POTENTIAL ENERGY. 17 



mechanical effects, measures the heat consumed by these 

 mechanical effects. But it is not easy to carry out exact calori- 

 metric experiments under these conditions. 



10. However, the heat disengaged measures the maximum 

 work which the explosive can accomplish acting under atmo- 

 spheric pressure. It suffices to multiply this quantity of heat 

 by 425, the mechanical equivalent of it to express this work in 

 kilogrammetres. This is the value of its potential energy. 



The potential energy of an explosive must not be confounded, 

 as has sometimes been done, with the heat of combustion of a 

 substance combustible by air or oxygen ; for example, in com- 

 paring what has been called the potential of coal with the 

 potential of .powder. For the energy of the powder is contained 

 entirely in itself, while the energy of coal in combustion resides 

 not in the inflammable body alone, but in a system composed 

 of this body and the air necessary to bum it. Even in the case 

 of an explosive the total heat disengaged at the ordinary tem- 

 perature is not in general that which regulates the pressure 

 developed at the moment of explosion. This latter quantity of 

 heat corresponds solely to the formation of compounds actually 

 existing at the temperature and in the conditions of the 

 explosion ; that is to say, it is subordinate to dissociation. For 

 example, if at the temperature of explosion the carbonic acid is 

 dissociated to the extent of one-third into carbonic oxide and 

 oxygen, it would be necessary to deduct from the heat trans- 

 formable into work, the heat corresponding to the metamor- 

 phosis of this third consisting of carbonic oxide. 



11. From what has been said it will be seen that it is very 

 interesting to compare the potential of an explosive with the 

 work which the gases developed by its explosion could accom- 

 plish in the ease of an indefinite expansion. This point has 

 hitherto only been experimentally studied in the case of powder ; 

 the discussion of the results observed would lead to questions in 

 mechanics which are foreign to the chief subject of this book. 

 It will only be added that, according to the most recent experi- 

 ments those of Sebert and Hugoniot l the ratio between the 



total and potential work for powder would be ; ', or 



305 kgm. 



44 per cent. This ratio coincides approximately with the ratio 

 in weight of gaseous products to the saline products of the 

 explosion. In practice the limit of work which 1 kgm. of 

 powder can effect falls to 90,000 kgms., that is to say, below 

 one-third of its potential energy. 



1 " Memorial de 1'Artillerie de Marine," torn. x. p. 184. 1882. 



