Part 8

287. If you understand this, you have already mastered the cause of the moraine-ridges. They are not produced by any swelling of the ice upwards. But the ice underneath the rocks and rubbish being protected from the sun, the glacier right and left melts away and leaves a ridge behind.

288. Various other appearances upon the glacier are accounted for in the same way. Here upon the Mer de Glace we have flat slabs of rock sometimes lifted up on pillars of ice. These are the so-called _Glacier Tables_. They are produced, not by the growth of a stalk of ice out of the glacier, but by the melting of the glacier all round the ice protected by the stone. Here is a sketch of one of the Tables of the Mer de Glace.

289. Notice moreover that a glacier table is hardly ever set square upon its pillar. It generally leans to one side, and repeated observation teaches you that it so leans as to enable you always to draw the north and south line upon the glacier. For the sun being south of the zenith at noon pours its rays against the southern end of the table, while the northern end remains in shadow. The southern end, therefore, being most warmed does not protect the ice underneath it so effectually as the northern end. The table becomes inclined, and ends by sliding bodily off its pedestal.

290. In the figure opposite we have what maybe called an ideal Table. The oblique lines represent the direction of the sunbeams, and the consequent tilting of the table here shown resembles that observed upon the glaciers.

291. A pebble will not rise thus: like Franklin's single bit of cloth, a dark-coloured pebble sinks in the ice. A spot of black mould will not rest upon the surface, but will sink; and various parts of the Glacier du Géant are honeycombed by the sinking of such spots of dirt into the ice.

292. But when the dirt is of a thickness sufficient to protect the ice the case is different. Sand is often washed away by a stream from the mountains, or from the moraines, and strewn over certain spaces of the glacier. A most curious action follows: the sanded surface rises, the part on which the sand lies thickest rising highest. Little peaks and eminences jut forth, and when the distribution of the sand is favourable, and the action sufficiently prolonged, you have little mountains formed, sometimes singly, and sometimes grouped so as to mimic the Alps themselves. The _Sand-Cones_ of the Mer de Glace are not striking; but on the Görner, the Aletsch, the Morteratsch, and other glaciers, they form singly and in groups, reaching sometimes a height of ten or twenty feet.

§ 44. _The Glacier Mills or Moulins._

293. You and I have learned by long experience the character of the Mer de Glace. We have marched over it daily, with a definite object in view, but we have not closed our eyes to other objects. It is from side glimpses of things which are not at the moment occupying our attention that fresh subjects of enquiry arise in scientific investigation.

294. Thus in marching over the ice near Trélaporte we were often struck by a sound resembling low rumbling thunder. We subsequently sought out the origin of this sound, and found it.

295. A large area of this portion of the glacier is unbroken. Driblets of water have room to form rills; rills to unite and form streams; streams to combine to form rushing brooks, which sometimes cut deep channels in the ice. Sooner or later these streams reach a strained portion of the glacier, where a crack is formed across the stream. A way is thus opened for the water to the bottom of the glacier. By long action the stream hollows out a shaft, the crack thus becoming the starting-point of a funnel of unseen depth, into which the water leaps with the sound of thunder.

296. This funnel and its cataract form a glacier Mill or _Moulin_.

297. Let me grasp your hand firmly while you stand upon the edge of this shaft and look into it. The hole, with its pure blue shimmer, is beautiful, but it is terrible. Incautious persons have fallen into these shafts, a second or two of bewilderment being followed by sudden death. But caution upon the glaciers and mountains ought, by habit, to be made a second nature to explorers like you and me.

298. The crack into which the stream first descended to form the moulin, moves down with the glacier. A succeeding portion of the ice reaches the place where the breaking strain is exerted. A new crack is then formed above the moulin, which is thenceforth forsaken by the stream, and moves downward as an empty shaft. Here upon the Mer de Glace, in advance of the _Grand Moulin_, we see no less than six of these forsaken holes. Some of them we sound to a depth of 90 feet.

299. But you and I both wish to determine, if possible, the entire depth of the Mer de Glace. The Grand Moulin offers a chance of doing this which we must not neglect. Our first effort to sound the moulin fails through the breaking of our cord by the impetuous plunge of the water. A lump of grease in the hollow of a weight enables a mariner to judge of a sea bottom. We employ such a weight, but cannot reach the bed of the glacier. A depth of 163 feet is the utmost reached by our plummet.

300. From July 28 to August 8 we have watched the progress of the Grand Moulin. On the former date the position of the Moulin was fixed. On the 31st it had moved down 50 inches; a little more than a day afterwards it had moved 74 inches. On August 8 it had moved 198 inches, which gives an average of about 18 inches in twenty-four hours. No doubt next summer upon the Mer de Glace a Grand Moulin will be found thundering near Trélaporte; but like the crevasse of the Grand Plateau, already referred to (§ 16), it will not be our Moulin. This, or rather the ice which it penetrated, is now probably more than a mile lower down than it was in 1857.

§ 45. _The Changes of Volume of Water by Heat and Cold._

301. We have noticed upon the glacier shafts and pits filled with water of the most delicate blue. In some cases these have been the shafts of extinct moulins closed at the bottom. A theory has been advanced to account for them, which, though it may be untenable, opens out considerations regarding the properties of water that ought to be familiar to enquirers like you and me.

302. In our dissection of lake ice by a beam of heat (§ 11) we noticed little vacuous spots at the centres of the liquid flowers formed by the beam. These spots we referred to the fact that when ice is melted the water produced is less in volume than the ice, and that hence the water of the flower was not able to occupy the whole space covered by the flower.

303. Let us more fully illustrate this subject. Stop a small flask water-tight with a cork, and through the cork introduce a narrow glass tube also water-tight. It is easy to fill the flask with water so that the liquid shall stand at a certain height in the glass tube.

304. Let us now warm the flask with the flame of a spirit-lamp. On first applying the flame you notice a momentary sinking of the liquid in the glass tube. This is due to the momentary expansion of the flask by heat; it becomes suddenly larger when the flame is first applied.

305. But the expansion of the water soon overtakes that of the flask and surpasses it. We immediately see the rise of the liquid column in the glass tube, exactly as mercury rises in the tube of a warmed thermometer.

306. Our glass tube is ten inches long, and at starting the water stood in it at a height of five inches. We will apply the spirit-lamp flame until the water rises quite to the top of the tube and trickles over. This experiment suffices to show the expansion of the water by heat.

307. We now take a common finger-glass and put into it a little pounded ice and salt. On this we place the flask, and then build round it the freezing mixture. The liquid column retreats down the tube, proving the contraction of the liquid by cold. We allow the shrinking to continue for some minutes, noticing that the downward retreat of the liquid becomes gradually slower, and that it finally ceases altogether.

308. Keep your eye upon the liquid column; it remains quiescent for a fraction of a minute, and then moves once more. But its motion is now _upwards_ instead of downwards. _The freezing mixture now acts exactly like the flame._

309. It would not be difficult to pass a thermometer through the cork into the flask, and it would tell us the exact temperature at which the liquid ceased to contract and began to expand. At that moment we should find the temperature of the liquid a shade over 39° Fahr.

310. At this temperature, then, water attains _its maximum density_.

311. Seven degrees below this temperature, or at 32° Fahr., the liquid begins to turn into solid crystals of ice, which you know swims upon water because it is bulkier for a given weight. In fact, this halt of the approaching molecules at the temperature of 39°, is but the preparation for the subsequent act of crystallisation, in which the expansion by cold culminates. Up to the point of solidification the increase of volume is slow and gradual; while in the act of solidification it is sudden, and of overwhelming strength.

312. By this force of expansion the Florentine Academicians long ago burst a sphere of copper nearly three quarters of an inch in thickness. By the same force the celebrated astronomer Huyghens burst in 1667 iron cannons a finger breadth thick. Such experiments have been frequently made since. Major Williams during a severe Quebec winter filled a mortar with water, and closed it by driving into its muzzle a plug of wood. Exposed to a temperature 50° Fahr. below the freezing point of water, the metal resisted the strain, but the plug gave way, being projected to a distance of 400 feet. At Warsaw howitzer shells have been thus exploded; and you and I have shivered thick bombshells to fragments, by placing them for half an hour in a freezing mixture.

313. The theory of the shafts and pits referred to at the beginning of this section is this: The water at the surface of the shaft is warmed by the sun, say to a temperature of 39° Fahr. The water at the bottom, in contact with the ice, must be at 32° or near it. The heavier water is therefore at the top; it will descend to the bottom, melt the ice there, and thus deepen the shaft.

314. The circulation here referred to undoubtedly goes on, and some curious effects are due to it; but not, I think, the one here ascribed to it. The _deepening_ of a shaft implies a quicker melting of its bottom than of the surface of the glacier. It is not easy to see how the fact of the solar heat being first absorbed by water, and then conveyed by it to the bottom of the shaft, should make the melting of the bottom more rapid than that of the ice which receives the direct impact of the solar rays. The surface of the glacier must sink _at least_ as rapidly as the bottom of the pit, so that the circulation, though actually existing, cannot produce the effect ascribed to it.

§ 46. _Consequences flowing from the foregoing Properties of Water. Correction of Errors._

315. I was not much above your age when the property of water ceasing to contract by cold at a temperature of 39° Fahr. was made known to me, and I still remember the impression it made upon me. For I was asked to consider what would occur in case this solitary exception to an otherwise universal law ceased to exist.

316. I was asked to reflect upon the condition of a lake stored with fish and offering its surface to very cold air. It was made clear to me that the water on being first chilled would shrink in volume and become heavier, that it would therefore sink and have its place supplied by the warmer and lighter water from the deeper portions of the lake.

317. It was pointed out to me that without the law referred to this process of circulation would go on until the whole water of the lake had been lowered to the freezing temperature. Congelation would then begin, and would continue as long as any water remained to be solidified. One consequence of this would be to destroy every living thing contained in the lake. Other calamities were added, all of which were said to be prevented by the perfectly exceptional arrangement, that after a certain time the _colder_ water becomes the _lighter_, floats on the surface of the lake, is there congealed, thus throwing a protecting roof over the life below.

318. Count Rumford, one of the most solid of scientific men, writes in the following strain about this question:--"It does not appear to me that there is anything which human sagacity can fathom, within the wide-extended bounds of the visible creation, which affords a more striking or more palpable proof of the wisdom of the Creator, and of the special care He has taken in the general arrangement of the universe, to preserve animal life, than this wonderful contrivance.

319. "Let me beg the attention of my readers while I endeavour to investigate this most interesting subject; and let me at the same time bespeak his candour and indulgence. I feel the danger to which a mortal exposes himself who has the temerity to explain the designs of Infinite Wisdom. The enterprise is adventurous, but it surely cannot be improper.

320. "Had not Providence interfered on this occasion in a manner which may well be considered as _miraculous_, all the fresh water within the polar circle must inevitably have been frozen to a very great depth in winter, and every plant and tree destroyed."

321. Through many pages of his book Count Rumford continues in this strain to expound the ways and intentions of the Almighty, and he does not hesitate to apply very harsh words to those who cannot share his notions. He calls them hardened and degraded. We are here warned of the fact, which is too often forgotten, that the pleasure or comfort of a belief, or the warmth or exaltation of feeling which it produces, is no guarantee of its truth. For the whole of Count Rumford's delight and enthusiasm in connexion with this subject, and the whole of his ire against those who did not share his opinions, were founded upon an erroneous notion.

322. Water is _not_ a solitary exception to an otherwise general law. There are other molecules than those of this liquid which require more room in the solid crystalline condition than in the adjacent molten condition. Iron is a case in point. Solid iron floats upon molten iron exactly as ice floats upon water. Bismuth is a still more impressive case, and we could shiver a bomb as certainly by the solidification of bismuth as by that of water. There is no fish, to be taken care of here, still the "contrivance" is the same.

323. I am reluctant to mention them in the same breath with Count Rumford, but I am told that in our own day there are people who profess to find the comforts of a religion in a superstition lower than any that has hitherto degraded the civilized human mind. So that the _happiness_ of a faith and the _truth_ of a faith are two totally different things.

324. Life and the conditions of life are in necessary harmony. This is a truism, for without the suitable conditions life could not exist. But both life and its conditions set forth the operations of inscrutable Power. We know not its origin; we know not its end. And the presumption, if not the degradation, rests with those who place upon the throne of the universe a magnified image of themselves, and make its doings a mere colossal imitation of their own.

§ 47. _The Molecular Mechanism of Water-Congelation._

325. But let us return to our science. How are we to picture this act of expansion on the part of freezing water? By what operation do the molecules demand with such irresistible emphasis more room in the solid than in the adjacent liquid condition? In all cases of this kind we must derive our conceptions from the world of the senses, and transfer them afterwards to a world transcending the range of the senses.

326. You have not forgotten our conversation regarding "atomic poles" (§ 10), and how the notion of polar force came to be applied to crystals. With this fresh in your memory, you will have no great difficulty in understanding how expansion of volume may accompany the act of crystallisation.

327. I place a number of magnets before you. They, as matter, are affected by gravity, and, if perfectly free, they would move towards each other in obedience to the attraction of gravity.

328. But they are not only matter, but _magnetic_ matter. They not only act upon each other by the simple force of gravity, but by the polar force of magnetism. Imagine them placed at a distance from each other, and perfectly free to move. Gravity first makes itself felt and draws them together. For a time the magnetic force issuing from the poles is insensible; but when a certain nearness is attained, the polar force comes into play. The mutually attracting points close up, the mutually repellent points retreat, and it is easy to see that this action may produce an arrangement of the magnets which requires more room. Suppose them surrounded by a box which exactly encloses them at the moment the polar force first comes into play. It is easy to see that in arranging themselves subsequently the repelled corners and ends of the magnets may be caused to press against the sides of the box, and even to burst it, if the forces be sufficiently strong.

329. Here then we have a conception which may be applied to the molecules of water. They, like the magnets, are acted upon by two distinct forces. For a time while the liquid is being cooled they approach each other, in obedience to their general attraction for each other. But at a certain point new forces, some attractive, some repulsive, _emanating from special points_ of the molecules, come into play. The attracted points close up, the repelled points retreat. Thus the molecules turn and rearrange themselves, demanding, as they do so, more space, and overcoming all ordinary resistance by the energy of their demand. This, in general terms, is an explanation of the expansion of water in solidifying: it would be easy to construct an apparatus for its illustration.

§ 48. _The Dirt Bands of the Mer de Glace._

330. Pass from bright sunshine into a moderately lighted room; for a time all appears so dark that the objects in the room are not to be clearly distinguished. Hit violently by the waves of light (§ 3) the optic nerve is numbed, and requires time to recover its sensitiveness.

331. It is for this reason that I choose the present hour for a special observation on the Mer de Glace. The sun has sunk behind the ridge of Charmoz, and the surface of the glacier is in sober shade. The main portion of our day's work is finished, but we have still sufficient energy to climb the slopes adjacent to the Montanvert to a height of a thousand feet or thereabouts above the ice.

332. We now look fairly down upon the glacier, and see it less foreshortened than from the Montanvert. We notice the diet overspreading its eastern side, due to the crowding together of its medial moraines. We see the comparatively clean surface of the Glacier du Géant; but we notice upon this surface an appearance which we have not hitherto seen. It is crossed by a series of grey bent bands, which follow each other in succession, from Trélaporte downwards. We count eighteen of these from our present position. (See sketch, page 128.)

333. These are the _Dirt Bands_ of the Mer de Glace; they were first observed by Professor Forbes in 1842.

334. They extend down the glacier further than we can see; and if we cross the valley of Chamouni, and climb the mountains at the opposite side, to a point near the little auberge, called La Flégère, we shall command a view of the end of the glacier and observe the completion of the series of bands. We notice that they are confined throughout to the portion of the glacier derived from the Col du Géant. (See sketch, page 129.)

335. We must trace them to their source. You know how noble and complete a view is obtained of the glacier and Col du Géant from the Cleft Station above Trélaporte. Thither we must once more climb; and thence we can see the succession of bands stretching downwards to the Montanvert, and upwards to the base of the ice-cascade upon the Glacier du Géant. The cascade is evidently concerned in their formation. (See sketch opposite.)

336. And how? Simply enough. The glacier, as we know, is broken transversely at the summit of the ice-fall, and descends the declivity in a series of great transverse ridges. At the base of the fall, the chasms are closed, but the ridges in part remain forming protuberances, which run like vast wrinkles across the glacier. These protuberances are more and more bent because of the quicker motion of the centre, and the depressions between them form receptacles for the fine mud and débris washed by the little rills from the adjacent slopes.

337. The protuberances sink gradually through the wasting action of the sun, so that long before Trélaporte is reached they have wholly disappeared. Not so the dirt of which they were the collectors: it continues to occupy, in transverse bands, the flat surface of the glacier. At Trélaporte, moreover, where the valley becomes narrow, the bands are much sharpened, obtaining there the character which they afterwards preserve throughout the Mer de Glace. Other glaciers with cascades also exhibit similar bands.

§ 49. _Sea Ice and Icebergs._

338. We are now equipped intellectually for a campaign into another territory. Water becomes heavier and more difficult to freeze when salt is dissolved in it. Sea water is therefore heavier than fresh, and the Greenland Ocean requires to freeze it a temperature 3½ degrees lower than fresh water. When concentrated till its specific gravity reaches 1.1045, sea water requires for its congelation a temperature 18⅓ degrees lower than the ordinary freezing-point.[E]

[E] Scoresby.

339. But even when the water is saturated with salt, the crystallising force studiously rejects the salt, and devotes itself to the congelation of the water alone. Hence the ice of sea water, when melted, produces fresh water. The only saline particles existing in such ice are those entangled 'mechanically in its pores. They have no part or lot in the structure of the crystal.

340. This _exclusiveness_, if I may use the term, of the water molecules; this entire rejection of all foreign elements from the edifices which they build, is enforced to a surprising degree. Sulphuric acid has so strong an affinity for water that it is one of the most powerful agents known to the chemist for the removal of humidity from air. Still, as shown by Faraday, when a mixture of sulphuric acid and water is frozen, the crystal formed is perfectly sweet and free from acidity. The water alone has lent itself to the crystallising force.

341. Every winter in the Arctic regions the sea freezes, roofing itself with ice of enormous thickness and vast extent. By the summer heat, and the tossing of the waves, this is broken up; the fragments are drifted by winds and borne by currents. They clash, they crush each other, they pile themselves into heaps, thus constituting the chief danger encountered by mariners in the polar seas.

342. But among the drifting masses of flat sea-ice, vaster masses sail, which spring from a totally different source. These are the _Icebergs_ of the Arctic seas. They rise sometimes to an elevation of hundreds of feet above the water, while the weight of ice submerged is about seven times that seen above.