

 pansion, when this expansion or rarefaction does not amount to more than 4 

 times its bulk ; that such a power ought not to be very great, we know by the 

 force employed in ordinary wind guns and condensing machines ; whereas no 

 condensing machine has yet been contrived by which air could be condensed to 

 any thing approaching the state of condensation of this fluid as it exists in the 

 substance of nitre. 



It might be here objected, that air compressed to -^ in a wind gun, possesses 

 a power not much short of gunpowder, though only -rVor -jV of it 's let loose at 

 a time ; and that thus inflammable air, though expanded only 4 times in the 

 moment of inflammation, may exert a force approaching that of the wind gun, 

 the whole mass of the charge being employed in one and the same explosion. 

 This comparison is very inadequate ; for in the case of a wind gun the air com- 

 pressed to ~ is ready to exert all the force of elasticity existing in the whole 

 mass, and may therefore be compared to a strong spring forcibly bent. But the 

 inflammable air is far from exerting the force of expansion and elasticity through 

 its whole mass at the same instant: for the inflanmiation is propagated through 

 it successively, beginning where the electrical spark kindles it, and reaching gra- 

 dually farther till the whole is consumed. Now as I have demonstrated, that 

 inflammable air is reduced to more than half its bulk by inflammation, it must 

 follow, that that portion of it which is consumed the first by the inflammation, 

 leaving more room by its diminution, diminishes in proportion the propelling 

 powers of what remains still to be inflamed. 



The very great difference between the explosive force of the 2 compounds is 

 illustrated by what happens after their inflammation. The compound of inflam- 

 mable with common or dephlogisticated air, is very much reduced in bulk after 

 inflammation. I found this by the following experiment : I fired a brass inflam- 

 mable air pistol, which had a piston in the cylinder, by which a proper quantity 

 of respirable and inflammable air was drawn in. I had rammed into the barrel, 

 adapted to it, a leaden bullet wrapped up in a piece of leather so strongly, that I 

 did not expect the resistance could be overcome by the explosion. I fired it by 

 an electrical spark ; the inflammation took place, the pistol became hot, the 

 ball was not propelled, and the piston was driven more than half way down the 

 cylinder by the pressure of the atmosphere, acting on it when the explosive air 

 was consumed by the inflammation. The case is quite different in the firing of 

 gunpowder, as there remains after its inflammation a mass of air which occupies 

 about 250 times the former bulk, according to Mr. Robins. 



