54 DURATION OF EXPLOSIVE REACTIONS. 



5. Effects vary similarly according to whether the substance 

 is pure or associated with foreign substances, and again accord- 

 ing to the structure of the latter. This is shown by dynamite, 

 a mixture of nitroglycerin with silica ; which has lost a great 

 part of its sensitiveness to ordinary shock, yet it remains explo- 

 sive under the shock of a ball, and particularly under the shock 

 of mercury fulminate. 



6. Gun-gotton impregnated with water or paraffin becomes 

 equally non-sensitive to a shock. In order to explode it a small 

 supplementary cartridge of dry gun-cotton powder primed with 

 fulminate must be employed. 



If a small quantity of camphor be incorporated with nitro- 

 cellulose its susceptibility to explosion by shock is almost 

 completely annihilated, at least at the ordinary temperature ; to 

 such an extent that this association constitutes a substance now 

 employed for various purposes under the name of celluloid. 



7. Blasting gelatin, which is made of nitroglycerin and nitro- 

 cotton, sometimes with the addition of camphor, constitutes an 

 elastic mass very slightly sensitive to shock, and which in like 

 manner requires an auxiliary cartridge of dry gun-cotton also 

 primed with fulminate. 



8. The change introduced by the camphor and resinous 

 matters in the explosive properties of such substances, results 

 from the modification supervening in the cohesion of the mass. 

 The mass has acquired a certain elasticity and solidity of parts, 

 by reason of which the initial shock of the detonator is at. once 

 propagated throughout a much larger mass (see p. 38). Besides, 

 a portion of the effects of the shock is expended in the work of 

 tearing up and separation, a small portion of it remains, which 

 is susceptible of heating the parts directly struck, and this 

 heating is, moreover, distributed throughout a large mass. 

 Hence a sudden and local rise in temperature, capable of deter- 

 mining chemical and consecutive mechanical actions, becomes 

 more difficult ; it will require the employment of a detonator 

 of much greater weight. This results from the preceding theory. 



9. But camphor, on the other hand, should not, and does not, 

 in fact, exercise, as experience proves, any specific action on a 

 discontinuous powder, such as potassium chlorate powders. It 

 is this which also explains the fact that blasting gelatin, when 

 frozen, recovers a sensitiveness to shock which may be compared 

 with that of nitroglycerin ; the continuity of it has been 

 destroyed by the crystallisation of this substance. 



10. Hence we see the importance of primers, until recently 

 regarded as simple agents intended to communicate flame to 

 powder. In fact these primers, provided only their bulk 

 be sufficient, regulate by their nature the character of the 

 initial shock, and consequently the character of the entire 

 explosion. In this case they take the name of detonators, 



