260 Mr Skinner, Note on the action of 



Note on the action of Radium rays on Mercurous Salts. By 

 S. Skinner, M.A., Christ's College. 



[Bead 26 October 1903.] 



When making Clark cells it is well known that the mercurous 

 sulphate in the depolarising paste rapidly changes from white 

 to a dark colour if a cell is left exposed to light. This change also 

 occurs when dry mercurous sulphate is exposed to light. Some 

 years ago I investigated the conditions of this change with the 

 view of deciding whether it was sufficiently extensive to cause 

 such an alteration in the chemical nature of the cell that the elec- 

 tromotive force might be affected ; and among the experiments I 

 then made some of the salt was submitted to the action of the 

 Becquerel rays from uranium, but the result was negative. The 

 much greater radio-activity of radium salts caused me to think 

 their action would shew itself in some way. This has recently 

 been tried, and an action similar to that of light was obtained. 



As the interest of these experiments lies in the comparison 

 of the effects produced by various agents I venture now to put 

 together the earlier experiments with those made recently. The 

 white mercurous sulphate when exposed to sunlight turns at first 

 light brown, and with continued exposure passes through stages of 

 gradually increasing darkness until it becomes dark brown or 

 black. It was found that this occurred when the salt was exposed 

 in daylight (a) in open air, (b) in a vacuum, (c) in shellac varnish, . 

 (d) in linseed oil, (e) under water, (/) gummed on paper. These 

 experiments seem to indicate that the presence of air was not 

 essential. 



To find out which of the rays in sunlight caused the change an 

 arc-light spectrum was cast by a quartz lens and prism on paper 

 coated with the salt. Here it was found that the action was 

 caused by the violet and ultra-violet rays. To further test this 

 a vacuous tube containing some of the salt was taken to a height 

 of 4,500 feet and exposed to the action of sunlight, and it was 

 found that the action was much more intense in comparison with 

 that at low altitudes. These results shewed that ultra-violet light 

 was the chief agent in producing the action. 



Now the question arises as to what the action is. Is it a 

 conversion of the mercurous salt into a mercuric with the 

 separation of mercury ? This view has been put forward, specially 

 by Haga 1 , who has offered evidence that most mercurous salts 

 undergo this change. 



1 Chemical Society's Journal Trans., 1896, p. 1673. 



