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-- Nitro-glycerine 1,784 heat units. Gun-cotton 1,123 " Potassic picrate 840 " which, multiplied by the mechanical equivalent per unit, gives-- Nitro-glycerine 778 metre tons per kilogramme. Gun-cotton 489 " " Picrate of potash 366 " "