CONDITIONS OF CHEMICAL CHANGE 659 



The two experiments, taken in conjunction with one another, 

 prove that the phenomenon is due neither to heating nor to 

 delayed dissolution of the hydrogen chloride and can therefore 

 only be occasioned by the non-formation of hydrogen chloride 

 during the first part of the illumination. In a later paper by 

 the same author {Phil. Mag. 1845, 26, 473), a still more con- 

 vincing experiment relating to the same point is described. 



There are therefore two distinct stages to be considered, 

 viz. the stage during which the gases from some unknown 

 cause refuse to interact at their maximum rate, named by Bunsen 

 and Roscoe the " period of induction " ; and a subsequent stage 

 during which interaction has been shown to take place in 

 conformity with definite laws; this latter may be called the 

 11 normal period." 



Draper in his first paper (Phil. Mag. 1843, 23, 401) described 

 observations he had made with the aid of the tithonometer as 

 to the behaviour of the mixed gases during the normal period 

 and showed that, in the first place, the rate of formation of 

 hydrogen chloride, other conditions remaining the same, 

 depended upon the composition of the mixture ; the maximum 

 sensitiveness was attained when the gases were present in 

 equivalent proportions, a small excess of either being accom- 

 panied by a considerable diminution in the rate of chemical 

 change. This peculiar effect was more closely investigated by 

 Bunsen and Roscoe (Trans. Roy. Soc. 1857, 147, 390), who found 

 that the sensitiveness of a normal mixture is reduced by the 

 presence of an additional 3 parts of hydrogen in 1,000 from 

 100 to 37"8 and by that of 10 parts of chlorine in 1,000 from 100 

 to 6o'2. The same observers also demonstrated the remarkable 

 catalytic effect of small quantities of oxygen, the presence of 

 5 parts of this gas in 1,000 reducing the rate of change from 100 

 to 97. It is here of interest to observe that it has been shown 

 by A. Jodlbauer and H. V. Tappeiner (Berichte der deutschen 

 Chemischen Gesellschaft, 1905, 37, 2602) that oxygen exercises a 

 remarkable retarding effect on the reduction of mercuric chloride 

 by ammoniun oxalate, an interaction studied by Eder, which pro- 

 ceeds only in the light, in the manner represented by the 

 following equation : 



2HgCl 2 + C 2 4 (NH 4 ) 2 = Hg 2 Cl a + 2C0 2 + 2NH 4 C1. 

 Occasion will arise later on to refer to the part played by 

 this class of impurities during the induction period. 



