I have found the protonitrate of iron, as also the protosulphate, and not seldom the pyrogallic, so difficult of application, that I have stained and spoiled very good pictures. I have therefore used, and with perfect success, a tray of gutta percha a little longer than the glass (say one-fourth of an inch), and one-fourth of an inch deep; sliding from one end the glass into the tray (supplied immediately before using it), by which means the glass is all covered at once.
I think the Rev. Mr. Sisson's suggestion, viz. to send you some of our specimens with collodion, a very proper one, if not declined on your own part, and shall, for one, feel great pleasure in acting in accordance with it.
You will, I trust, pardon any foregoing hints for beginners, as I well know that I have lost several pictures by hypo-crystals, and very many by the difficulty in developing.
L. Merritt.
Maidstone.
P.S.—I always find collodion by Dr. Diamond's formula capital, and with it from five to ten seconds is time enough.
Mr. Weld Taylor's cheap Iodizing Process.—I have no doubt Mr. Weld Taylor will be kind enough to explain to me two difficulties I find in his cheap iodizing process for paper.
In the first place, whence arises the caustic condition of his solution, unless it be through the decomposition of the cyanide of potassium which is sometimes added? and if such caustic condition exists, does it not cause a deposition of oxide of silver together with the iodide, thereby embrowning the paper?
Why does the caustic condition of the solution require a larger dose of nitrate of silver, and does not this larger quantity of nitrate of silver more than outbalance the difference between the new process and the old, as regards price? I pay 1s. 3d. for an ounce of iodide of potassium of purest quality; the commoner commercial quality is cheaper.
F. Maxwell Lyte.