Regelation will consist of the passage, under small changes of pre. sure and temperature, of crystalloid into colloid ice, and its re-conver sion into the former, the " cementation " and " reilintegration " of the colloid intervening ; and thus Faraday's own interpretation of his experiments and Professor Thomaon's will be brought into harmony The theories of glacier motion of Forbes, Thomson, and Tyndall (in cluding the explanation of the plasticity of ice by the latter), and the views respecting it of Hopkins and Whewell, may prove to be equally true, or some of them unnecessary. Under the reciprocal changes of pressure and temperature going on in a glacier, it will follow that every part of it must be perpetually changing from a truly viscous or plastic, colloidal, dynamic state, maintained only for a short time, to the rigid, crystalloidal, statical condition of ordinary ice ; which, under the same changes will again transiently assume the colloidal form, only to undergo the corresponding change, and so on in a succession which can terminate only with the final liquefaction of the ice, and its separation from the glacier by flowing away in the form of water. All the conditions of viscosity or plasticity, and rigidity, which the phenomena of glaciers and the known properties of ice require to be fulfilled, may thus be capable of explanation agreeably to the laws .of nature. But many of 31r. Graham's results are independent of his assumption respecting ice. It is stated by him, as above, that colloidal bodies affect the vitreous structure ; and lie recognises the vitreous form of silica as the colloidal one. It is now evident, indeed, that what, looking primarily from organic substances, he has termed the colloidal state, is identical, allowing fur inherent difference of properties, with that, which, looking primarily from inorganic bodies, has been termed the vitreous or glassy state. Every description of glass must, agreeably to Mr. Graham's inductions, be a colloidal body, and the union of two surfaces of it at all temperatures, while the solid condition is maintained, must be that cementation and redintegratieri which is physically identical with regelation itself; and thus Mr. Brayley's arguments for the virtual identity with regelation of the process by which two plates of plate-glass become one, which Dr. Faraday's new results appeared to invalidate, will be confirmed; while regelation, as also suggested by him, may be found to be universal, as respects all bodies which can assume the colloid form, Dr. Faraday's experiments indi cating its non-extension having been confined to crystalloids. The mutability and continued metastasis of colloids are manifested in the unstable condition of arrested liquidity or potential solidity recognised by Mr. Brayley; and Mr. Graham's comparison of a colloid in that respect "to water while existing liquid at a temperature below its usual freezing point, or to a supersaturated saline solution," is a repetition of Mr. Brayley's view of the molecular constitution of glass already cited. Colloidal ice (if it shall be proved to exist), and unfrozen water below 32°, aro evidently degrees of the ;same condition, and thus Mr. Bmyley'e suggested homologue of the glassy condition of water is almost equivalent to the former, and if, as seems conformable with known facts, we assume that the colloid state of water cannot begin until it is reduced to the temperature of its greatest density, that homologue will include, almost explicitly, Mr. Graham's colloidal con dition of ico, Some nice questions of temperature will, however, have to be settled by experiment; and indeed the subject is now ripe for those quantitative determinations in which the existing discussions of regelation are remarkably deficient, and which must necessarily be of a minute and delicate description.
But here we must conclude on this enbject ; having merely indicated how wide a field for experimental research, observation in nature, the verification of hypotheses, and mathematical investigation, all relating to ice and water, has probably been opened to science by Mr. Graham's researches on liquid diffusion.
From the preceding view of the obvious characters and actual nature of ice, we proceed to describe some of its properties in relation to heat, and some also of water in its two fluid states. The dilatation of ice by heat was measured in the years 1845 and 1846, at the Imperial Obser vatory of Pnlkowa, by Schumacher and his associates, and the par ticulars of their experiments were communicated by M. Struve to the Academy of St. Petersburg, in 1848, and were afterwards published in
its' Memoirs.' The measurements had reference to observed tempera tures of the block of ice employed, varying from It. to — 22° R. (+5'175 to Fahr.) After applying the requisite corrections, it resulted from them that the coefficient of expansion of ice is for 1° R. (2°.25 Fahr.) '00006466 ; which, according to the Rev. Canon Moseley, F.R.S., " is nearly twice as great as the coefficient of dilatation of lead, and more than twice as great as that of any other solid." [HEAT, col. 637.] We do not know the modulus of elasticity of ice,* or the pressure under which it disintegrates ; but Mr. Moseley has observed that " If it were as elastic as slate and did not resist crushing more than hard brick, a block of it placed with its ends between two immoveable obstacles, would crumble when its temperature was raised one degree of Fahrenheit. It is its great dilatability which gives to ice this ten dency to disintegrate, when, not being free to dilate, its temperature is raised, even so slightly as this. Agassiz describes a disintegration of the transparent ice of the blue bands of glaciers when laid bare, which appears to be due to its expansion." Bulletin [Bibliotlickple] do Geneve,' vol. ally., p. 142; (` Proc. of Roy. Sec.' vol. xi., pp. 171, 172.) According to the experiments of Melloni on the transmission of radiant heat, ice transmits none (absorbs all) of the calorific rays issuing from copper at 212° or at 752° Fahr., nearly approaching a red heat ; and transmits only 0-5 of those from incandescent platinum, and only 6 per cent. of such rays from the Locatelli lamp. In these cases the heat is absorbed in the internal liquefaction of the ice.
The colour of liquid water varies, according to the thickness of the quantity examined, from a yellowish green of all degrees of intensity through green and blue-green to intense blue, such as that observed in great depths of the sea. Professor Tyndall has introduced into British demonstrative science, if indeed be has not devised, an experiment in which the colour of water is exhibited by passing the light from the voltaic lamp through a long tube of water closed by glass at both ends, and receiving the image on a screen. In this experiment, with no greater thickness than twenty feet, the colour of water is seen to be yellowish green. Ice, probably, has the same range of colour ; being, like water, colourless in small masses; it is greenish or bluish in large masses. Pure aqueous vapour is colourless in the greatest thicknesses in which it has been examined.
Many important facts, and inductions from them, relating to the electrical properties of water in all its three states of aggregation, will be found referred to under their respective appellations in the Indexes to Faraday's Experimental Researches in Electricity,' and in Che mistry and Physics,'—indexes which are enhanced in value by having been constructed by the author of those researches himself.
The process of the solidification of water by depression of tempera ture is noticed under FREEZING, and FREEZING AND MELTING POINTS. The lowering of its freezing-point by pressure, as discovered by Pro. fessor J. Thomson, is stated in the article cited, and has been referred to in the present article, and also under ICE. The theory and quantitative calculation he originally gave respecting it will he found in the ' Transactions of the Royal Society of Edinburgh,' vol. :v., and the Cambridge and Dublin Mathematical Journal' for November 1850. Ice, as a crystalline substance has been described under HAIL, HOAR-FROST, and SNOW. Its specific gravity is stated in the last. Mr. J. Chapman, Professor of Mineralogy in the University of Toronto, in the Canadian Journal of Science' for 1861, has questioned the truth of referring the crystallisations of ice to the rhombohedral system. A mode of investigating the process of formation of the stellar and other aggregations of crystals so characteristic of snow, under circumstances more convenient than those of observations which must he made at a temperature below 32° Fehr., has been pointed out by Mr. Joseph Spencer, and adopted by Mr. Glashier. It consists in observing the crystallisation of camphor, in which similar aggregates are produced ; and has been described in papers read before the Greenwich Natural Ilistory Club, in the year 1856, and issued by the British Meteorological Society. The compressibility and ELASTICITY of water have both been treated of under the latter head.