_Cyanide of Potassium._--It may be interesting to our photographic friends to know that cyanide of potassium is capable of replacing hyposulphite of soda in all collodion processes. If used of the strength of five grains to one ounce of water, no danger need be apprehended from it. Its merits are cleanliness, quickness of operation, and the minute quantity of water required for washing the picture fixed therewith. J. B. HOCKIN. _Mode of exciting Calotype Paper._--I forgot inserting this plan of exciting in my paper: it is very clean and convenient, simple and sure. Obtain a piece of plate glass, two or three inches larger than your paper, level it on a table with a few bits of wood, pour on it your exciting mixture (say aceto-nitrate and gallic acid, solution of each 20 minims, distilled water 1 ounce), and spread it evenly over with a scrap of blotting-paper. Float your paper two minutes, remove and blot off; this ensures perfect evenness, especially if the paper is large. You may thus excite half a dozen papers with little more trouble than one. THOS. L. MANSELL. _The Double Iodide Solution--Purity of Photographic Chemicals._--The observations of MR. LEACHMAN upon the solvent powers of iodide of potassium (Vol ix., p. 182.) are perfectly correct, but I believe our photographic chemicals are often much adulterated. The iodide of potassium is frequently mixed with the carbonate. DR. MANSELL writes me word, in a comment upon your note upon his communication, "What I used was _very_ pure, having been prepared by Mr. Arnold with great care: it was some that had gone to the Great Exhibition as a sample of Guernsey make, and obtained a medal." I have this day used exactly seven ounces avoirdupois to make a pint of the iodizing solution, which, within a few grains, agrees with my former results. Nitrate of silver, I am informed upon a most respectable authority, has been adulterated thirty per cent., and without careful testing has eluded detection; but I am inclined to think our cheapest article has come in for its largest share of mixture. I have lately perfectly failed in the removal of the iodide of silver with a _saturated_ solution of what I purchased as hyposulphite of soda, but which could have been little else than common Glauber's salts; for upon applying a similar solution of some which was made by M. Butka of Prague, and supplied me by Messrs. Simpson and Maule, the effect was almost immediate, demonstrating how much we are misled in our conclusions, from believing we are manipulating with the same substances, when in fact they are quite different. HUGH W. DIAMOND. _Hyposulphite of Soda Baths._--Is there any objection to using the same bath (saturated solution of hyposulphite) for fixing both paper calotype _negatives_ and positives printed on albumenized paper from glass collodion negatives? C. E. F. * * * * * Replies to Minor Queries.