The Determination of the Relative Strength of Explosives.—­Explosives may be roughly divided into two divisions, viz., those which when exploded produce a shattering force, and those which produce a propulsive force.  Explosives of the first class are generally known as the high explosives, and consist for the most part of nitro compounds, or mixtures of nitro compounds with other substances.  Any explosive whose detonation is very rapid is a high explosive, but the term has chiefly been applied to the nitro-explosives.

The effectiveness of an explosive depends upon the volume and temperature of the gases formed, and upon the rapidity of the explosion.  In the high explosives the chemical transformation is very rapid, hence they exert a crushing of shattering effect.  Gunpowder, on the other hand, is a low explosive, and produces a propelling or heaving effect.

The maximum work that an explosive is capable of producing is proportionate to the amount of heat disengaged during its chemical transformation.  This may be expressed in kilogrammetres by the formula 425Q, where Q is the number of units of heat evolved.  The theoretical efficiency of an explosive cannot, however, be expected in practice for many reasons.

In the case of blasting rock, for instance:[A]—­1.  Incomplete combustion of the explosive. 2.  Compression and chemical changes induced in the surrounding material operated on. 3.  Energy expended in the cracking and heating of the material which is not displaced. 4.  The escape of gas through the blast-hole, and the fissures caused by the explosion.  The proportion of useful work has been estimated to be from 14 to 33 per cent. of the theoretical maximum potential.

[Footnote A:  C.N.  Hake, Government Inspector of Explosives, Victoria, Jour.  Soc.  Chem.  Ind., 1889.]

For the purposes of comparison, manufacturers generally rely more upon the practical than the theoretical efficiency of an explosive.  These, however, stand in the same relation to one another, as the following table of Messrs Roux and Sarrau will show:—­

MECHANICAL EQUIVALENT OF EXPLOSIVES.

Theoretical Work Relative
in Kilos.  Value.

Blasting powder (62 per cent.  KNO_{3})               242,335        1.0
Dynamite (75 per cent. nitro-glycerine)              548,250        2.26
Blasting gelatine (92 per cent. nitro-glycerine)     766,813        3.16
Nitro-glycerine                                      794,563        3.28

Experiments made in lead cylinders give—­
  Dynamite 1.0
  Blasting gelatine 1.4
  Nitro-glycerine 1.4

Sir Frederick Abel and Captain W.H.  Noble, R.A., have shown that the maximum pressure exerted by gunpowder is equal to 486 foot-tons per lb. of powder, or that when 1 kilo, of the powder gases occupy the volume of 1 litre, the pressure is equal to 6,400 atmospheres; and Berthelot has calculated that every gramme of nitro-glycerine exploded gives 1,320 units of heat.  MM.  Roux and Sarrau, of the Depot Centrales des Poudres, Paris, by means of calorimetric determinations, have shown that the following units of heat are produced by the detonation of—­