388 EXPLOSIVE GASES AND DETONATING GASEOUS MIXTURES. 



With hydrocarbon gases it is three times that of powder, and 

 one and a half times as much as that of nitroglycerin. How- 

 ever, the advantages which might result from the potential 

 energy of explosive gaseous mixtures compared to that of solids 

 and liquids are counterbalanced in practice by the difficulties 

 arising from the greater volume of the gaseous mixtures and the 

 necessity of keeping them in resisting envelopes. From this 

 point of view of the potential energy of gaseous mixtures, 

 referred to the unit of weight, no coinbustive, generally speak- 

 ing, rivals pure oxygen, seeing that every other oxidising com- 

 pound contains inactive elements (useless weight), which share 

 the heat without supplying sufficient compensating energy at 

 the moment of the destruction of the oxidising compound. 



4. We must remark that the theoretical pressures calculated 

 for the various explosive mixtures scarcely vary, except from 

 single to double, these being limits which we shall find by-and- 

 by, between the pressure really observed, notwithstanding the 

 diversity of composition and the condensation of the gases 

 taken into consideration. 



5. Moreover, the pressures calculated are purely theoretical, 

 and only intended to serve as terms of comparison. 



In fact, the figures measured by observers are much lower, 

 which is explained either by the short duration of the state of 

 integral combination which seems to correspond to the explosive 

 wave, or by the inaccurate estimation of the specific heats 

 employed in the calculations, or, finally, by dissociation. 



Let us follow up this question. 



It suffices to admit the existence of a certain dissociation in 

 order to reduce the pressures by one-half, or even one-third, of 

 the calculated values. 



Nevertheless, the rapidity of propagation of the explosive 

 wave as it has been measured (p. 101) seems to indicate that at 

 the moment of its production the explosive system contains all 

 the heat liberated by an integral combination. The propagation 

 of the wave is, however, so rapid that the pressure observed 

 probably corresponds in every kind of apparatus to a system 

 which is already partially cooled, and it is this reduced pressure 

 which seems to correspond to the case of ordinary combustion. 

 We might .also explain the results observed by accepting the 

 variation of specific heats, especially if we double the mean 

 specific heat of water vapour or of carbonic acid. 1 



Experience has not yet expressed a definite opinion respecting 

 these different manners of conceiving the phenomenon. It 

 tends, however, to show that the part played by dissociation had 

 been exaggerated at first. 



6. Let us now cite the figures really observed for pressures 

 subject to the reservations just named. 



1 See " Essai de Me'canique Chimique," torn. i. pp. 344 et 346. 



