390 EXPLOSIVE GASES AND DETONATING GASEOUS MIXTUKES. 



pressures observed do not differ very much from the theoretical 

 pressures, so that, if necessary, the latter may be employed in 

 the comparisons, at least for a first approximation. 



7. By replacing pure oxygen by its mixture with nitrogen, 

 that is, by atmospheric air, in order to effect the combustion of 

 the gases and vapours, we obtain systems which are very 

 interesting in their applications. In fact, it is a similar mixture 

 of air and methane which constitutes the fire-damp so much 

 dreaded in mines. 



A similar mixture, composed of air and coal gas, has often 

 given rise to serious accidents in houses and sewers. 



The vapour of ether, carbon disulphide, and petroleum 

 spirit, associated with air, have more than once produced fires 

 and explosions in manufactories and laboratories. Let us now 

 examine more closely the effects of this substitution of air for 

 oxygen. 



8. It does not change the heat liberated, and consequently it 

 does not affect the maximum work which can be developed by 

 a given weight of the combustive body. 



9. On the contrary, it modifies the pressures, and that in two 

 ways. In fact, at first sight it may be conceived that the 

 theoretical pressures should decrease by one-half, or even more, 

 owing to the necessity of heating the nitrogen, and even the 

 excess of oxygen, which lowers the temperature. For instance, 

 hydrogen mixed with five times its volume of air would not 

 develop, according to theory, more than 8'5 atm., instead of 

 20 atm., and only 5'1 atm. with ten times its volume of 

 air. 



10. These figures are still above the real values, for the same 

 reasons that lower the pressures with pure oxygen, that is to 

 say, on account of dissociation, or rather, the increase of the 

 specific heats (p. 388). 



However, the influence of these causes is limited by the 

 lowering of the temperature. Thus, according to Bunsen, one- 

 half of the mixture of carbonic oxide and oxygen would burn, 

 instead of one-third, as soon as the temperature falls below 

 2560. Below 1146 the quantity burnt would again increase, 

 and continue to do so until total combustion took place. 

 Nevertheless, the last figures must be looked upon as doubtful. 

 In fact, they have been derived from observed pressures, assum- 

 ing the specific heats to be constant, which is not admissible ; l 

 now the effects observed can be explained equally by the varia- 

 tion of the specific heats, a variation which cannot be disputed 

 for compound gases. 



For instance, since the specific heat of carbonic acid increases 

 with the temperature, the gaseous mixture which contains it is 

 brought to a lower temperature by a given quantity of heat, 

 1 See " Annales de Chimie et de Physique," 5" se*rie, torn. xii. p. 305. 



