534 
POPULAR SCIENCE REVIEW. 
upon which the successful result so much depends. In order to restore to 
proper working order a hath which exhibits these erratic properties we 
should do well in following the instructions of Mr. Divine. The silver 
solution is to be transferred to a glass vessel, and freely exposed to the 
sun’s rays, when any contamination arising from the presence of oxidizable 
organic matter, or from an excess of the oxide of silver itself, is quickly 
destroyed with reduction of dark-grey metallic silver ; at this stage a 
small quantity of a dilute solution of common salt is introduced, which will 
have the effect of carrying down with it these finely-divided particles, 
which no ordinary filter is capable of separating. After thorough agita- 
tion the liquid may be filtered, and the clear solution returned to the 
dipping-bath. 
The preparation of pure nitrate of silver for photographic purposes 
is in some cases facilitated by employing ammonia as a means of sepa- 
rating traces of copper from reduced metallic silver. In proceeding to 
make use of this plan, the silver and copper alloy (standard silver) is 
to be dissolved in nitric acid, and the clear solution precipitated by common 
salt, when a moderately pure form of chloride of silver is obtained after 
careful washing. This precipitate is then dissolved in ammonia, and again 
thrown down in the state of reduced metallic silver by the action of a rod 
of copper ; by washing now this finely-divided silver with dilute ammonia, 
every trace of copper is removed, and the pure metal may be dried and 
dissolved at once in nitric acid ; the solution being either evaporated to the 
crystallizing point, or carried to diyness and partially fused, to furnish 
the neutral description of nitrate of silver. 
The use of the proto-salts of iron, as developing agents in the collodion 
process, appears once more to be gaining the ascendancy over pyrogallic 
acid. M. Adolphe Martin has given an excellent suggestion for the re- 
moval of the small quantity of free sulphuric acid which is usually contained 
in the crystals of proto-sulphate of iron, and which interferes with the 
production of delicate half-tones when this agent is used in the prepara- 
tion of a developing solution. 
Acetate of lead is weighed out in the proportion of one part to four of the 
sulphate of iron, and these salts being separately dissolved in water are 
mixed, when a heavy precipitate of sulphate of lead is formed with pro- 
duction of an equivalent amount of the acetate of iron. Under these 
conditions it is impossible for any free sulphuric acid to exist in the solu- 
tion ; a small quantity of acetic acid will be liberated, which serves a use- 
ful purpose in keeping the iron dissolved without hindering its developing 
qualities. 
With a similar object, a writer from Elberfeld recommends the addition 
of nitrate of potash (saltpetre) to the sulphate of iron developing solution. 
This expedient was, however, suggested as far back as 1853, in the “Jour- 
nal of the Photographic Society,” by Mr. Spiller, who described this as a 
ready means of preparing a solution having all the properties of the 
proto-nitrate of iron, and capable of developing the half-tones in great 
perfection. 
