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SCIOPTICON MANUAL. 85 of copper, and mix it well with the water of the tank, then with the pipette run in, with more or less force, some diluted ammonia, pausing from time to time to observe the progress of the effect. On the screen will be observed the gathering of a tempest of black storm- clouds, which twirl around in violent commotion, as if urged by a tornado of wind, but as the action continues, these clouds will melt away, and leave the entire field of a serene and beautiful sky-blue. By now throwing in some diluted sulphuric acid, the same changes can be reproduced, and so on alternately for a number of times. Then when the tank is clear, with an excess of acid, let fall a few drops of a solution of ferrocyanide of potassium from a small pipette, and rich red curdled clouds of ferrocyanide of copper will form with a beautiful appearance. Experiment 2d. Having rinsed the tank, or taken a fresh one with water in it as before, add to this some solution of litmus, until the whole acquires a purplish- blue tint. Now throw in very gently a little very dilute acid, and allow it to diffuse. On the screen will appear the image of a beautiful sunset sky, with its changing tints of drifting clouds. When all has changed to red, add ammonia, and so reverse the change, which may then be repeated. Experiment 3d. Proceed exactly as in the last case, but with a solution of cochineal in place of litmus. The red color will then be changed by the acid to a brilliant yellow, and by ammonia to a rich purple. Experiment 4th. Into a tank of water drop slowly a strong solution of the acid perchloride of tin. This on the screen will resemble the eruption of a submarine volcano. When a pretty strong solution has thus been made in