126 PHOTOGRAPHIC FACTS AND FORMULAS 
may be tested as suggested above. About three days is suffi- 
cient, as a rule, to throw down the whole of the silver. 
One of the best methods of regaining the silver is to 
procure a sheet of brass, not too thin, and place in the barrel 
or jar at an angle, so that both sides are presented to the 
liquid. After about 48 hours, the whole of the silver will be 
precipitated in the metallic form on the brass and can usually 
be removed by bending a corner sharply two or three times, 
when the silver will spring off. The brass can then be put 
back into the vessel and allowed to remain another 24 or 48 
hours, when practically the last trace of silver will be ex- 
tracted. This thin coat is difficult to remove and may be left 
on until the next batch of residues is to be treated. 
Paper and print clippings and waste prints should be burnt, 
and the ashes collected and mixed with the silver sulphide 
from the old fixing baths. 
Platinum Residues——The developer and acid fixing baths 
from the platinotype process should be collected and prefer- 
ably boiled down to about one-fourth the volume. Then some 
saturated solution of ferrous sulphate should be added, the 
mixture boiled for an hour, and the platinum filtered out. 
Gold baths.—Old gold toning baths may be treated in the 
same way as the platinum solutions, after acidulating with 
hydrochloric acid. 
It does not pay a photographer to refine his own residues. 
All the silver residues should be mixed together, and the gold 
and platinum kept separate, and sent to a refiner, who will 
allow market value of the metals minus a small charge for 
refining. 
Hypo ELImMInators.—It is an open question whether the 
use of chemicals to destroy the last traces of hyposulphite of 
soda and the hyposulphites of silver is justifiable if perma- 
nency of results be the aim. Their action is probably in most 
