﻿Ignition of Gases by Sudden Compression. 95 



The initial rise of temperature is therefore given by 

 dT „ ^ r, /Qg 



§-Q.-Qo=Qo(J-i) ... (9) 



/n 1 dT=^dt, 



(Ht _ -, \ c v 



or 





But since Q = ue v (T — 0), 



we have 



7Q^-<|) = a (VV- 



(10) 



V. Can this equation be used to determine the delay, or 

 period of slow combustion, that occurs before the temperature 

 suddenly rises very rapidly, i. e. before explosion takes 

 place ? Strictly speaking, this could only be done if Q, the 

 rate of evolution of heat during the initial reaction, depended 

 only on temperature and not on the concentration of the 

 reacting substances. According to all ordinary theories of 

 reaction, this would not be true ; the rate of reaction should 

 depend in some way on the concentration of the reactants. 

 If we consider such a reaction as the combustion of heptane 



C 7 H 16 + 110 2 = 7C0 2 + 8H 2 0, 



we can hardly suppose it is necessary, before the initial re- 

 action — whatever is its nature — can occur, for 1 molecule of 

 heptane and 11 molecules^ of oxygen to collide ; but it is 

 reasonable to assume that the rate of reaction must at least 

 be proportional either to the concentration of the heptane or 

 to that of the oxygen, or to the product of the two. It is 

 necessaiw, however, to point out that there is little, if any, 

 satisfactory evidence that homogeneous reactions in gases 

 obey what is ordinarily understood by the law of mass action. 

 In fact, evidence from the ignition of gaseous mixtures by 

 sudden compression points rather to the reverse. Dixon 

 and Crofts' experiments * on the ignition of mixtures of 

 hydrogen and oxygen are difficult to explain by any reasonable 

 assumption as to the mechanism of the reaction based on the 



* Dixon & Crofts, Trans. Chem. Soc. 1914, p. 2036. 



