CONDITIONS OF CHEMICAL CHANGE 665 



It will also be perceived that for the first time water vapour is 

 conceived as playing an important part in the change. 



After Dixon {B.A. Reports, 1880, 593) had found that an 

 explosion cannot be propagated through a dried mixture of 

 carbon monoxide and oxygen, several attempts were made to 

 discover whether moisture exercises a similar influence over 

 the interaction of hydrogen and chlorine. The experiments of 

 Pringsheim {Weid. Ann. der Physik. 1887, 32, 384), Dixon and 

 Harker {Manchester Memoirs, 1889 (iv.), 3, n8; 1890 (iv.) 4, 1), 

 Baker {Trans. 1894, 65, 611) and of Mellor and Russell {Trans. 

 1902, 81, 1272) relating to this point are in fair accord : the 

 result appears to prove that the presence of moisture accelerates 

 the interaction of chlorine and hydrogen when it is brought about 

 by the influence of light but is without effect on the explosive 

 combination of the gases. 1 



Another striking peculiarity of a sensitive mixture of chlorine 

 and hydrogen is noticed by Draper in his description of the 

 tithonometer. When the mixture is exposed for an instant to 

 light it suddenly expands and then contracts : the phenomenon 

 is most strikingly seen under the action of an electric spark. 

 Pringsheim {loc. cit), who appears to have been unacquainted 

 with Draper's work, found the expansion produced by a spark 

 of given length to be independent of the state of induction 

 of the gas and of the amount of hydrogen chloride that is 

 formed ; he therefore argued that it is not a consequence of heat 

 disengaged by the union of the gases but is caused by the 

 dissociation of the molecules into atoms. Bevan has questioned 

 the accuracy of Pringsheim's observations and has brought 

 forward experimental evidence, which in our opinion is con- 

 clusive, that the expansion in question is nothing but the 

 thermal effect of a sudden formation of hydrogen chloride. 

 The expansion was, in fact, found to be greater in the case of 

 an insolated than in that of an uninsolated mixture and also 

 roughly proportional to the rise in temperature and to the 

 amount of hydrogen chloride formed. 



The Draper or, as it is sometimes called, the Pringsheim 

 effect, must not, as Pringsheim pointed out— and here there 

 are no two opinions — be confused with a remarkable property, 

 discovered by Budde (Pogg, 1871, 144, 213; Erg. 1873, vi. 477), 

 which chlorine itself possesses. Budde observed that chlorine 



1 The rate of explosion in the dried mixture is a little faster than in the moist 



