“591. In using these test papers with a fluid suspected to contain free acid or alkali, or knowing that one of these substances is predominant, to ascertain which is so, all that is necessary is to moisten them with the liquid, and observe the change: if the fluid be acid, the blue colour of the litmus will immediately become red; if alkaline, the yellow colour of the turmeric will be changed to a brown. The moistening may be effected by dipping the paper into the liquid; but a better method is to touch the edge of the slip with a rod dipped in the fluid. In the latter case there is no risk of contamination to the fluid from the paper, and only a very minute quantity of the liquid is used at once. “592. These trials must be made by day-light; artificial light not permitting that just estimation of the changes by which the presence of a small excess of acid or alkali is to be determined. As the proportion of free acid or alkali diminishes, the intensity of the new tint produced upon the paper is also diminished; and when in very small quantity, it requires considerable attention before a decision can be arrived at. The test paper should occasionally be touched with pure water in the immediate neighbourhood of the part where the solution has been applied, for any change in appearance that may have occurred, not due to mere moistening, is then readily perceived. “593. Although acid is generally tested for by litmus paper, and alkali by turmeric paper, yet the former is sometimes used advantageously for the latter purpose, being first slightly reddened, either by exposure to the air, or by momentary contact with muriatic acid fumes. When the [p280] paper thus modified is used to detect a free alkali, instead of turmeric paper, that substance is indicated by the restoration of the original blue colour. Litmus paper is best slightly reddened for this use, by putting a drop or two of muriatic acid into a large jar, allowing it to stand a few minutes, and then bringing the paper towards the mouth of the jar, or carefully placing it within: so soon as the blue tint has become slightly reddened, the paper should be removed for use. If too much acid be imparted to the paper, the delicacy of its indications is injured, because of the greater quantity of alkali required to neutralize the acid, and restore the blue colour. For the same reason a paper free from alkali or carbonate of lime has been recommended for the preparation of these tests: for these impurities, combining with a minute portion of acid, neutralize it, and thus prevent that delicacy of indication which the test paper ought and may be made to possess.”