1352 
The light sensitiveness of the mixture of HgCl^ and 
was first discovered by Planche (Jour. de Pliarm. 1815 p. 49). 
Becquere]. and Fremy (“La Inniière, ses causes et ses etfefs”, 1868, 
2 Bde, 2, p. 69) used a mixture containing 6.5 grams of HgCl^ and 
12.5 grams of oxalic acid dissolved in 100 c.c. of water for the 
construction of their cliemical photometer. 
In an interesting work “Etude sur la force chimique continue dans 
la lumière du soleil”, 1875, Marchand observed that the rate of 
decomposition of oxalic acid and mercuric chloride in presence of 
light, is uot regular. 
Euer (loc. cit.) in 1879 made a thorough study of this photometer 
and introduced some corrections, by wliich this photometer can be 
used in raeasuring accurately the Chemical intensity of light. A light- 
sensitive mixture of ammonium oxalate and mercuric chloride is 
known as Eder’s solutiou. 
The action of light on a mixture of mercuric chloride and any 
other reducijig organic acid seems not to have been studied. 
It has uow been observed that many other organic acids can 
reduce mercuiic chloride in presence of snnlight ; thus it has been 
observed that tartaric acid, citric acid, lactic acid, malie acid, glycollic 
acid, malonic acid, mandelic acid, mucic acid, mono-chlor-acetic acid 
etc. as well as their soluble salts slowly reduce mercuric chloride 
in presence of sunlight. 
In presence of ditfused light there is no appreciable reduction ; so 
a certain volume of the reacting solution was exposed to bright 
sunlight for a few hours each day and this procedure was continued 
for several days and the amount of mercurous chloride forrned was 
estimated by the method already described. 
Volume of solution used 
Time of 
exposure 
Mercurous chloride 
forrned in grams 
1) 25 c.c. ^ Potassium oxalate j 
5 [ 
N 1 
and 10 c.c. mercuric chloride j 
165' 
0.12508 
iV [ 
25 c.c. Potassium oxalate and j 
5 \ 
N i 
20 c.c. ^ mercuric chloride ) 
25 c.c. ^ Potassium oxalate and j 
220' 
0.17854 
5 \ 
10 c.c. jQ mercuric chloride | 
80' 
1 
0.06225 
