The light sensitiveness of the mixture of HgCl, and (NH,),C,O, 
was first discovered by Prancne (Jour. de Pharm. 1815 p. 49). 
BECQCEREL and Fremy (“Loa lumière, ses causes et ses effets”, 1868, 
2 Bde, 2, p. 69) used a mixture containing 6.5 grams of HeCl, and 
12.5 grams of oxalic acid dissolved in 100 ¢.c. of water for the 
construction of their chemical photometer. 
In an interesting work “Etude sur la force chimique continue dans 
la lumière du soleil”, 1875, Marcuanp observed that the rate of 
decomposition of oxalic acid and mercuric chloride in presence of 
light, is not regular. 
Eper (loc. cit.) in 1879 made a thorough study of this photometer 
and introduced some corrections, by which this photometer can be 
used in measuring accurately the chemical intensity of light. A light- 
sensitive mixture of ammonium oxalate and mercuric chloride is 
known as Eper’s solution. 
The action of light on a mixture of mercuric chloride and any 
other reducing organic acid seems not to have been studied. 
It has now been observed that many other organie acids can 
reduce mercuric chloride in presence of sunlight; thus it has been 
observed that tartaric acid, citric acid, lactic acid, malic acid, glyeollie 
acid, malonie acid, mandelie acid, mucic acid, mono-chlor-acetic acid 
ete. as well as their soluble salts slowly reduce mercurie chloride 
in presence of sunlight. 
In presence of diffused 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 formed was 
estimated by the method already described. 
| Time of Mercurous chloride 
Volume: of solution used | exposure | formed in grams 
| 
1) e253 cics 2 Potassium oxalate {| 
5 | 165 | 0.12508 
and 10 c.c. Tô mercuric chloride | | 
N | 
25, GC: 5 Potassium oxalate and | | 
N ii 220’ 0.17854 
20 c.c. ID mercuric chloride | 
N | | 
2DRCEC: 5 Potassium oxalate and | 
N 80’ 0.06225 
10 c.c. 10 mercuric chloride | 
