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.