1889.] A. Podlov—Vola/iliiy of some of Ike Oom-poimds of Mercury. 191 



The volatility of certain compounds of mercury has also beou 

 brought to my notice in the course of a number of experiments which I 

 liavo been carrying out on the action of light on various chemical com- 

 pounds. The usual method of experimenting has been as follows : — A 

 small quantity of a compound is placed in a glass tube sealed at one 

 end, and if a solid, a small plug of cotton wool is placed above it. The 

 tube is then considerably narrowed before the blowpipe at about 3 to 4 

 inches from the closed end, and above the constricted part a stick of 

 caiistic soda or a piece of metallic sodium or other reagent is placed, to 

 decompose or combine with any vapour which may be evolved by the 

 compound which is being tested. The tube is thou rendered vacuous 

 by ii Sprengel piimp and hermetically sealed, the whole tube being 8 to 

 10 inches long. It is afterwards exposed to the sunlight or kept in the 

 dark as the case may require. 



The following mercuric compounds have been tested : — mercuric 

 oxide, mercuric cyanide, mercuric iodide, mercuric oxalate, mercuric 

 chloride (corrosive sublimate), and mercuric bromide. Of the mei-curous 

 salts only mcrcurous chloride was tested, for it is well-known that such 

 compounds as mercui-ous oxide, mcrcurous iodide, etc. are very readily 

 decomposed. 



Rod mercuric oxide was placed in a scaled tube in vacuo with 

 a few freshly cut pieces of metallic sodium in the upper part of the 

 tube. The experiment was commenced in March 1887, and the tube 

 was exposed daily to sunlight for about a year. On examination the 

 pieces of sodium were all found completely covered with a decided 

 layer of oxide, and on removing the oxide, part of the sodium surfaces 

 appeared like a piece of sodium amalgam. After acting on the sodium 

 with water a considerable number of distinct and moderate sized 

 globules of mercury were obtained. 



The oxide of mercury, which was of a bright red colour at the 

 commencement of the experiment, was examined. It was of a dark 

 chocolate brown colour, and on examination it was found to consist of 

 unchanged red mercuric oxide with a distinct quantity of mcrcurous 

 oxide. Metallic mercury was tested for but none could be detected. 

 The experiment therefore proves conclusively that mercuric oxide is 

 feebly volatile at temperatures such as those obtained in sunlight, for 

 the only way in which the mercury could have passed to the sodium is 

 by volatilination of the oxide. No metallic mercury was found in the 

 oxide after the action of sunlight, and therefore no upward distillation 

 of mercury as such could have happened. 



Observations have already boon made showing that mercuric oxide 

 when exposed to suushine turns black suporlicially and partially de- 

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