With reference to his process for iodizing Canson's paper, I presume his meaning to be as follows, viz.: Mix half an ounce of a _forty-grain_ solution of _nitrate of silver_ with an equal quantity {245} of a _fifty-grain_ solution of _iodide of potassium_, by which a precipitate of iodide of silver will be formed, the supernatant fluid containing the excess of iodide of potassium and the nitrate of potash formed by the decomposition. _Add_ drop by drop a solution of the cyanide of potassium, until the iodide of silver is redissolved, and the liquid becomes limpid, and then _four ounces_ more of distilled water, making up five ounces altogether. The paper should then be washed over with the above and _dried_, after which it may be floated on water slightly acidulated with sulphuric acid for a few minutes, and after being again dried, either wholly, or else partially with blotting-paper, may be rendered sensitive with a weak solution of nitrate of silver. Here are two or three points admitting doubt: first, Would it not be better to wash away the nitrate of potash and free iodide of potash first, and then dissolve the iodide of silver in solution of cyanide of potassium? Secondly, Would not a slight soaking in plain water after the acidulated bath be of advantage? Thirdly, Is it better to dry the paper again before rendering it sensitive? and fourthly, What strength of nitrate of silver solution should be used to render it sensitive; and ought it to have any acetic or gallic acid, or both? 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.