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1858.] or $1⁄1000$ of a degree would be far less: and if we suppose that only, $1⁄20$th of a piece of ice is brought into the condition of melting before the rest of the mass, and that the salts in that proportion were originally in the whole of the water, then its quantity there may be so small as to escape detection except by very careful analysis. However, it would be desirable to examine the water chemically which is produced by ice distinguished by having in its interior much, that liquefies before the rest.

It is easy to make ice perfectly free from air, and, as I believe, from salts, by a process I formerly described. It would be interesting to see if such ice had within it portions melting at a lower temperature than the general mass. I think it ought not.

Royal Institution, Dec. 9, 1857.

On Regelation.

subject of reg elation has of late years acquired very great interest through the experimental investigations of Tyndall, J. Thomson, Forbes and others, and in its present state will perhaps justify a few additional remarks on my part as to the cause. On the first observation of the effect eight years ago, I attributed it to the greater tendency which a particle of fluid water had to assume the solid state, when in contact with ice on two or more sides, above that it had when in contact on one side only. Since then Mr. Thomson has shown that pressure lowers the freezing-point of water, and has pointed out how such an effect occurring at the places where two masses of ice press against each other, may lead first to fusion and then union of the ice at those places; and so he explains the fact of reg elation. Prof. J. D. Forbes does not think that pressure causes reg elation in this manner, though it favours it by moulding the touching surfaces to each other. He admits Person's view of the gradual liquefaction of ice, and assumes that ice must be essentially colder than ice-cold water, i. e. the water in contact with it.