IO2 
MISCELLANEOUS. 
PERMANENT PHOTOGRAPHS.—The use of the peroxide of hy- 
drogen for eliminating from photographs the last trace of the de- 
structive hyposulphites, has found opponents. Mr. F. W. Hart 
points out, that as the peroxide of hydrogen decomposes when in 
contact with gold or silver, and also contains hydrochloric acid 
(used for its preservation), he cannot but regard its proposed use 
as dangerous if not certainly destructive to the photograph. In 
reply, Mr. Dawson, of King’s College, states that using the per- 
oxide of hydrogen used by Mr. Robbin, he found the trace of 
hydrochloric acid contained in it to be so trifling, that in a ten- 
volume solution, which is the strength at which it is used, blue 
litmus-paper remained unchanged. He also asserts, as the result 
of careful experiment, that the preparation does not decompose 
when in contact with a silver print. Dr. J. Emerson Reynolds is 
also conducting analysis connected with this subject, the results 
of which he promises to make public. At present he is inclined 
to believe that the application of peroxide of hydrogen to a print 
is only another mode of giving it a wash in a very weak solution 
of sulphide of sodium, and thereby facilitating fading. Thus 
*‘ doctors differ.” Tempus omnia revelat. The nature of that in- 
visible image which light fixes on a surface photographically pre- 
pared, is a mystery which has baffled the best efforts of our most 
persevering and distinguished physicists and chemists. Mr. M. 
Carey Lea (British Journal of Photography) described a series of 
experiments bearing on the subject, in the course of which many 
errors were demonstrated, and some new evidence of great value 
made clear. ‘These tended to prove that pure iodide of silver is 
iot chemically affected by hight ; that although, when exposed to 
light, it yields an image capable of development, yet the chemical 
composition of the iodide remains unaltered. ‘The true action has 
therefore yet to be determined, and the controversy still continues 
between those who uphold the molecular or physical theory, and 
those who still retain their faith in the chemical theory. Some of 
the more recent efforts in this direction are due to Dr. Reissig, — 
and certain experiments, which seem to show that pure iodide of 
silver exposed to light under a pure aqueous solution of the 
nitrate releases oxygen and renders the solution acid, are im- 
portant. In the course of some remarks on the precipitation of 
silver, Mr. Carey Lea describes a curious fact in relation to the 
colour of the deposited silver. He says, “A plate was covered 
with a considerable thickness of ammonia nitrate solution, to 
which was added a dilute solution of Rochelle salt. The plate 
was then placed in sunlight and left for some time. Reduction 
took place, and the evaporation, which went on simultaneously, — 
had extended over about one-half the plate, when it was removed 
