removed, c, or it may form a triangular prism or a rhombic prism, fig. d. On standing over a sheet of ice, and giving it a sudden blow with the end of a walking-stick, we may often produce six-rayed stars of great regularity. Professor Tyndall examined some clear plates of ice, taken from under the water in the cavity of a glacier, and found in them numerous liquid cells, the most common shape of which was a regular hexagon: many cells had also crimped borders, intimating that their primitive form was that of a flower with six leaves.*

The same observer states that, on watching the formation of artificial ice in Harrison's freezing machine, he has seen small six-rayed stars of thin ice forming and rising to the surface of the liquid. A similar arrangement of particles must be going on at the surface of a freezing lake, and large masses of ice must be built up under this star-like arrangement. On sending a beam of solar light through a block of pure Wenham Lake ice, in the form of a diverging cone of rays formed by a lens, "the ice became studded with lustrous spots, evidently formed by the beam, as if minute reflectors had been suddenly established within the mass, from which the light flashed when it met them. On examining the cube afterwards, I found that each of these spots was surrounded by a liquid flower of six petals; such flowers were distributed in hundreds through the ice, being usually clear and detached from each other, but sometimes * Philosophical Transactions for 1858.

crowded together into liquid bouquets, through which, however, the six-starred element could be plainly traced. At first the edges of the leaves were unbroken curves, but when the flowers expanded under a long-continued action, the edges became serrated. When the ice was held at a suitable angle to the solar beams, these liquid blossoms, with their central spots shining more intensely than burnished silver, presented an exhibition of beauty not easily described." We may

remark that an appearance, very nearly identical with these six-petal flowers, is sometimes to be traced on a frozen window-pane, as the light shines strongly through it; not on the parts which exhibit the beautiful frost figures of winter, but on those which are uniformly frozen over, and resemble, at a general view, a pane of ground glass.

On sending the sunbeam through the transparent ice, Professor Tyndall noticed that the appearance of the lustrous spots was accompanied by a faint clicking sound, as if the ice had been ruptured inwardly. Such, however, does not appear to be the case. Water holds a quantity of air in solution, the effect of which greatly weakens the cohesion of its particles. If this air be thoroughly got rid of its cohesion is so much increased, together with its adhesion to other surfaces, that, as M. Donny has shown, a long glass tube may be filled with such water, and inverted without the water falling out. So also water deprived of air will not boil at 212°, but may be heated to nearly 300° without boiling; but the experiment is a dangerous one, on account of the water suddenly exploding into steam. When water freezes, the whole of the air, together with any salts or other impurities, is expelled, as already shown in Faraday's experiments, so that the melting of such a piece of ice is a very good method of obtaining pure water. But in the very act of melting, air is absorbed, unless it be melted out of contact with the air. If a piece of ice be dropped into oil heated to 250° or 260°, it will liquefy without absorption of air, and remain without boiling, as in M. Donny's experiment; but Professor Faraday, to whom this observation is due, has shown that if the oil be heated to 300°, the water will not only liquefy, but explode.

Professor Tyndall applies these facts to the explanation of the clicking sound already referred

to. On sending a sunbeam through the ice, liquid cavities are suddenly formed within it, and are completely separated from contact with the air; but the water formed by the melting ice occupies less space than the ice which produces it, so that the water of a cavity not being able to fill it entirely, there must be a void space in the cell. Mr. Tyndall thinks that "the strong cohesion between the walls of the cell, and the drop within it, augments the volume of the latter a little, so as to compel it to fill the cell; but, as the quantity of liquid becomes greater, the shrinking force augments, until finally the particles snap asunder like a broken spring." We should doubt whether the water fills the cell at all, since the lustrous spot, which is indeed the vacuum, appears at the moment the click is heard: we should rather be disposed to refer this clicking sound to the fall of the particles of water against each other and against the walls of the cell, just as the water in a water-hammer produces a faint clicking sound when gently handled, in consequence of the particles of the nearly incompressible liquid striking against each other in the vacuum, and against the sides of the glass.

That these lustrous spots in the ice are not air bubbles, but void spaces, can be proved by plunging the ice containing them into hot water, when, as the walls of the cells break down, no trace of air can be seen rising to the surface of the water.

The crystalline ice flowers observed by Professor Tyndall were also noticed by Sir E. Belcher,* while wintering in the Arctic Regions in December 1852, in the course of some interesting experiments on the freezing of water at low temperatures. We may bring together these experiments in this place. The first point to be decided was the amount of vapour thrown off by water in the act of freezing, by exposure to a low degree of cold. It has already been noticed, at page 52, that water may be lowered many degrees below its freezing point without becoming solid, but if agitated it will suddenly freeze, and in doing so will rise to 32°. If the surrounding air be much below this point, the water thus warmed in freezing, will throw off vapour or frost smoke, as noticed at page 66. In order to determine the amount of vapour thus thrown off, Belcher constructed a delicate balance, fitted with gutta percha scale pans of 9 square inches surface, and inch in depth. Six drachms of distilled water at 56° were operated on, in a kind of screen cabin erected on deck. The water was exposed to the temperature of the air, viz. -17°, and in five minutes it was frozen, at the same time throwing off copious vapours. In ten minutes more the ice was hard, opaque, and raised. The loss of weight was 5.725 grains. The same experiment on salt water gave

"The Last of the Arctic Voyages," 8vo. London, 1855. It will be observed that the title of this book is now incorrect, since the celebrated voyage of the Fox, described in "Winter in the Arctic Regions."