192 A Pedlov—Volatilikj of some of tlie Compounds of Mercury. [No. 2, 



composes. Guibert* states that the decomposition yields mercury and 

 oxygen, while Donovan* states that merourous oxide and oxygen are 

 formed. Donovan's explanation therefore of the reaction appears to be 

 the correct one, and not that advanced by Guibert. 



Jacob Myers (Deut. Chem. Ges. Ber. VI. 11) has also shovrn that 

 mercuric oxide apparently commences to decompose in vacuo in a very 

 feeble manner at about 150''G, but that even at 240° the tension of the 

 evolved oxygen is only 2 m.m., but it is clear that the phenomenon 

 described in the previous experiments is quite different from that 

 alluded to by Myers. 



Mercuric cyanide in vacuo appeared quite non-volatile, and, even 

 after exposure to the direct rays of the sun for a month in the hot 

 season of the year, the compound itself seemed to be quite unaltered. 



Mercuric iodide appeared also quite non-volatile at ordinary air 

 temperatures in vacuo, even when exposed to the rays of the Indian sun, 

 and it also appeared practically unchanged in composition by the direct 

 action of sunlight, and, though there was a slight darkening in colour 

 under the influence of light, the change was to a groat extent temporary, 

 and disappeared when the iodide was exposed to the air. 



Mercuric oxalate was exposed to the direct rays of the sun in 

 vacuo for seven and a half months during the hot season of the year. 

 There was no evidence whatever of the substance being volatile, but 

 the colour of the salt darkened rapidly under the influence of the sun 

 and changed to a deep grey. The grey mass after treatment with 

 solution of potassic cyanide, and afterwards with ammonic chloride, left 

 a black residue from which globules of mercury wore obtained by the 

 usual process, and which black residue gave all the tests for metallic 

 mercury. This salt, though not volatile at ordinary temperatures in 

 vactio, appears therefore to decompose by sunlight according to the 

 equation : — 



HgOoO, = Hg + 2C02. 

 Mercuric chloride was the next salt tested, and this yielded rather 

 important results. One of the experiments may be described as fol- 

 lows :— On March 6th, 1886, a glass tube f of an inch internal dia- 

 meter was filled at its closed end with 10-876 grams of pure mercuric 

 chloride. It was drawn out, &c., in the usual manner, and a stick 

 of caustic soda placed in the upper part of the tube, which was then 

 rendered vacuous and hermetically sealed. After sealing, it was daily 

 exposed to the sun's rays with the end of the tube containing the 

 caustic soda upwards. After a few days a grey deposit of metallic 



Gmeliu's Chemistry, Yol. VI, p. 10. 



