VELOCITY OF COMBUSTION. 49 



but every diminution in pressure retards it. In 1855, Mitchell 

 observed that fuses burn slower on high mountains ; some very 

 exact experiments were made in this respect by Frankland in 

 1861 in his own laboratory, and afterwards by De Saint-Robert 

 in the Alps. Under pressures varying between 0'722 metre 

 and O405 metre, that is to say, below atmospheric pressure, the 

 velocity of the combustion of powder would be practically 

 represented, according to De Saint-Robert, by such a formula as 

 the following : 



V = A/, 



A being a constant, and p expressing the pressure. 



These effects are to be attributed to the greater or less velocity 

 with which the heated gases escape before having had time to 

 heat the adjacent portions of the solid matter. This is equiva- 

 lent to saying that pressure lessens the number of gaseous 

 particles brought up to a high temperature which at every 

 moment come in contact with the solid particles not yet ignited 

 and share with them their energy, so as to place themselves in 

 equilibrium of temperature. 



Whatever the pressure, if it operates under a constant 

 volume, the initial temperature of these particles remains prac- 

 tically the same ; at least, so long as the chemical reaction is 

 not modified. But if operating under a constant pressure, the 

 case will be otherwise, the temperature being lowered lay the 

 expansion of the gases. 



6. On the other hand, the combustion velocity of powder 

 increases with great rapidity, once we obtain the considerable 

 pressures which are produced in cannons and guns. Thus, for 

 instance, Captain Castan estimated the combustion velocity of 

 powder, in the interior of large bore cannons, at 0*32 metre 

 per second, instead of about O'l metre in the open air. 



7. The combustion velocity of other explosive substances has 

 not formed the object of such numerous experiments as that of 

 black powder ; moreover, it gives rise to fresh observations and 

 to a theory of an entirely different order, as will presently be 

 stated. 



Piobert estimated the combustion velocity of non-compressed 

 gun-cotton at eight times more than that of service powder; which 

 estimate was applied to a progressive combustion effected 

 without detonation. 



8. These researches were extended to explosive gaseous 

 mixtures. In 1867, Bunsen 1 estimated the velocity of combus- 

 tion for electrolytic gas (hydrogen and oxygen) at 34 metres per 

 second, and at only one metre per second for the mixture of 

 carbonic oxide and oxygen in equivalent proportions, these 

 mixtures being taken under atmospheric pressure. He deter- 



1 " Annalee de Chimie et de Physique," 4 e se*rie, torn. xiv. p. 449. 



E 



