_Photographic Gun-Cotton._--The "doctors differ" not a little in their
prescriptions for preparing the best gun-cotton for photographic use. How
shall the photographer decide between them?
DR. DIAMOND ("N. & Q.," Vol. vi., p. 277.) says (I quote briefly), "_Pour
upon_ 100 grains of cotton an ounce and a half of nitric acid, previously
mixed with one ounce of strong sulphuric acid. Knead it with glass rods
_during five minutes_," &c.
Mr. Hunt, quoting, (apparently with approbation) from MR. ARCHER, says (p.
260., 3rd edit.), "Take one ounce by measure of nitric acid, mixed with one
ounce by measure of ordinary sulphuric acid, and _add to them_ eighty
grains of cotton; well stir," &c., "_for not more than_ FIFTEEN SECONDS,"
&c. "It will be seen that the cotton is not exposed to the action of the
mixed acids in this last mode longer than is necessary to saturate the
cotton; should the action be continued further, the solubility of the
cotton is entirely lost."
Not only is the order of manipulation different (a point probably not
material), but the time between "five minutes" and "fifteen seconds" must
exercise a most important influence on the result. Who is right?