Mix, and lay the paper on this solution for not less than five minutes, and if English paper, double that time. The hyposulphite to be used may be a very strong solution of twenty to twenty-five per cent., and this mode of treatment will always be found to produce fine tints. After some time it will be found that the nitrate bath will lose its acidity, and a drachm of acetic acid may be again added, when the prints begin to take a red tone: this will again restore the blacks. Lastly, the bath may of itself get too weak, and then it will be best to place it on one side, and recover the silver by any of the usual methods, and make a new bath. One word about the addition of the bromide of {158} silver to the double iodide, as recommended by DR. DIAMOND. I tried this, and feel most confident that it produces no difference; as soon as the bromide of silver comes in contact with the iodide of potassium, double decomposition ensues, and iodide of silver is formed. Indeed, farther, this very double decomposition, or a similar one, is the basis of a patent I have just taken for at the same time refining silver and manufacturing iodide of potassium; a process by which I much hope the enormous present price of iodide of potassium will be much lowered. F. MAXWELL LYTE. Hôtel de l'Europe, à Pau, Basses Pyrénées. P.S.--Since writing the former part of this letter, I see in _La Lumière_ a paper on the subject of printing positives, in part of which the addition of nitric acid is recommended to the bath; but as my experiments have been quite independent of theirs, and my process one of a different nature, I still send it to you. When I have an opportunity, I will send a couple of specimens of my workmanship. I had prepared some for the Exhibition, but could not get them off in time. I may add that the developing agent I use is the same in every way as that I have before indicated through the medium of your pages; but where formic acid cannot be got, the best developer is made as follows: Pyrogallic acid 27 grs. Acetic acid 6 drs. Water 9 oz. _On Sensitive Collodion._--As I have lately received many requests from friends upon the subject of the most sensitive collodion, I am induced to send you a few words upon it. Since my former communication, I believe a greater certainty of manufacture has been attained, whereby the operator may more safely rely upon uniformity of success.