EFFECTS OF SHOCK. 51 



wrapped in a sheet of platinum and placed on an anvil, will be 

 sufficient to give rise to the formation of very sensible traces of 

 potassium chloride, whereas under similar conditions potassium 

 sulphate gives no indication whatever of decomposition. But 

 the decomposition of potassium sulphate into potassium sulphide 

 and oxygen absorbs heat, whereas the decomposition of potas- 

 sium chlorate into potassium chloride disengages heat (11,000 

 cal. for KClOa). 



11. This condition, however, is not sufficient for shock to 

 produce a detonation. It is further necessary that the energy 

 developed by the decomposition of the first portions should be 

 able to communicate itself to the neighbouring parts, so as to 

 determine step by step the decomposition of the whole mass. 

 The most favourable condition is evidently that one in which 

 the particles of the explosive substance are in movement and 

 animated by an energy of such a nature that their sudden 

 stoppage would transform this force into heat in the interior of 

 the substance itself. The substance is thus heated in a uniform 

 and sudden manner ; if the proper temperature be attained, the 

 explosion occurs immediately. Such conditions may be realized 

 on the sudden stoppage of a bomb-shell charged with dynamite 

 which meets with a resisting surface (see p. 43). In an opposite 

 sense it may be noted that the shock of a hammer which is 

 hardly sufficient to produce on some isolated points the desired 

 conditions with pure potassium chlorate is, on the contrary, 

 very efficacious with nitroglycerin. Even the fall of a weight 

 of 4*7 kgms. from a height of 0*25 metre on to a drop of nitro- 

 glycerin covering a surface of 2 sq. cms. is sufficient to cause 

 the explosion of this substance. 1 On the other hand, nitro- 

 glycerin mixed with a silicious earth constitutes dynamite, a 

 substance which is very slightly susceptible to shock, because 

 the porous and cellular structure of the silica militates against 

 the immediate and local communication of energy to a very 

 small portion of nitroglycerin when regarded apart from 

 the rest. 



Further, the explosion of black powder causes nitroglycerin 

 to explode, whereas it does not produce the explosion of 

 dynamite, at least in the open air and with weak charges. 



But this inertness disappears under the influence of certain 

 particularly violent shocks, such as that of mercury fulminate. 

 Again, the explosion of nitroglycerin is very different, according 

 as it is pure or mixed with another body or is operated on by a 

 simple shock, by contact with a body in weak ignition, or in 

 active ignition, or, again, by the aid of an ordinary fuse; or, 

 finally, by the contact of a strong priming of mercury fulminate. 



1 Ch. Girard, Millot et Vogt (" Comptes rendus des stances de 1'Acade'mie 

 des Sciences," torn. Ixxi. p. 691). 



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