109. VARIOUS CHEMICAL TESTS.--When water is suspected to hold any foreign substance in solution, various means may be used to detect and ascertain the quality of the substances combined; thus, acids may be detected by immersing in the water, a slip of litmus colored paper, which, if acid be present, will be changed to red. In the same manner, alkalies may be detected by a strip of turmeric yellow paper, which will be also changed to red by alkalies. These tests are sensible to the presence of an acid or alkali in the proportion of one to ten thousand. Iron may be detected by a drop of infusion of galls, which will give to the water (if iron be present) a brown tinge. A drop of sulphuric acid, precipitates barites in the form of a white powder. Clear transparent lime-water (water in which lime has been slaked and then suffered to settle) will indicate the presence of carbonic acid by a milky whiteness. On the same principle, a solution of super-carbonate of potass will detect lime. A few drops of nitrate of silver will instantly discover muriatic acid, by a white flaky precipitate. Muriatic acid, consequently, is a good test for silver. Acetate of lead, in solution, is a test for sulphureted hydrogen, which occasions a precipitate of a black colour. Nitrate of mercury is an excellent test for ammonia, one part of which, with 30,000 parts of water is indicated by a blackish yellow tinge on adding the test. Liquid ammonia is a very sensible test for copper, with which it strikes a fine blue colour. Nitro-muriate of gold will discover the presence of tin, by a beautiful purple precipitate. Nitro-muriate of tin is, on the same principle, an excellent test for gold. 110. TO PRODUCE A PICTURE INSTANTLY, IN A VARIETY OF COLOURS.--Paint any picture on paper in the usual way, only instead of colours, use the following substitutes: for green, use a solution of nitro-muriate of cobalt, for blue, a solution of sulphate of iron--for yellow, a solution of nitrate of bismuth--and for a brown, a solution of sulphate of copper. Any of these solutions may be more or less diluted, as the respective parts of the picture are to be light or dark, but none of them must be strong enough to colour the paper. This pictture is invisible: but when it is required to appear, the paper may be tacked up on the wall, and having a glass of the transparent solution of prussiate of potass (which by sight cannot be distinguished from clear water) dashed suddenly upon it, the picture will instantly appear in its full colours. A similar effect may be produced, by drawing the picture with infusion of galls, and sub-carbonate of potass; this is revived by a solution of sulphate of iron, and appears in a yellow and a brown colour.