HENRY H. HELE. _Cyanide of Potassium_ (Vol. ix., p. 230.).--I have for a long time been in the habit of using a solution of the above-named substance for fixing collodion _positives_, because the reduced silver has a much _whiter_ appearance when thus fixed, than when the hyposulphite of soda is used for the same purpose; but I cannot quite agree with MR. HOCKIN that it is _equally_ applicable to negatives, though in many cases it will do very well. I find the reduced metal is more pervious to light when fixed with the cyanide solution, particularly in weak negatives. Lastly, I find that a small quantity of the {255} silver salts being added to the solution before using, produces less injury to the half-tones, and this not by merely weakening the solution, as one of double the strength with the silver is better than one without it, though only half as powerful. Your correspondent C. E. F. (_ibid._) will find his positives will not stand a saturated solution of hyposulphite of soda, unless he prints them so intensely dark that all traces of a picture by reflected light are obliterated; but I have sometimes accidentally exposed my positives a _whole day_, and retained a fair proof by soaking the apparently useless impressions in such a solution. GEO. SHADBOLT. * * * * * Replies to Minor Queries. _Saw-dust Recipe_ (Vol. ix., p. 148.).--See Herschel's _Discourse on the Study of Natural Philosophy_, published in Lardner's _Cyclopædia_, p. 64., where he says: "That sawdust itself is susceptible of conversion into a substance bearing no remote analogy to bread; and though certainly less palatable than that of flour, yet no way disagreeable, and both wholesome and digestible, as well as highly nutritive." To which passage the following note is appended: "See Dr. Prout's account of the experiments of Professor Autenrieth of Tubingen, _Phil. Trans._, 1827, p. 331. This discovery, which renders famine next to _impossible_, deserves a higher degree of celebrity than it has obtained." J. M. W. Though not exactly the recipe for _saw-dust biscuits_ which I have heard of, there is an account of the process of making bread from bark in Laing's "Norway" (Longman's _Traveller's Lib._), part ii. p. 219., where, on the subject of pine-trees, it is stated: