George Shadbolt.
Animal Charcoal in Photography.—Perhaps you or one of your photographic correspondents would inform me whether the animal charcoal, recommended for the aceto-nitrate of silver solution, should be used as a filter, or simply allowed to remain in the bottom of the bottle?
A. B. C.
Oxford.
Sir W. Newton on Use of Common Soda and Alum.—In reply to W. Adrian Delferier, who is desirous of knowing the "rationale of the action of the common soda and powdered alum, &c.," my motive for using common soda to cleanse the negatives is, that it not only removes the hyposulphite of soda more readily, but any impurities which may be in the paper, as well as the whole of the size, such being absolutely necessary for the after waxing process; which, when done, the negative should appear nearly as transparent as glass.
The reason why I prefer alum for the positives is, that while it has the effect of removing the hyposulphite of soda and other impurities in the paper, it does not act upon the size, which in this instance it is desirous to retain.
I have been induced to make a series of experiments, with a view to prevent the fading of the positives, or, indeed, that any portion should be, as it were, eaten away in parts; and since I have adopted the foregoing, in no one instance has any change taken place whatever.
W. J. Newton.
6. Argyle Street.
Difficulties in Photographic Practice.—Having met with some of the difficulties that your correspondent G. H. mentions in his communication (Vol. vii., p. 218.), I beg to offer a few hints which I think will be of service to those who are trying the waxed-paper process.