Properties of the Gelatine Compounds.—­Blasting gelatine is generally composed of 93 to 95 parts nitro-glycerine, and 5 to 7 parts of nitro-cellulose, but the relative proportions of explosive base and nitro-glycerine, &c., in the various forms of the gelatine explosives do not always correspond to those necessary for total combustion, either because an incomplete combustion gives rise to a greater volume of gas, or because the rapidity of decomposition and the law of expansion varies according to the relative proportions and the conditions of application.  The various additions to blasting gelatine generally have the effect of lowering the strength by reducing the amount of nitro-glycerine, but this is sometimes done in order to change a shattering agent into a propulsive force.  If this process be carried too far, we of course lose the advantages due to the presence of nitro-glycerine.  There is therefore a limit to these additions.[A]

[Footnote A:  Mica is said to increase the rapidity of explosion when mixed with gelatine.]

The homogeneousness and stability of the mixture are of the highest importance.  It is highly essential that the nitro-glycerine should be completely absorbed by the substances with which it is mixed, and that it should not subsequently exude when subjected to heat or damp.  It is also important that there should be no excess of nitro-glycerine, as this may diminish instead of augment the strength, owing to a difference in the mode of the propagation of the explosive wave in the liquid, and in the mixture.  Nitro-glycerine at its freezing point has a tendency to separate from its absorbing material, in fact to exude.  When frozen, too, it requires a more powerful detonation to explode it, but it is less sensitive to shock.  The specific gravity of blasting gelatine is 1.5 (i.e., nearly equal to that of nitro-glycerol); that of gun-cotton (dry) is 1.0.

Blasting gelatine burns in the air when unconfined without explosion, at least in small quantities and when not previously heated, but it is rather uncertain in this respect.  It can be kept at a moderately high temperature (70 deg.  C.) without decomposition.  At higher temperatures the nitro-glycerine will partially evaporate.  When slowly heated, it explodes at 204 deg.  C. If, however, it contains as much as 10 per cent. of camphor, it burns without exploding.  According to Berthelot,[A] gelatine composed of 91.6 per cent. nitro-glycerine and 8.4 per cent. of nitro-cellulose, which are the proportions corresponding to total combustion, produces by explosion 177CO_{2}+ 143H_{2}O + 8N_{2}.

[Footnote A:  Berthelot, “Explosives and their Powers.”]

He takes C_{24}H_{22}(NO_{3}H)_{9}O_{11} as the formula of the nitro-cellulose, and 51C_{3}H_{2}(NO_{3}H)_{3} + C_{24}H_{22}(NO_{3}H)_{9}O_{11} as the formula of the gelatine itself, its equivalent weight being 12,360 grms.  The heat liberated by its explosion is equal to 19,381 calories, or for 1 kilo. 1,535 calories.  Volume of gases reduced temperature equals 8,950 litres.  The relative value[A] of blasting gelatine to nitro-glycerine is as 1.4 to 1.45, kieselguhr dynamite being taken as 1.0.