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86 SCIOPTICON MANUAL. the tank, put in it a strip of sheet zinc, and long leaf- like blades of metallic tin will at once be seen to shoot out in all directions. Experiment 5th. Make a concentrated solution of crystals of urea in alcohol of about 95 per cent. (The common 85 per cent^ alcohol will not answer.) Let a few drops of this fall on a glass plate, and with the finger spread it rapidly over the surface, and then at once place it as an object in the lantern. After about a minute, blow gently on the plate with a bellows (not with the breath), and at once on the screen will be seen the growth as of frost crystals shooting over the field in all directions. Experiment 6th. If sulphate of copper in solution is mixed with enough gum-arabic water to make the solu- tion form a continuous film, when flowed like collodion on a clean glass, and such plates are allowed to dry slowly in a nearly horizontal position, a very beautiful crystalline vegetation will set in, which varies in its character with the proportion of gum used, and will make objects well fitted for exhibition with the lantern. In place of sulphate of copper, we may use nitre, or ferrocyanide of potassium, with the production of an entirely new class of forms. By placing the plates so covered with crystals over a leaden dish, in which is a little fluor-spar, moistened with sulphuric acid, and warmed slightly (giving off fumes of hydrofluoric acid), permanent etchings may bo prepared, which are also very beautiful objects for the lantern. These are only a few of the experiments of this char- acter which can be performed with the lantern, but they will indicate the direction in which each one can be a discoverer and inventor for himself.