_Cyanide of Potassium_ (Vol. ix., p. 230.).--I have for a long time been in
the habit of using a solution of the above-named substance for fixing
collodion _positives_, because the reduced silver has a much _whiter_
appearance when thus fixed, than when the hyposulphite of soda is used for
the same purpose; but I cannot quite agree with MR. HOCKIN that it is
_equally_ applicable to negatives, though in many cases it will do very
well. I find the reduced metal is more pervious to light when fixed with
the cyanide solution, particularly in weak negatives. Lastly, I find that a
small quantity of the {255} silver salts being added to the solution before
using, produces less injury to the half-tones, and this not by merely
weakening the solution, as one of double the strength with the silver is
better than one without it, though only half as powerful.
Your correspondent C. E. F. (_ibid._) will find his positives will not
stand a saturated solution of hyposulphite of soda, unless he prints them
so intensely dark that all traces of a picture by reflected light are
obliterated; but I have sometimes accidentally exposed my positives a
_whole day_, and retained a fair proof by soaking the apparently useless
impressions in such a solution.