With regard to the point, whether the free iodine is the sole cause of the deterioration of old collodion, I should say decidedly not, at least in a theoretical view; the liberation of free iodine necessitates some other changes in the collodion, and the result must be influenced by these in one way or another, but practically I have as yet found nothing to warrant the supposition that they perceptibly interfere with the sensitiveness of the film.
In the above I have endeavoured as much as possible to avoid technicalities, in order to make it intelligible to amateurs; but if there be any part which may be considered obscure, on its being pointed out to me, I will endeavour to solve the difficulty.
William Crookes.
Hammersmith.
Photographic Queries.—1. Would you, Sir, or Dr. Diamond (Dr. Mansell is too far off), be kind enough to inform your readers whether Dr. Mansell's process, recommended in No. 225., is equally applicable to inland as to sea-side operations; or must we, in the one case, follow Dr. Diamond, and in the other Dr. Mansell, and thus be compelled to prepare two sets of papers?
2. Dr. Mansell recommends, as a test for the iodized paper, a strong solution of bichloride of mercury; may we ask how strong?
3. Mr. Sisson's developing fluid has undergone so many changes, and has been so much written about, that we are at a loss to discover or to determine whether it has been at length settled, in the mind of the inventor, that it will do equally well for negatives as for positives.
Four Photographic Readers.
[1. Both papers are equally available for both purposes. In actual practice we have not ourselves experienced any difference in their results.
2. It is quite immaterial. A drachm of bichloride dissolved in one ounce of spirits of wine will cause a cloudiness and a precipitate, if a very few drops are added to the tested water.