Part 29 (1/2)

REGELATION.

(23.)

[Sidenote: FARADAY'S FIRST EXPERIMENT.]

I was led to the foregoing results by reflecting on an experiment performed by Mr. Faraday, at a Friday evening meeting of the Royal Inst.i.tution, on the 7th of June, 1850, and described in the 'Athenaeum'

and 'Literary Gazette' for the same month. Mr. Faraday then showed that when two pieces of ice, with moistened surfaces, were placed in contact, they became cemented together by the freezing of the film of water between them, while, when the ice was below 32 Fahr., and therefore _dry_, no effect of the kind could be produced. The freezing was also found to take place under water; and indeed it occurs even when the water in which the ice is plunged is as hot as the hand can bear.

A generalisation from this interesting fact led me to conclude that a bruised ma.s.s of ice, if closely confined, must re-cement itself when its particles are brought into contact by pressure; in fact, the whole of the experiments above recorded immediately suggested themselves to my mind as natural deductions from the principle established by Faraday. A rough preliminary experiment a.s.sured me that the deductions would stand testing; and the construction of the box-wood moulds was the consequence. We could doubtless mould many solid substances to any extent by suitable pressure, breaking the attachment of their particles, and re-establis.h.i.+ng a certain continuity by the mere force of cohesion.

With such substances, to which we should never think of applying the term viscous, we might also imitate the changes of form to which glaciers are subject: but, superadded to the mere cohesion which here comes into play, we have, in the case of ice, the actual regelation of the severed surfaces, and consequently a more perfect solid. In the Introduction to this book I have referred to the production of slaty cleavage by pressure; and at a future page I hope to show that the lamination of the ice of glaciers is due to the same cause; but, as justly observed by Mr. John Ball, there is no tendency to cleave in the _sound_ ice of glaciers; in fact, this tendency is obliterated by the perfect regelation of the severed surfaces.

[Sidenote: RECENT EXPERIMENTS OF FARADAY.]

Mr. Faraday has recently placed pieces of ice, in water, under the strain of forces tending to pull them apart. When two such pieces touch at a single point they adhere and move together as a rigid piece; but a little lateral force carefully applied breaks up this union with a crackling noise, and a new adhesion occurs which holds the pieces together in opposition to the force which tends to divide them. Mr.

James Thomson had referred regelation to the cold produced by the liquefaction of the pressed ice; but in the above experiment all pressure is not only taken away, but is replaced by tension. Mr. Thomson also conceives that, when pieces of ice are simply placed together without intentional pressure, the capillary attraction brings the pressure of the atmosphere into play; but Mr. Faraday finds that regelation takes place _in vacuo_. A true viscidity on the part of ice Mr. Faraday never has observed, and he considers that his recent experiments support the view originally propounded by himself, namely, that a particle of water on a surface of ice becomes solid when placed between two surfaces, because of the increased influence due to their joint action.

CRYSTALLIZATION AND INTERNAL LIQUEFACTION.

(24.)

[Sidenote: HOW CRYSTALS ARE ”NURSED.”]

In the Introduction to this book I have briefly referred to the force of crystallization. To permit this force to exercise its full influence, it must have free and unimpeded action; a crystal, for instance, to be properly built, ought to be suspended in the middle of the crystallizing solution, so that the little architects can work all round it; or if placed upon the bottom of a vessel, it ought to be frequently turned, so that all its facets may be successively subjected to the building process. In this way crystals can be _nursed_ to an enormous size. But where other forces mingle with that of crystallization, this harmony of action is destroyed; the figures, for example, that we see upon a gla.s.s window, on a frosty morning, are due to an action compounded of the pure crystalline force and the cohesion of the liquid to the window-pane. A more regular effect is obtained when the freezing particles are suspended in still air, and here they build themselves into those wonderful figures which Dr. Scoresby has observed in the Polar Regions, Mr. Glaisher at Greenwich, and I myself on the summit of Monte Rosa and elsewhere.

Not only however in air, but in water also, figures of great beauty are sometimes formed. Harrison's excellent machine for the production of artificial ice is, I suppose, now well known; the freezing being effected by carrying brine, which had been cooled by the evaporation of ether, round a series of flat tin vessels containing water. The latter gradually freezes, and, on watching those vessels while the action was proceeding very slowly, I have seen little six-rayed stars of thin ice forming, and rising to the surface of the liquid. I believe the fact was never before observed, but it would be interesting to follow it up, and to develop experimentally this most interesting case of crystallization.

[Sidenote: DISSECTION OF ICE BY SUNBEAM.]

The surface of a freezing lake presents to the eye of the observer nothing which could lead him to suppose that a similar molecular architecture is going on there. Still the particles are undoubtedly related to each other in this way; they are arranged together on this starry type. And not only is this the case at the surface, but the largest blocks of ice which reach us from Norway and the Wenham Lake are wholly built up in this way. We can reveal the internal const.i.tution of these ma.s.ses by a reverse process to that which formed them; we can send an agent into the interior of a ma.s.s of ice which shall take down the atoms which the crystallizing forces had set up. This agent is a solar beam; with which it first occurred to me to make this simple experiment in the autumn of 1857. I placed a large converging lens in the sunbeams pa.s.sing through a room, and observed the place where the rays were brought to a focus behind the lens; then shading the lens, I placed a clear cube of ice so that the point of convergence of the rays might fall within it. On removing the screen from the lens, a cone of sunlight went through the cube, and along the course of the cone the ice became studded with l.u.s.trous spots, evidently formed by the beam, as if minute reflectors had been suddenly established within the ma.s.s, 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 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 s.h.i.+ning more intensely than burnished silver, presented an exhibition of beauty not easily described. I have given a sketch of their appearance in Fig. 34.

[Sidenote: LIQUID FLOWERS IN ICE.]

[Ill.u.s.tration: Fig. 34. Liquid Flowers in lake ice.]

I have here to direct attention to an extremely curious fact. On sending the sunbeam through the transparent ice, I often noticed that the appearance of the l.u.s.trous spots was accompanied by an audible clink, as if the ice were ruptured inwardly. But there is no ground for a.s.suming such rupture, and on the closest examination no flaw is exhibited by the ice. What then can be the cause of the noise? I believe the following considerations will answer the question:--

Water always holds a quant.i.ty of air in solution, the diffusion of which through the liquid, as proved by M. Donny, has an immense effect in weakening the cohesion of its particles; recent experiments of my own show that this is also the case in an eminent degree with many volatile liquids. M. Donny has proved that, if water be thoroughly purged of its air, a long gla.s.s tube filled with this liquid may be inverted, while the tenacity with which the water clings to the tube, and with which its particles cling to each other, is so great that it will remain securely suspended, though no external hindrance be offered to its descent. Owing to the same cause, water deprived of its air will not boil at 212 Fahr., and may be raised to a temperature of nearly 300 without boiling; but when this occurs the particles break their cohesion suddenly, and ebullition is converted into explosion.

Now, when ice is formed, every trace of the air which the water contained is squeezed out of it; the particles in crystallizing reject all extraneous matter, so that in ice we have a substance quite free from the air, which is never absent in the case of water; it therefore follows that if we could preserve the water derived from the melting of ice from contact with the atmosphere, we should have a liquid eminently calculated to show the effects described by M. Donny. Mr. Faraday has proved by actual experiment that this is the case.

[Sidenote: WATER DEPRIVED OF AIR SNAPS ASUNDER.]

Let us apply these facts to the explanation of the clink heard in my experiments. On sending a sunbeam through ice, liquid cavities are suddenly formed at various points within the ma.s.s, and these cavities are completely cut off from atmospheric contact. But the water formed by the melting ice is less in volume than the ice which produces it; the water of a cavity is not able to fill it, hence a vacuous s.p.a.ce must be formed in the cell. I have no doubt that, for a time, 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 quant.i.ty of liquid becomes greater the shrinking force augments, until finally the particles snap asunder like a broken spring.