fluid which evaporates readily is forced to do so under such conditions that the heat which it must have and render latent in its vapor will be extracted from a limited quantity of water, and this process being made continuous, so much heat will finally be abstracted from the water that its molecules can no longer stay mobile, but fly together into crystals-the water freezes.

The evaporating fluid used in practice may be sulphuric ether or sulphurous acid or ammonia. The last is perhaps nowadays most often used. These fluids are kept in strong close pipes and receivers, and can under no conditions come into contact with the water to be frozen.

Suppose we consider the ammonia freezing machines very briefly only, for into the details of the process it is not necessary for us here to

enter.

But in order to understand this operation one more physical principle must be stated, which is that the degree of pressure to which a substance is exposed has a great influence upon the temperature at which it passes from the fluid to the gaseous condition. Water at the ordinary altitudes boils-that is, is converted into vapor-at 100° centigrade (212° Fahrenheit). But on a high mountain it will boil at a lower temperature than this, because the atmospheric pressure is less there.

On the other hand, if you put a gas into a receiver, and expose it to a sufficiently heavy pressure by a powerful pump, or in some other way, it will in most cases become a fluid forth with, and the heat which had been latent in it will be given out. Now this in a general way is what is done to the ammonia in getting it ready to freeze water. Pure liquid ammonia boils

that is, passes from the liquid into the gaseous state-at a temperature about 240° Fahrenheit lower than water does. Hence ammonia is a substance which at ordinary temperatures is a gas. That which we buy at the drug stores as ammonia is simply a solution of the gas in water, and from this, as every one knows, it is readily given off. It is volatile, that is, it tends under ordinary conditions to get into the gaseous form. Now in ice factories powerful engines are used to force the ammonia gas by pressure into the liquid state, and the heat which is thus set free is carried off by cool water pouring over the coils of stout iron pipe in which it is confined. This fluid ammonia

kept fluid by the pressure to which it is subjected, a pressure varying from 125 to 175 pounds to the square inch-is carried in pipes to coils in the freezing-tanks. These tanks are simply great vats filled

with brine, and covered over with a floor. Into this brine, cans filled with the water to be frozen are placed and carefully covered. These cans are usually between three and four feet deep, about one foot thick, and nearly two feet wide. Coils of pipe communicating with the ammonia pipes are immersed in the brine, which is kept in motion by paddle-wheels moved by machinery.

The brine acts as a carrier of the heat from the water in its tight metallic cans to the ammonia pipes. Brine is used because it does not as readily freeze as water does, and acts as a good conductor or distributer of the heat.

Now, when all is ready, the fluid ammonia under its great pressure is allowed to escape into the coils of pipe which pass about in the brine, and in which the pressure is so much less that the ammonia as it rushes in becomes at once a gas. But to do this it must have heat. It must have it. From the walls of the pipes into which it rushes, it seizes it first. These take it from the brine which bathes them until its temperature goes down, down, and it begins to draw through their iron walls upon the heat stock of the water in the cans. And so the mimic but relentless warfare goes on. The ammonia vapor is constantly pumped away from the cold pipes in which it had expanded to be used over again, while fresh liquid ammonia is as constantly forced in from behind.

Some hours pass, and the heat stock in the water is growing scantier and scantier. It can stand it in this way but little longer; it is down wellnigh to zero centigrade, and the wild insatiable vapor raging for heat in the pipes not far off is still sucking it away. The only thing which can be done now to furnish more is for the water to give up its latent heat, and that is to sign its own death-warrant as water; for, if one may use such a turn of phrase, without its latent heat water is ice.

Well, at last there is nothing for it, and that happens which is happening this clear winter night on which I write at the edge of every lake and pool and pond out-of-doors hereabouts-little transparent spiculæ shoot out from the cooled surfaces, and the water slowly, as if unwilling, yields itself into its crystal bonds. The ice layers on all sides slowly thicken, and at last, in about sixty hours, all is

solid. The watchful attendant raises with a crane the great beautiful ice block, still in its galvanized iron can, out of its cold bath, and trundles it off to make way for another molecular battle and another victory.

Water turns into ice out-of-doors in winter because it must give up the heat which it had slowly gathered from the sunbeams, at the demands of gelid space, as if an ice age in little and in brief had come again. But it is the sun's heat itself, lain dormant in the coal for ages, which, under man's directing finger and for his weal, sets free the molecular furies raging to suck from the water its motion and its simulate life.

When water freezes out-of-doors on still pools or on streams, the ice-forming does not usually go on steadily and without interruption. Warm currents in the water, sweeping under the thickening films, now and again undo the work which has been accomplished. In clear winter weather, and especially on clear winter nights, the freezing goes on best, because then space claims more eagerly its dole of heat. On cloudy nights freezing is not so rapid, since even so light and airy a blanket as a cloud keeps in the earth's heat in large measure. For the same reason a newspaper spread over a plant will often protect it from the early frosts. Snow on the ice makes, too, a blanket which retards ice-formation underneath. Warm days come when the top melts a little-and so altogether the formation of ice out-of-doors in winter in these latitudes is an irregular one, and the crystalbuilding is subject to many and varied vicissitudes.

Not so the artificial winter which man calls into being in his little separate ironwalled pools. Here the cooling and the freezing go steadily and relentlessly on, as regularly as the stroke of the piston in the great engines, for every one of whose throbs a myriad aqueous molecules sink into rest.

The result of this uniformity in the freezing and the regular shapes of the cans is that the artificial ice presents some interesting features in its crystalline structure to which we shall now devote some little attention.

As clearness and transparency are desirable qualities in ice designed for household use, a good deal of care and expense is requisite to free the water which is to

be frozen as much as possible from the air which it holds in solution before the freezing begins. If this were not done, the ice crystals would force it out in bubbles, which, caught here and there between them, would impair the clearness of the product. Although boiling will in large measure free water of the dissolved air, this is not as efficient as distillation, which is now commonly practised, not only for the purpose of removing air, but for the destruction and removal of any bacteria or other impurities which might be in the water.

The cans being filled with distilled water and covered, and the freezing begun, it will be found, if one watches the process, that the ice crystals shoot out at right angles to the cooling surfaces, that is, to the sides and bottom of the can. Just as in natural freezing, they form, except at first, at right angles to the surface of the water. But now a very curious thing is to be noticed in the canfreezing, which is, that as the can has four sides and a bottom, the ice crystals cannot grow out into the water very far without running afoul of other crystals which have grown out from adjacent sides at right angles to those from which they sprang. How this is may be seen in Fig. 1. Crystals shooting into the water from the surfaces A and B will soon meet at right angles, and their formation be interfered with, disturbed, and stopped along a line which runs from the corner to the line of the centre. Crystals forming from the surfaces C and D will go on inwards until they meet at the central line of the ice block, E.

Now the fact is that even distilled water

does, in the manipulation to which it is subjected in getting distributed into the freezing-cans, absorb a certain amount of air and certain other gases due to the decomposition by heat of organic matter in the water, and this air and these gases are necessarily forced out again by the forming crystals. The points where this squeezed-out air shows itself in tiny bubbles is just where the ice crystals finally meet end to end, that is, most abundantly along the central meeting line, E, and along diagonals running from this line to the corners of the can, f, f, Fig. 1.

Great skill is required in freezing ice in the cans, and when the most perfect result is obtained, these lines of collision of the ice crystals are almost wholly invisible. But ordinarily one may see a whitish layer of small bubbles running lengthwise through the middle of the block. Frequently one sees, too, faint lines of bubbles just within each corner

The block is shown standing as it is frozen in the can, and in addition to the features which I have mentioned, one sees that at the bottom of the block a considerable number of squeezed-out bubbles have been caught on the points of the ice crystals, and form a wedgeshaped mass.

If one thinks for a moment of the condition of affairs as the ice is forming, the reason for this bottom bubble wedge will be plain. Here the ice crystals bearing the bubbles at their tips are coming not only inward from the four sides of the can, but also upward from the bottom. These five different sets of ice crystals meet in planes which correspond to the four-sided peaked roof of a square or oblong house, and here the bubbles are caught, making the wedge-shaped mass. It is said by expert manufacturers that this bottom bubbly mass may be avoided by distributing the cooled brine so that the freezing may not be too rapid at the bottom.

On the top of the block the conditions are different, the cooling brine does not cover the iron can, so that when the ice has formed at the sides and well up through the water, the very top layers are frozen by the cold ice below and in thin laminæ. We thus have the general structure of an artificial ice block outlined in bands and streaks of tiny air bubbles, which are caught in certain places as the crystals form and meet. How thin the central band of bubbles is may be seen in Fig. 3, where the block is looked at from the edge.

If you peep into an ice wagon which is distributing the artificial ice through our streets, you will find the ice blocks always standing top end up as they were formed, because if the top end, which bulges from the expanding as the water freezes, were down, the blocks would wobble, and perhaps fall over.

But the story of the crystals in an artificial ice cake cannot always be made out by simple inspection when the telltale air bubbles are not imprisoned in considerable numbers. We have, however, at command in the sunbeams a magician of such delicacy and power that at his lightest touch the primal forces which have held the molecules in leash yield their sceptre, and along cleavage lines and planes of the most exquisite delicacy and beauty the ice melts, and a whole block

of ice may be seen riven into prisms as sharp and distinct and delicate as any rock-crystal.

More than this, along every cleavage plane one may see with a lens the footprints of the sunbeams in bands of frondlike structures changing every instant, and at last flowing together as each crystal prism becomes separated from its neighbor by a sheet of water almost as delicate as the film of a bubble.

It is a very interesting and beautiful experiment to put a piece of artificial ice in a large pan, and setting it in the sun, watch its silent disintegration step by step. Much of interest and beauty can be seen with the unaided eye, but a hand magnifying glass will reveal new and unexpected pictures.

One does well, in watching this experiment, to remember that under his very eyes Nature is working one of her most fascinating miracles. Here are the molecules of the ice fast locked in that rest which is crystallization, and clinging together with all the tenacity of their primal attractions. But at your will this placid state is invaded by the dancing sunbeams. By that vibratile witchery akin to music, but so hard to understand, they enter and permeate every recess of this crystal stillness. But the dance of the sunbeams in this austere domain is not for long a solitary sport. The dormant instinct of motion in the ice molecules soon awakes at the touch of this new Circe, heat, all-powerful dancing daughter of the sun, and one by one they spring into motion, and join the silent music which underlies all movements of the sea, all flow of streams, all pictures in the clouds. Of all this elemental music, the ear hears no strain. But the eye soon catches its record in faint and exquisite shining lines which flash out here and there, weaving in and out through the crystal mass, fine as a spider's thread, and stretching in long, graceful, and often interlacing sweeps, every where through the ice from its surfaces to its utmost depths.

As these lines grow slowly larger and more abundant, one can see that they are not scattered at random in the ice, but range inward from the surface of the block, sloping towards one another as they go, until they are lost in a mazy network at the centre of the block or along certain curious lines of junction which

They are

run inward from its corners. first formed along the lines and planes where the crystal masses join.

It is not easy to picture these early lines of sun-disintegration of artificial ice, owing to their extreme delicacy and fineness. But Dr. Leaming has photographed a corner of a little slab shaved out from near the bottom of an artificial ice block which had been exposed to the sunbeams for about an hour on a winter day. See Fig. 4. The direction of freezing is well shown here by the cleavage lines which the sunbeams have called forth. One sees also how along the line from the corners to the wedge of bubbles the crystals have met, and so squeezed one another in their struggle for room that they have been a little bent away from their line of contact and run in