I58 the art of projecting. To water made slightly acid, add enough litmus solution to turn it red and project it ; then drop a lump of carbonate of aihmonia into it. It will dissolve rapidly with effervescence, and the solution will be made blue about the crystal, and if there is enough of it the whole solution will ultimately become blue. The gradual solution of substances in water may be nicely shown by filling the tank with pure water and dropping a crystal of alum or sulphate of zinc or sul¬ phate of copper into it. Where the substance is dis¬ solved the solution will be denser, and its refractive powers changed, which will be manifest by gently stir¬ ring it with a glass rod. A dilute solution of copper sulphate may be placed in the tank. With a pipette, force into the solution some ammonia water : A dense precipitate will at first be formed, which will afterwards be dissolved if am¬ monia enough has been added, leaving the solution a beautiful blue color. A few drops of sulphuric acid will reproduce the precipitate and destroy the color; and when the solution again becomes clear, a few drops of ferrocyanide of potassium added will produce a brownish-red bulky precipitate, which will present a fine appearance upon the screen. In like manner all of the characteristic reactions of inorganic chemistry may be projected, and often with much less expenditure of materials than would be used if large vessels were employed to demonstrate the same things. One who has projected a number of these phenomena will find no difficulty in projecting any reaction that may be observed in a test-tube. Pictures of chemical apparatus, of processes, etc., will be very convenient for projection when instruction is given in chemistry.