Mr. Hunt, quoting, (apparently with approbation) from MR. ARCHER, says (p. 260., 3rd edit.), "Take one ounce by measure of nitric acid, mixed with one ounce by measure of ordinary sulphuric acid, and _add to them_ eighty grains of cotton; well stir," &c., "_for not more than_ FIFTEEN SECONDS," &c. "It will be seen that the cotton is not exposed to the action of the mixed acids in this last mode longer than is necessary to saturate the cotton; should the action be continued further, the solubility of the cotton is entirely lost." Not only is the order of manipulation different (a point probably not material), but the time between "five minutes" and "fifteen seconds" must exercise a most important influence on the result. Who is right? COKELY. _Sealing-wax for Baths._--I notice in your answers to correspondents (No. 176., p. 274.), that you inform H. HENDERSON that glass may be cemented for baths with sealing-wax. May I recommend to H. HENDERSON the use of gutta percha, instead of glass, for that purpose? Sheet gutta percha is now very cheap, and the baths are most easily made. I have had one of my own making in constant use since last July, having never emptied it but twice, to filter the nitrate of silver solution. It is not liable to breakage. The joinings are much less liable to leakage. And when it is necessary to heat slightly the silver solution (as it has been during the late cold weather), I have adopted the following simple plan: Heat moderately a stout piece of plate glass; plunge it into the bath; repeat the operation according to the size of bath. It is very useful to make a gutta percha cap to cover over the bath when not in use; it protects it from dust and evaporation, and saves the continual loss of materials arising from pouring the solution backwards and forwards. For home-work I have reduced {315} the whole operation to a very simple system. My bath, hypo-soda, developing fluid (of which, as it keeps so long, I make ten ounces at a time), are always ready in a small closet in my study. These I arrange on my study-table: a gutta percha tray, a brass levelling-stand upon it, a jug of soft water, and half-a-dozen small plates to place my pictures on, after treating them with the hypo-solution (for, to save time, I do not finish washing them until I have done all the pictures I require). All these things I can prepare and arrange in less than ten minutes, and can as easily return them to their places afterwards.