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228 OPTICAL PROJECTION so that the bubble will be in focus when blown, and the sauce* of solution is lifted to the funnel when required, and then with- drawn. If this bubble be pricked, it does not disappear as a soap- bubble would, but' hangs in rags' to the funnel, as it were. And secondly, if the air be sucked rapidly out from the bubble, it does not contract smoothly, but hangs in folds like a wet linen bag. That this is owing to extraordinary surface viscosity can be shown in a tank like fig. 117 laid on the vertical attachment, or less perfectly by direct projection in a cubical tank. Tanks like fig. 117 are made by cementing large metal rings or cylinders any required depth, on pieces of plate glass. The tank being supplied with saponine solution, it can first be shown by projection that a magnet suspended above the surface is readily moved about by another magnet. Being then lowered to the surface, hardly any movement can be produced. And being finally lowered below the surface, movement is free and easy again. 120. Cohesion Figures and Motions.— In such tanks as fig. 117 all the experiments usually known as ' Tomlinson's Cohesion Figures' * are readily projected by the vertical method; also the movements due to varying tension caused by dropping camphor, alcohol, &c., on the surface of water. The water must be perfectly clean for each new experiment; but cleaning or a fresh tank may often be dispensed with by neatly tossing, as it were, the contents of the tank perpen- dicularly out of it into a convenient receptacle. Another class of cohesion figures are produced in an ordinary slide-tank projected direct, by filling with water or alcohol, and dropping on the surface, or against one of the glass sides, with a glass rod, solutions of aniline dyes. Still another class are produced by providing pairs of pieces of plate-glass, bevelled towards the inside surfaces, and between » For detaila see Phil. Mag. 1861.