GREAT ENERGY OF GASEOUS MIXTURES. 537 



carbon gases. But it is far beyond that of all solid or liquid 

 compounds. For instance, in the case of hydrogen and oxygen, 

 the potential energy is four times what it is in ordinary gun- 

 powder, and double what it is in nitroglycerin. In most of the 

 hydrocarbons associated with oxygen it scarcely attains to two- 

 thirds of the energy of an oxyhydric mixture ; acetylene alone 

 approaches hydrogen. 



But these advantages are discounted by the considerable 

 volume of gaseous mixtures and by the necessity for preserving 

 them in strong receptacles. 



We have given the theoretical pressures and the pressures 

 observed for these different mixtures. By comparing these we 

 may observe that the theoretical pressures exceed the real pres- 

 sures by double and sometimes even more, probably owing to 

 the dissociation of the compounds, water and carbonic acid, and 

 to the increase in the specific heats with the temperature. 



In fact, the pressures observed with total combustion mixtures 

 have not exceeded 20 atm., and in most cases they were con- 

 siderably below the figure. These pressures are very far 

 inferior to those of solid or liquid explosive substances, this 

 inferiority being due to the lesser condensation of the sub- 

 stance. 



In the case of liquefied gases, or of analogous bodies, such as 

 hyponitric acid, we obtain a nearer approach to solid substances. 

 The table at page 398 furnishes a certain number of details on 

 -this point. 



Finally, we have examined the mixtures of gases and com- 

 bustible dusts to which numerous accidents in mines have been 

 attributed, and we have briefly summarised both the theoretical 

 data and the facts which have come under notice. 



5. We now come to liquid or solid explosive compounds. 

 In the case of each of these we have given the physical 

 properties, the temperature of decomposition, the heat liberated, 

 the volume of gases, the permanent pressure, the theoretical 

 pressure at the moment of the explosion, in fact, the results 

 of experiments made recently in order to measure the real 

 pressures and the time necessary for the propagation of the 

 explosion. 



6. All these particulars are shown in the following table, 

 which summarises the characteristic details of the principal 

 explosive substances (see next page). 



According to this table, gaseous mixtures, such as hydrogen 

 and oxygen, or acetylene and oxygen, represent those systems 

 whose potential energy is the greatest ; nitroglycerin and nitro- 

 mannite, which are the most powerful among solid or liquid 

 powders, do not attain the half of the proportions referred to 

 gases; gun-cotton one-third; potassium picrate slightly over 

 one-fourth, and black powder does not even reach one-fourth. 



