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 same time, and we are quite at a loss to explain how it is that the motion of a mass of ice conforms to that of an imperfect fluid.

Professor Tyndall has recently cleared up the mystery, and has shown that ice may be plastic without being viscous. Some time ago, Professor Faraday discovered that two pieces of ice when placed in contact, would freeze together, even under hot water, and that any number of fragments would unite into a solid mass, provided sufficient pressure were applied to bring their surfaces together. The plasticity of ice has since been established beyond all question by the beautiful experiments of the younger philosopher. Spheres of ice have been flattened into cakes, cakes have been formed into transparent lenses, a block of ice has been moulded into a crystal cup, and a straight bar six inches long has been bent into a semi-ring. Ice can be forced into a mould and made to take what shape we please, not because it is an imperfect fluid like plaster of Paris, but because it possesses the peculiar property of re-uniting by the contact of adjoining surfaces, after having been broken into fragments. In forcing a cube of ice into a cup-shaped mould, we crush it to a powder, but the particles composing this powder immediately freeze together again into a solid and transparent cup. The plasticity of ice may therefore be explained as the effect of breakage and re-freezing, or in scientific language, fracture and regelation.