Part 35 (2/2)
11. The nearer the ice is to its melting point in temperature, the more easily are the above results obtained; when ice is many degrees below its freezing point it is crushed by pressure to a white powder, and is not capable of being moulded as above.
12. Two pieces of ice at 32 Fahr., with moist surfaces, when placed in contact freeze together to a rigid ma.s.s; this is called Regelation.
13. When the attachments of pressed ice are broken, the continuity of the ma.s.s is restored by the regelation of the new contiguous surfaces.
Regelation also enables two tributary glaciers to weld themselves to form a continuous trunk; thus also the creva.s.ses are mended, and the dislocations of the glacier consequent on descending cascades are repaired. This healing of ruptures extends to the smallest particles of the ma.s.s, and it enables us to account for the continued compactness of the ice during the descent of the glacier.
14. The quality of viscosity is practically absent in glacier-ice. Where pressure comes into play the phenomena are suggestive of viscosity, but where tension comes into play the a.n.a.logy with a viscous body breaks down. When subjected to strain the glacier does not yield by stretching, but by breaking; this is the origin of the creva.s.ses.
15. The creva.s.ses are produced by the mechanical strains to which the glacier is subjected. They are divided into marginal, transverse, and longitudinal creva.s.ses; the first produced by the oblique strain consequent on the quicker motion of the centre; the second by the pa.s.sage of the glacier over the summit of an incline; the third by pressure from behind and resistance in front, which causes the ma.s.s to split at right angles to the pressure [strain?].
16. The moulins are formed by deep cracks intersecting glacier rivulets.
The water in descending such cracks scoops out for itself a shaft, sometimes many feet wide, and some hundreds of feet deep, into which the cataract plunges with a sound like thunder. The supply of water is periodically cut off from the moulins by fresh cracks, in which new moulins are formed.
17. The lateral moraines are formed from the debris which loads the glacier along its edges; the medial moraines are formed on a trunk-glacier by the union of the lateral moraines of its tributaries; the terminal moraines are formed from the debris carried by the glacier to its terminus, and there deposited. The number of medial moraines on a trunk glacier is always one less than the number of tributaries.
18. When ordinary lake-ice is intersected by a strong sunbeam it liquefies so as to form flower-shaped figures within the ma.s.s; each flower consists of six petals with a vacuous s.p.a.ce at the centre; the flowers are always formed parallel to the planes of freezing, and depend on the crystallization of the substance.
19. Innumerable liquid disks, with vacuous spots, are also formed by the solar beams in glacier-ice. These empty s.p.a.ces have been hitherto mistaken for air-bubbles, the flat form of the disks being erroneously regarded as the result of pressure.
20. These disks are indicators of the intimate const.i.tution of glacier-ice, and they teach us that it is composed of an aggregate of parts with surfaces of crystallization in all possible planes.
21. There are also innumerable small cells in glacier-ice holding air and water; such cells also occur in lake-ice; and here they are due to the melting of the ice in contact with the bubble of air. Experiments are needed on glacier-ice in reference to this point.
22. At a free surface within or without, ice melts with more ease than in the centre of a compact ma.s.s. The motion which we call heat is less controlled at a free surface, and it liberates the molecules from the solid condition sooner than when the atoms are surrounded on all sides by other atoms which impede the molecular motion. Regelation is the complementary effect to the above; for here the superficial portions of a ma.s.s of ice are made virtually central by the contact of a second ma.s.s.
23. The dirt-bands have their origin in the ice-cascades. The glacier, in pa.s.sing the brow, is transversely fractured; ridges are formed with hollows between them; these transverse hollows are the princ.i.p.al receptacles of the fine debris scattered over the glacier; and after the ridges have been melted away, the dirt remains in successive stripes upon the glacier.
24. The ice of many glaciers is laminated, and when weathered may be cloven into thin plates. In the sound ice the lamination manifests itself in blue stripes drawn through the general whitish ma.s.s of the glacier; these blue veins representing portions of ice from which the air-bubbles have been more completely expelled. This is the veined structure of the ice. It is divided into marginal, transverse, and longitudinal structure; which may be regarded as complementary to marginal, longitudinal, and transverse creva.s.ses. The latter are produced by tension, the former by pressure, which acts in two different ways: firstly, the pressure acts upon the ice as it has acted upon rocks which exhibit the lamination technically called cleavage; secondly, it produces partial liquefaction of the ice. The liquid s.p.a.ces thus formed help the escape of the air from the glacier; and the water produced, being refrozen when the pressure is relieved, helps to form the blue veins.
APPENDIX.
COMPARATIVE VIEW OF THE CLEAVAGE OF CRYSTALS AND SLATE-ROCKS.
A LECTURE DELIVERED AT THE ROYAL INSt.i.tUTION, ON FRIDAY EVENING THE 6TH OF JUNE, 1856.[A]
When the student of physical science has to investigate the character of any natural force, his first care must be to purify it from the mixture of other forces, and thus study its simple action. If, for example, he wishes to know how a ma.s.s of water would shape itself, supposing it to be at liberty to follow the bent of its own molecular forces, he must see that these forces have free and undisturbed exercise. We might perhaps refer him to the dew-drop for a solution of the question; but here we have to do, not only with the action of the molecules of the liquid upon each other, but also with the action of gravity upon the ma.s.s, which pulls the drop downwards and elongates it. If he would examine the problem in its purity, he must do as Plateau has done, withdraw the liquid ma.s.s from the action of gravity, and he would then find the shape of the ma.s.s to be perfectly spherical. Natural processes come to us in a mixed manner, and to the uninstructed mind are a ma.s.s of unintelligible confusion. Suppose half-a-dozen of the best musical performers to be placed in the same room, each playing his own instrument to perfection: though each individual instrument might be a well-spring of melody, still the mixture of all would produce mere noise. Thus it is with the processes of nature. In nature, mechanical and molecular laws mingle, and create apparent confusion. Their mixture const.i.tutes what may be called the _noise_ of natural laws, and it is the vocation of the man of science to resolve this noise into its components, and thus to detect the ”music” in which the foundations of nature are laid.
The necessity of this detachment of one force from all other forces is nowhere more strikingly exhibited than in the phenomena of crystallization. I have here a solution of sulphate of soda. Prolonging the mental vision beyond the boundaries of sense, we see the atoms of that liquid, like squadrons under the eye of an experienced general, arranging themselves into battalions, gathering round a central standard, and forming themselves into solid ma.s.ses, which after a time a.s.sume the visible shape of the crystal which I here hold in my hand. I may, like an ignorant meddler wis.h.i.+ng to hasten matters, introduce confusion into this order. I do so by plunging this gla.s.s rod into the vessel. The consequent action is not the pure expression of the crystalline forces; the atoms rush together with the confusion of an unorganized mob, and not with the steady accuracy of a disciplined host.
Here, also, in this ma.s.s of bis.m.u.th we have an example of this confused crystallization; but in the crucible behind me a slower process is going on: here there is an architect at work ”who makes no chips, no din,” and who is now building the particles into crystals, similar in shape and structure to those beautiful ma.s.ses which we see upon the table. By permitting alum to crystallize in this slow way, we obtain these perfect octahedrons; by allowing carbonate of lime to crystallize, nature produces these beautiful rhomboids; when silica crystallizes, we have formed these hexagonal prisms capped at the ends by pyramids; by allowing saltpetre to crystallize, we have these prismatic ma.s.ses; and when carbon crystallizes, we have the diamond. If we wish to obtain a perfect crystal, we must allow the molecular forces free play: if the crystallizing ma.s.s be permitted to rest upon a surface it will be flattened, and to prevent this a small crystal must be so suspended as to be surrounded on all sides by the liquid, or, if it rest upon the surface, it must be turned daily so as to present all its faces in succession to the working builder. In this way the scientific man nurses these children of his intellect, watches over them with a care worthy of imitation, keeps all influences away which might possibly invade the strict morality of crystalline laws, and finally sees them developed into forms of symmetry and beauty which richly reward the care bestowed upon them.
In building up crystals, these little atomic bricks often arrange themselves into layers which are perfectly parallel to each other, and which can be separated by mechanical means; this is called the cleavage of the crystal. I have here a crystallized ma.s.s which has thus far escaped the abrading and disintegrating forces which, sooner or later, determine the fate of sugar-candy. If I am skilful enough, I shall discover that this crystal of sugar cleaves with peculiar facility in one direction. Here, again, I have a ma.s.s of rock-salt: I lay my knife upon it, and with a blow cleave it in this direction; but I find on further examining this substance that it cleaves in more directions than one. Laying my knife at right angles to its former position, the crystal cleaves again; and, finally placing the knife at right angles to the two former positions, the ma.s.s cleaves again. Thus rock-salt cleaves in three directions, and the resulting solid is this perfect cube, which may be broken up into any number of smaller cubes. Here is a ma.s.s of Iceland spar, which also cleaves in three directions, not at right angles, but obliquely to each other, the resulting solid being a rhomboid. In each of these cases the ma.s.s cleaves with equal facility in all three directions. For the sake of completeness, I may say that many substances cleave with unequal facility in different directions, and the heavy spar I hold in my hand presents an example of this kind of cleavage.
Turn we now to the consideration of some other phenomena to which the term cleavage may be applied. This piece of beech-wood cleaves with facility parallel to the fibre, and if our experiments were fine enough we should discover that the cleavage is most perfect when the edge of the axe is laid across the rings which mark the growth of the tree. The fibres of the wood lie side by side, and a comparatively small force is sufficient to separate them. If you look at this ma.s.s of hay severed from a rick, you will see a sort of cleavage developed in it also; the stalks lie in parallel planes, and only a small force is required to separate them laterally. But we cannot regard the cleavage of the tree as the same in character as the cleavage of the hayrick. In the one case it is the atoms arranging themselves according to organic laws which produce a cleavable structure; in the other case the easy separation in a certain direction is due to the mechanical arrangement of the coa.r.s.e sensible ma.s.ses of stalks of hay.
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