EFFECTS OF DIFFERENT PRIMINGS. 59 



the explosive wave can only be produced with difficulty in such 

 a substance. 



Compressed gun-cotton itself is less compact than nitro- 

 glycerin owing to its structure. This is the reason why 

 pressures which are due to shocks should become sensibly 

 attenuated by the existence of interstices. Gun-cotton is there- 

 fore more difficult to explode than nitroglycerin. Nitroglycerin 

 explodes by the fall of a weight from a lesser height, by the 

 use of a priming charged with gun-cotton, or a mixture of 

 fulminate and potassium chlorate, etc. ; whereas gun-cotton does 

 not explode under the influence of nitroglycerin, nor under the 

 influence of a mixture of fulminate and chlorate, it requires the 

 more sudden shock of pure mercury fulminate. 



This latter agent is also less efficacious if it be employed 

 exposed than if it be placed in a thick copper or tin covering ; 

 it is less efficacious in an envelope made of paper or tinfoil, 

 than in a copper envelope; it is still less efficacious if the 

 priming be not in contact with the gun-cotton. Finally, if it 

 T5e~plaeed in a leaden tube, an elastic substance which at once 

 yields to pressure, its effect becomes nullified. 



Nitroglycerin is less explosive under the influence of a 

 priming of fulminate if it be inflamed before the explosion of 

 the fulminate, the previous inflammation producing a certain 

 void between the two (p. 56). The absence of immediate contact 

 between the dynamite contained in the cartridges and the 

 priming of fulminate is prejudicial for the same reason, the 

 shock being partially deadened by the interposed air. The 

 sensitiveness to the action of the fulminate is greater in 

 dynamite, containing liquid nitroglycerin, than in that con- 

 taining frozen nitroglycerin, which is similarly explained by 

 the absence of homogeneity in congealed dynamite, in which 

 nitroglycerin is partially separated from the porous silica owing 

 to its solidification. 



10. All these phenomena are explained by the more or less- 

 considerable value of initial pressures, by their more or less 

 sudden development, and by their more or less easy communica- 

 tion to the rest of the mass ; that is to say, by the conditions 

 which regulate the energy transformed into heat in a given time 

 in the interior of the first layers of the explosive substance 

 which are reached by the shock (see pp. 52, 53). 



The quantity of energy thus transformed depends therefore 

 both on the suddenness of the shock, and on the greatness of 

 the work which it is capable of developing, Now here we have 

 two data, which vary with each explosive substance. For 

 instance, the most suitable primings are not always those in 

 which the explosion is the most instantaneous. Abel has 

 recognised that nitrogen chloride is not very efficacious in 

 inflaming gun-cotton; nitrogen iodide, so sensitive to the least 



