168 Proceedings of Royal Society of Edinburgh. [sess. 
to bright yellow, and ultimately the solution is found to contain 
chromic acid and no chromic salt. 
Ammonium persulphate is now made and employed technically 
on a considerable scale. Possibly the employment of small quan- 
tities of silver compounds in conjunction with it may extend its 
applicability as an oxidising agent to cases where by itself it would 
be ineffective. 
There is another point of interest in connection with the use 
of ammonium persulphate solution as a ‘ reducer ’ in photography. 
A solution which has been once used for this purpose is bound 
to contain sufficient silver salt to accelerate enormously the rate of 
decomposition and render the solution very soon unfit for use, 
although in its unused condition it might be kept for a consider- 
able time without undergoing decomposition to a serious extent. 
It is also possible that the metallic silver of the film is more 
rapidly attacked once there is some of the peroxidic compound 
present in the solution. It has been stated, indeed, that pure 
solution of ammonium persulphate does not attack the film, 
and that the action only commences once a small quantity of 
ozone has been formed by decomposition. If that is so, then 
probably the addition of a small quantity of silver nitrate solution 
to a { reducer ’ freshly prepared from pure ammonium persul- 
phate would make it immediately active. 
