426. The question of regelation immediately connects itself with that of "latent heat," already referred to (383), but which we must now subject to further examination. To melt ice, as already stated, a large amount of heat is necessary, and in the case of the glaciers this heat is furnished by the sun. Neither the ice so melted nor the water which results from its liquefaction can fall below 32° Fahrenheit. The freezing point of water and the melting point of ice touch each other, as it were, at this temperature. A hair's-breadth lower water freezes; a hair's-breadth higher ice melts. 427. But if the ice could be caused to melt without this supply of solar heat, a temperature lower than that of ordinary thawing ice would result. When snow and salt, or pounded ice and salt, are mixed together, the salt causes the ice to melt, and in this way a cold of 20 or 30 degrees below the freezing point may be produced. Here, in fact, the ice consumes _its own warmth_ in the work of liquefaction. Such a mixture of ice and salt is called "a freezing mixture." 428. And if by any other means ice at the temperature of 32° Fahrenheit could be liquefied without access of heat from without, the water produced would be colder than the ice. Now Professor James Thomson has proved that ice may be liquefied by mere _pressure_, and his brother, Sir William Thomson, has also shown that water under pressure requires a lower temperature to freeze it than when the pressure is removed. Professor Mousson subsequently liquefied large masses of ice by a hydraulic press; and by a beautiful experiment Professor Helmholtz has proved that water in a vessel from which the air has been removed, and which is therefore relieved from the pressure of the atmosphere, freezes and forms ice-crystals when surrounded by melting ice. All these facts are summed up in the brief statement _that the freezing point of water is lowered by pressure_.[I] [I] Professor James Thomson and Professor Clausius proved this independently and almost contemporaneously. 429. For our own instruction we may produce the liquefaction of ice by pressure in the following way:--You remember the beautiful flowers obtained when a sunbeam is sent through lake ice (§ 11), and you have not forgotten that the flowers always form parallel to the surface of freezing. Let us cut a prism, or small column of ice with the planes of freezing running across it at right angles; we place that prism between two slabs of wood, and bring carefully to bear upon it the squeezing force of a small hydraulic press.