J_78 S. HADA; HOW MERCUROUS AND 



need confirmation that mercuric sulphate and mercury, rubbed toge- 

 ther and moistened with water or alcohol and a little sulphuric acid, 

 combine readily, with the evolution of heat. The process is not a 

 convenient one for the preparation of pure mercurous sulphate, though 

 serviceable in the manufacture of calomel and in constructing the 

 Latimer-Clark galvanic cell. It is of great theoretical importance. 



Decomposition of mercurous sulphate by water. — It is familiar to 

 every chemist that, Avhereas mercuric sulphate turns bright yellow on 

 contact with water, mercurous sulphate remains white, even when 

 washed. But it seems to be unknown that there is a limit to this 

 stability quite easy to reach. Apparently, earlier observations of the 

 fact have been set aside as dependent upon the presence of mercuric 

 sulphate in the sulphate worked upon. 



Mercurous sulphate was precipitated from a solution of crystals of 

 mercurous nitrate, free from mercuric salt and nitrite, by adding 

 dilute sulphuric acid. It was well washed with water and remained 

 white. Of this moist salt about one gram was strongly shaken with 

 a liter of distilled water. This treatment rendered it distinctly 

 yellow, and repetition of the shaking with a fresh liter of water 

 deepened the colour to a clear though light yellow. 



Decomposition of mercurous sulphate by light. — In experimenting with 

 mercurous sulphate and water, as in the above case, light must be 

 avoided. It is known, I believe, that light acts upon mercurous 

 sulphate as it does upon the chloride. Mercurous sulphate when moist 

 becomes dark brown in a strong light. That which has been made 

 yellow by treatment with much water becomes first greenish under the 

 action of daylight and then dark brown. It seems safe to assume that 

 these colour changes are due to dissociation of the mercurous salt into 

 mercuric salt, and mercury or hypomercurous salt. The effect is only 

 superficial and too slight to admit of its nature being tested, besides 



