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244 OPTICAL PROJECTION give sufficient colour on the screen, which is the chief thing to be studied, and will depend upon the thickness of the tank: hence a tank of proportionate thickness enables any strength of solution to be used which may be suitable to the reaction. There is also room for ingenuity, in so arranging an experi- ment as to give the reactions in an attractive or striking manner. Thus, a tank may be filled with the well-known infusion of red cabbage, the top of the tank being long enough to extend all along the diameter of the condensers. Then, if we add a drop or two of potass solution at one end, of alum solution in the centre, and hydrochloric acid at the other end, the purple, green, and crimson colours will appear simultaneously. The reactions formed by adding potassic ferrocyanide to salts of iron, copper, bismuth, &c., are very impressive. Most of them are better shown with dilute solutions than the fore- going. Electric reactions are easily shown by filling a tank with sodium sulphate, coloured with cabbage or litmus, which pro- jects as a blue solution. If the terminals of a small battery are then introduced, the acid and alkaline reactions will appear at the poles. The action of heat upon salts of cobalt is prettily shown in the lantern by coating a glass plate with a saturated solution of the chloride in a solution of gelatine. The rosy tint will gradually change to a blue in the heat of the lantern. This may be varied by using with the plate a photographic slide, which if judiciously chosen will give a curious apparent change from day to moonlight; or if a design be sketched with a weaker solution, the gradual visibility of the so-called sympa^ thetic inks can be readily exhibited. The main facts of popular ' domestic chemistry' are readily demonstrated. Bleaching, for instance, may be illustrated by filling a tank with a solution of indigo in dilute sulphuric acid, and projecting the tank upon the screen. By adding a solution of calcium chloride, the colour will gradually disappear.