176 S. HADA; HOW MERCUROUS AND 



sphere. Where the exposure lasted only 19 days, the mercuric radical 

 formed was only five-sixths in the mercurial atmosphere of what it 

 was in the non-mercurial, in those cases where carbon dioxde was 

 used ; where air was used, the mercuric radical was only less by a 

 tenth in the mercurial atmosphere than what it was in the absence of 

 mercury. There can hardly be a second opinion as to the manner in 

 which the mercury exercises its inhibitory effect ; that this effect 

 should become so manifest is very remarkable. As pointed out in the 

 ntroductory section of this paper, the nitrous acid produced by the 

 light multiplies itself in making mercurous nitrate into mercuric salt. 

 Mercury vapour in the utmosphere over the solution, minute as it is in 

 quantity, is yet large enough, no doubt, to combine with an appreci- 

 able fraction of the minute quantities of the nitrite-ion generated by 

 light, and thus reduces the rate of its accumulation. Whether it 

 unites with it to produce mercurous or mercuric nitrite will not be 

 ascertainable, the quantity of salt produced being probably inappreci- 

 able. Were it not for the nitrous acid doubling itself in action, — 

 :HgN0 3 ) 2 + HN0 3 +HN0 2 =Hg(N03) 2 + Hg(NO«),+ OH 2 ; 

 Hg(N0 2 ) 2 + 2HN0 3 =Hg(N0 3 ) 2 + 2HN0 2) 

 its consumption by mercury would be too insignificant to have notice- 

 able effect. 



The inhibitory effect of mercury vapour is the same as that of 

 exposure to the open air, the reason being that the nitrous acid, 

 produced by lignt so slowly as it is, gets diffused away in the air 

 and its accumulation impeded. It must be remembered that, in such 

 solutions as those worked upon in these experiments, the amount of 

 free nitrous acid present must always be minute, much the greater part 

 of it being in the form of mercury nitrites, a consideration which will 

 explain the nearness, in amount of effect, of the action of mercury 

 vapour to that of the open air. 



