The discovery that certain of the per salts of iron when exposed to
light undergo decomposition and are reduced to proto salts, is
attributed to Sir John Herschell. But we are indebted to Poitevin
for numerous interesting developments in this department.
For instance, the per-chloride so exposed becomes reduced to the
proto-chloride; or, as Von Monckhoven more appropriately remarks, to
the state of oxy-chloride. For this purpose the sesqui-chloride must
be quite neutral. The ammonia tartrate, potassa tartrate and the
ammonia citrate of iron are much more sensitive to light than the
sesqui-chloride, and the latter salt (ammonia citrate) most of all.
The image formed by means of these salts is much fainter than that
with the chloride of silver; but it can be intensified by the
application of other metallic salts.
The mode of operation consists in floating the paper on the
solutions in question in the dark room, in allowing them to dry, and
then exposing them afterward beneath a negative, as usual with paper
prepared with chloride of silver.