11. The white blotting-paper used for some photographic purposes is not suitable for filtering solutions; that only should be employed which is made for this purpose, and is sold under the name of filtering-paper.
12. Hyposulphate of soda.—A great deal of rubbish is sold under the name of this salt. As a test of its quality, 11⁄2 drachms should entirely dissolve in 1 drachm of water, and this solution should dissolve rather more than 41⁄2 grains of iodide of silver.
13. Chemicals.—The purity of photographic chemicals cannot be too strongly urged; the cheapest are not always the most economical. The commercial preparations are generally not to be depended upon, as these, though perhaps unadulterated, are, strictly speaking, not chemically pure. It is best to procure them from well-known chemists, who understand the purpose for which they are intended, and make the preparation of these substances peculiarly a branch of their business.
14. Never leave chemical solutions exposed in dishes: when done with, pour them back into glass-stoppered bottles, and decant for use from any deposit, or filter if necessary.
15. In all photographic processes it is absolutely necessary to be chemically clean; and this sometimes is not easy. As a rule, never be satisfied with cleanly appearances only, but take such measures as shall insure the absence of all extraneous matter in preparing the solutions, cleaning the glasses, dishes, &c.
16. All stains on the hands, linen, &c. may be removed by means of cyanogen soap or cyanide of potassium, which should be applied without water at first, then thoroughly washed off. To assist the operation, the hands may be now gently rubbed with a fine piece of pumice-stone, when the stains quickly disappear.
For more perfect and complete directions the reader is referred to a most excellent work, “The Modern Practice of Photography,” by R. W. Thomas, F.C.S.
