MERCURY FULMINATE GIVES GREATEST PRESSURE. 539 



But this inequality is redeemed in practice by the impossi- 

 bility of raising gaseous mixtures to densities of charge compar- 

 able with those of other explosive substances. This observation 

 applies equally to the comparison of the gaseous volumes 

 developed by the two orders of substances. The absolute volume 

 of gases produced by one kgm. of matter is the maximum for 

 hydrogen mixed with oxygen; the other gaseous mixtures 

 scarcely attain the half of this. Among solid or liquid com- 

 pounds, gun-cotton and diazobenzene nitrate are those which 

 furnish the largest volume of gas, namely, two-fifths of the 

 volume produced by the oxyhydric mixture ; nitroglycerin is less 

 by one-sixth ; service powder does not attain to one-fourth the 

 volume furnished by the oxyhydric mixture, and is about one- 

 third the volume developed by nitroglycerin or gun-cotton. 



Any advantage, however, which gaseous mixtures appear to 

 offer according to these figures is not founded on the actual 

 measurements which have been made of specific pressures. In 

 fact, the most energetic mixtures, such as oxygen and hydrogen, 

 and methane and oxygen, barely attain the same pressures at a 

 given density of charge as nitroglycerin, nitromannite, and 

 gun-cotton, which substances are very similar to one another in 

 this respect. 



In truth, the specific pressures are deduced from experiments 

 made with gaseous mixtures at very small densities of charge. 

 Probably, if experimenting with gases compressed beforehand 

 so as to bring them up to densities comparable to those of 

 liquids, we might arrive at much higher specific pressures. At 

 all events the fact is one worth noting. 



The specific pressure of black powder under a density of 

 charge equal to unity would exceed the foregoing by about 

 one-half. Mercury fulminate does not go beyond this at this 

 density of charge. But its great specific weight (443) allows it 

 to attain four times this pressure when it detonates in its own 

 volume ; pressures to which no known body approaches. We 

 have said already that this circumstance plays a leading part 

 in the use of fulminate as a priming. 



In order to complete these ideas and to fully characterise 

 explosive bodies, we must further know the duration of the 

 decomposition in each of the substances, that is to say, the 

 specific velocity of their explosive wave. This velocity has, in 

 fact, been found equal to 2840 metres per second in oxyhydric 

 mixtures, and to 2400 metres in acetylene mixed with hydrogen. 

 The other combustible gases give similar velocities, with the 

 exception of carbonic oxide mixed with oxygen, which falls to 

 1089 metres. With solid or liquid substances similar data are 

 for the most part wanting, nevertheless velocities of 5000 metres 

 have been observed with dynamite, and 5000 to 6000 metres 

 with gun-cotton. These velocities are ample to account for the 



