SCIENTIFIC SUMMARY. 
575 
pulation. The difficulty in securing- uniform results, and the inconvenience 
arising from the diffusion of ammonia gas into the atmosphere of the 
operating-room are, on the other hand, manifest disadvantages. 
The double sulphate of iron and ammonia has taken a permanent 
position in the list of chemical salts required in photography. As a deve- 
loping agent this solution answers admirably, whilst it is less prone to 
oxidation than the simple sulphate. The formula of this compound salt, 
FeO, S0 3 , NIT 4 0, S0 3 -|- 6 HO, indicates a higher atomic value than 
the crystallized sulphate of iron, FeO, S0 3 -f- 7 HO, and a proportionately 
larger amount by weight must be employed in consequence. For its 
preparation, two parts of sulphate of iron and one of sulphate of ammonia 
may be dissolved together in four or five parts of hot distilled water ; on 
cooling, the double salt will crystallize out, and the supernatant liquid 
may be again used for the solution of further quantities of the individual 
salts. The crystals, drained and dried on blotting-paper, are still liable to 
be affected by exposure to air, but in stoppered bottles, and with ordinary 
precautions, they may be preserved for a long time unchanged. The 
aqueous solution soon acquires a pale brownish tint, which does not, 
however, appreciably affect its properties as a reducing agent. When 
required for the development of collodion negatives, the following formula 
may be adopted : — 
Sulphate of iron and ammonia 
Acetic acid .... 
Alcohol 
Water ..... 
25 grains 
15 minims 
quant, suff. 
1 ounce. 
The addition of alcohol is merely for the purpose of causing the liquid to 
flow evenly over the plate ; the quantity usually sufficient would be about 
thirty minims. 
M. Meynier’s proposal in reference to the employment of the sulpho- 
cyanide of ammonium as a fixing agent, has been favourably reported 
upon by >, committee consisting of MM. Perier, Bayard, Girard, and 
Davanne, and by others in this country ; but the encouragement of low 
cost is yet wanting as a stimulus to further experiment and more general 
application. 
The composition of photographic prints, as regards their metallic consti- 
tuents, is still under discussion. Dr. Schnauss maintains that in a 
properly fixed and toned print, there should not be any silver remaining, 
but that the whole of this metal is ordinarily dissolved out, and replaced 
by gold in the successive operations of toning and fixing the proof. 
MM. Girard and Davanne, Dr. Van Monckhoven, and Mr. Spiller, assert 
on the contrary that this substitution is not completely effected, and that 
under all circumstances a fair proportion of silver remains in the proof, 
and may easily be detected, by chemical analysis, in the ashes left on 
burning the paper. 
Mr. II. Cooper gives the following instructions for the preparation of a 
kind of gelatino-resinous paper, suitable for printing purposes : — Dissolve 
sheet india-rubber or gutta-percha in chloroform, and add to it benzole in 
such proportion as will make a solution of three grains to the ounce ; 
