1(58 A. Pedler — Volatility of Mercury and its compounds. [Mav 7 



tures and in the dark. When tested in this way it is found that mercuric 

 oxide when exposed to sunlight in vacuo is distinctly volatile. A small 

 portion of the mercuric oxide is also decomposed into mercurous oxide. 

 Mercuric cyanide appears to be quite non-volatile under these conditions, 

 the salt is also not decomposed by the action of sunlight. Mercuric oxalate 

 appears also to be non-valatile but under the influence of sunlight it 

 decomposes entirely into mercury and carbon dioxide. Mercuric iodide 

 was quite non- volatile, and it was also practically undecomposed by the 

 action of light, for though by exposure to sunlight it darkened, yet on 

 re-exposure to the air in diffused day-light its original colour was re- 

 gained. Mercuric bromide is very distinctly volatile in vacuo in sun- 

 light ; a small portion of the mercuric bromide is also decomposed into 

 mercurous bromide. Mercuric chloride is very decidedly volatile in 

 vacuo both in sunlight and also when kept in the dark. It is so volatile 

 indeed that the loss in weight can be easily determined. Mercuric 

 chloride also partly decomposes under the influence of light, and distinct 

 amounts of mercurous chloride (calomel) are formed. A similar decom- 

 position to the latter is also shewn to take place in solution, but this last 

 fact has been previously known. Mercurous chloride appears to be 

 volatile in vacuo in sunlight in an exceedingly feeble manner, but the 

 sunlight also decomposed it distinctly but partially into mercury and 

 free chlorine. 



Attention is prominently drawn to the importance of the fact of 

 the volatility of mercuric chloride inasmuch as this salt is very largely 

 used, particularly in India. Its uses are as an antiseptic and for 

 preserving perishable materials and for protecting books, specimens &c. 

 from the attacks of insects. 



Cases of mercurial poisoning from the use of mercuric chloride as 

 a preservative are quoted, which have apparently been put down to the 

 existence of solid particles of mercuric chloride floating in the air, and 

 which have been thrown off from surfaces like dust, but it is pointed out 

 that the volatility of the salt which has now been proved is a much 

 more probable cause of this poisonous action. 



The Peesidext remarked that in the light of Mr. Pedler's ex- 

 periments it seemed very probable that the fading of photographic nega- 

 tives that had been intensified with bichloride of mercury, in conjunction 

 either with alkaline iodides and sulphides or with liquor ammonia, as is 

 commonly the practice both for wet and dry plates, might be partly 

 due to the volatilisation of the mercurial compound formed. Such fad- 

 ing was very common in this country and high temperature would in- 

 crease the tendency if due to this cause. Bichloride of mercury was a 



