The Glaciers of the Alps Part 1

The Glaciers of the Alps.

by John Tyndall.

PREFACE.

In the following work I have not attempted to mix Narrative and Science, believing that the mind once interested in the one, cannot with satisfaction pa.s.s abruptly to the other. The book is therefore divided into Two Parts: the first chiefly narrative, and the second chiefly scientific.

In Part I. I have sought to convey some notion of the life of an Alpine explorer, and of the means by which his knowledge is acquired. In Part II. an attempt is made to cla.s.sify such knowledge, and to refer the observed phenomena to their physical causes.

The Second Part of the work is written with a desire to interest intelligent persons who may not possess any special scientific culture.

For their sakes I have dwelt more fully on principles than I should have done in presence of a purely scientific audience. The brief sketch of the nature of Light and Heat, with which Part II. is commenced, will not, I trust, prove uninteresting to the reader for whom it is more especially designed.

Should any obscurity exist as to the meaning of the terms Structure, Dirt-bands, Regelation, Interference, and others, which occur in Part I., it will entirely disappear in the perusal of Part II.

Two ascents of Mont Blanc and two of Monte Rosa are recorded; but the aspects of nature, and other circ.u.mstances which attracted my attention, were so different in the respective cases, that repet.i.tion was scarcely possible.

The numerous interesting articles on glaciers which have been published during the last eighteen months, and the various lively discussions to which the subject has given birth, have induced me to make myself better acquainted than I had previously been with the historic aspect of the question. In some important cases I have stated, with the utmost possible brevity, the results of my reading, and thus, I trust, contributed to the formation of a just estimate of men whose labours in this field were long anterior to my own.

J. T.

_Royal Inst.i.tution, June, 1860._

PREFATORY NOTE.

"Glaciers of the Alps" was published nearly six and thirty years ago, and has been long out of print, its teaching in a condensed form having been embodied in the little book called "Forms of Water." The two books are, however, distinct in character; each appears to me to supplement the other; and as the older work is still frequently asked for, I have, at the suggestion of my husband's Publishers, consented to the present reprint, which may be followed later on by a reprint of "Hours of Exercise."

Before reproducing a book written so long ago, I sought to a.s.sure myself that it contained nothing touching the views of others which my husband might have wished at the present time to alter or omit. With this object I asked Lord Kelvin to be good enough to read over for me the pages which deal with the history of the subject and with discussions in which he himself took an active part. In kind response he writes:--"... After carefully going through all the pa.s.sages relating to those old differences I could not advise the omission of any of them from the reprint. There were, no doubt, some keen differences of opinion and judgement among us, and other friends now gone from us, but I think the statements on controversial points in this beautiful and interesting book of your husband's are all thoroughly courteous and considerate of feelings, and have been felt to be so by those whose views were contested or criticised in them."

The current spelling of Swiss names has changed considerably since "Glaciers of the Alps" was written, but, except in the very few cases where an obvious oversight called for correction, the text has been left unaltered. Only the Index has been made somewhat fuller than it was.

L. C. T.

_January, 1896._

INTRODUCTORY.

In the autumn of 1854 I attended the meeting of the British a.s.sociation at Liverpool; and, after it was over, availed myself of my position to make an excursion into North Wales. Guided by a friend who knew the country, I became acquainted with its chief beauties, and concluded the expedition by a visit to Bangor and the neighbouring slate quarries of Penrhyn.

From my boyhood I had been accustomed to handle slates; had seen them used as roofing materials, and had worked the usual amount of arithmetic upon them at school; but now, as I saw the rocks blasted, the broken ma.s.ses removed to the sheds surrounding the quarry, and there cloven into thin plates, a new interest was excited, and I could not help asking after the cause of this extraordinary property of cleavage. It sufficed to strike the point of an iron instrument into the edge of a plate of rock to cause the ma.s.s to yield and open, as wood opens in advance of a wedge driven into it. I walked round the quarry and observed that the planes of cleavage were everywhere parallel; the rock was capable of being split in one direction only, and this direction remained perfectly constant throughout the entire quarry.

[Sidenote: CLEAVAGE OF SLATE ROCKS.]

I was puzzled, and, on expressing my perplexity to my companion, he suggested that the cleavage was nothing more than the layers in which the rock had been originally deposited, and which, by some subsequent disturbance, had been set on end, like the strata of the sandstone rocks and chalk cliffs of Alum Bay. But though I was too ignorant to combat this notion successfully, it by no means satisfied me. I did not know that at the time of my visit this very question of slaty cleavage was exciting the greatest attention among English geologists, and I quitted the place with that feeling of intellectual discontent which, however unpleasant it may be for a time, is very useful as a stimulant, and perhaps as necessary to the true appreciation of knowledge as a healthy appet.i.te is to the enjoyment of food.

On inquiry I found that the subject had been treated by three English writers, Professor Sedgwick, Mr. Daniel Sharpe, and Mr. Sorby. From Professor Sedgwick I learned that cleavage and stratification were things totally distinct from each other; that in many cases the strata could be observed with the cleavage pa.s.sing through them at a high angle; and that this was the case throughout vast areas in North Wales and c.u.mberland. I read the lucid and important memoir of this eminent geologist with great interest: it placed the data of the problem before me, as far as they were then known, and I found myself, to some extent at least, in a condition to appreciate the value of a theoretic explanation.

Everybody has heard of the force of gravitation, and of that of cohesion; but there is a more subtle play of forces exerted by the molecules of bodies upon each other when these molecules possess sufficient freedom of action. In virtue of such forces, the ultimate particles of matter are enabled to build themselves up into those wondrous edifices which we call crystals. A diamond is a crystal self-erected from atoms of carbon; an amethyst is a crystal built up from particles of silica; Iceland spar is a crystal built by particles of carbonate of lime. By artificial means we can allow the particles of bodies the free play necessary to their crystallization. Thus a solution of saltpetre exposed to slow evaporation produces crystals of saltpetre; alum crystals of great size and beauty may be obtained in a similar manner; and in the formation of a bit of common sugar-candy there are agencies at play, the contemplation of which, as mere objects of thought, is sufficient to make the wisest philosopher bow down in wonder, and confess himself a child.

[Sidenote: CRYSTALLIZATION THEORY.]

The particles of certain crystalline bodies are found to arrange themselves in layers, like courses of atomic masonry, and along these layers such crystals may be easily cloven into the thinnest laminae. Some crystals possess _one_ such direction in which they may be cloven, some several; some, on the other hand, may be split with different facility in different directions. Rock salt may be cloven with equal facility in three directions at right angles to each other; that is, it may be split into cubes; calcspar may be cloven in three directions oblique to each other; that is, into rhomboids. Heavy spar may also be cloven in three directions, but one cleavage is much more perfect, or more _eminent_ as it is sometimes called, than the rest. Mica is a crystal which cleaves very readily in one direction, and it is sufficiently tough to furnish films of extreme tenuity: finally, any boy, with sufficient skill, who tries a good crystal of sugar-candy in various directions with the blade of his penknife, will find that it possesses one direction in particular, along which, if the blade of the knife be placed and struck, the crystal will split into plates possessing clean and shining surfaces of cleavage.

[Sidenote: POLAR FORCES.]

Professor Sedgwick was intimately acquainted with all these facts, and a great many more, when he investigated the cleavage of slate rocks; and seeing no other explanation open to him, he ascribed to slaty cleavage a crystalline origin. He supposed that the particles of slate rock were acted on, after their deposition, by "polar forces," which so arranged them as to produce the cleavage. According to this theory, therefore, Honister Crag and the cliffs of Penrhyn are to be regarded as portions of enormous crystals; a length of time commensurate with the vastness of the supposed action being a.s.sumed to have elapsed between the deposition of the rock and its final crystallization.

When, however, we look closely into this bold and beautiful hypothesis, we find that the only a.n.a.logy which exists between the physical structure of slate rocks and of crystals is this single one of cleavage.

Such a coincidence might fairly give rise to the conjecture that both were due to a common cause; but there is great difficulty in accepting this as a theoretic truth. When we examine the structure of a slate rock, we find that the substance is composed of the debris of former rocks; that it was once a fine mud, composed of particles of _sensible magnitude_. Is it meant that these particles, each taken as a whole, were re-arranged after deposition? If so, the force which effected such an arrangement must be wholly different from that of crystallization, for the latter is essentially _molecular_. What is this force? Nature, as far as we know, furnishes none competent, under the conditions, to produce the effect. Is it meant that the molecules composing these sensible particles have re-arranged themselves? We find no evidence of such an action in the individual fragments: the mica is still mica, and possesses all the properties of mica; and so of the other ingredients of which the rock is composed. Independent of this, that an aggregate of heterogeneous mineral fragments should, without any a.s.signable external cause, so shift its molecules as to produce a plane of cleavage common to them all, is, in my opinion, an a.s.sumption too heavy for any theory to bear.

Nevertheless, the paper of Professor Sedgwick invested the subject of slaty cleavage with an interest not to be forgotten, and proved the stimulus to further inquiry. The structure of slate rocks was more closely examined; the fossils which they contained were subjected to rigid scrutiny, and their shapes compared with those of the same species taken from other rocks. Thus proceeding, the late Mr. Daniel Sharpe found that the fossils contained in slate rocks are distorted in shape, being uniformly flattened out in the direction of the planes of cleavage. Here, then, was a fact of capital importance,--the sh.e.l.ls became the indicators of an action to which the ma.s.s containing them had been subjected; they demonstrated the operation of pressure acting at right angles to the planes of cleavage.

[Sidenote: MECHANICAL THEORY.]

The more the subject was investigated, the more clearly were the evidences of pressure made out. Subsequent to Mr. Sharpe, Mr. Sorby entered upon this field of inquiry. With great skill and patience he prepared sections of slate rock, which he submitted to microscopic examination, and his observations showed that the evidences of pressure could be plainly traced, even in his minute specimens. The subject has been since ably followed up by Professors Haughton, Harkness, and others; but to the two gentlemen first mentioned we are, I think, indebted for the prime facts on which rests the _mechanical theory_ of slaty cleavage.[A]

[Sidenote: LECTURE AT THE ROYAL INSt.i.tUTION.]

The observations just referred to showed the co-existence of the two phenomena, but they did not prove that pressure and cleavage stood to each other in the relation of cause and effect. "Can the pressure produce the cleavage?" was still an open question, and it was one which mere reasoning, unaided by experiment, was incompetent to answer.

Sharpe despaired of an experimental solution, regarding our means as inadequate, and our time on earth too short to produce the result. Mr.

Sorby was more hopeful. Submitting mixtures of gypsum and oxide of iron scales to pressure, he found that the scales set themselves approximately at right angles to the direction in which the pressure was applied. The position of the scales resembled that of the plates of mica which his researches had disclosed to him in slate rock, and he inferred that the presence of such plates, and of flat or elongated fragments generally, lying all in the same general direction, was the cause of slaty cleavage. At the meeting of the British a.s.sociation at Glasgow, in 1855, I had the pleasure of seeing some of Mr. Sorby's specimens, and, though the cleavage they exhibited was very rough, still, the tendency to yield at right angles to the direction in which the pressure had been applied, appeared sufficiently manifest.

At the time now referred to I was engaged, and had been for a long time previously, in examining the effects of pressure upon the magnetic force, and, as far back as 1851, I had noticed that some of the bodies which I had subjected to pressure exhibited a cleavage of surpa.s.sing beauty and delicacy. The bearing of such facts upon the present question now forcibly occurred to me. I followed up the observations; visited slate yards and quarries, observed the exfoliation of rails, the fibres of iron, the structure of tiles, pottery, and cheese, and had several practical lessons in the manufacture of puff-paste and other laminated confectionery. My observations, I thought, pointed to a theory of slaty cleavage different from any previously given, and which, moreover, referred a great number of apparently unrelated phenomena to a common cause. On the 10th of June, 1856, I made them the subject of a Friday evening's discourse at the Royal Inst.i.tution.[B]

[Sidenote: ORIGIN OF RESEARCHES.]

Such are the circ.u.mstances, apparently remote enough, under which my connexion with glaciers originated. My friend Professor Huxley was present at the lecture referred to: he was well acquainted with the work of Professor Forbes, ent.i.tled 'Travels in the Alps,' and he surmised that the question of slaty cleavage, in its new aspect, might have some bearing upon the laminated structure of glacier-ice discussed in the work referred to. He therefore urged me to read the 'Travels,' which I did with care, and the book made the same impression upon me that it had produced upon my friend. We were both going to Switzerland that year, and it required but a slight modification of our plans to arrange a joint excursion over some of the glaciers of the Oberland, and thus afford ourselves the means of observing together the veined structure of the ice.

Had the results of this arrangement been revealed to me beforehand, I should have paused before entering upon an investigation which required of me so long a renunciation of my old and more favourite pursuits. But no man knows when he commences the examination of a physical problem into what new and complicated mental alliances it may lead him. No fragment of nature can be studied alone; each part is related to every other part; and hence it is, that, following up the links of law which connect phenomena, the physical investigator often finds himself led far beyond the scope of his original intentions, the danger in this respect augmenting in direct proportion to the wish of the inquirer to render his knowledge solid and complete.

[Sidenote: A BOY'S BOOK.]

When the idea of writing this book first occurred to me, it was not my intention to confine myself to the glaciers alone, but to make the work a vehicle for the familiar explanation of such general physical phenomena as had come under my notice. Nor did I intend to address it to a cultured man of science, but to a youth of average intelligence, and furnished with the education which England now offers to the young. I wished indeed to make it a boy's cla.s.s-book, which should reveal the mode of life, as well as the scientific objects, of an explorer of the Alps. The incidents of the past year have caused me to deviate, in some degree, from this intention, but its traces will be sufficiently manifest; and this reference to it will, I trust, excuse an occasional liberty of style and simplicity of treatment which would be out of place if intended for a reader of riper years.

FOOTNOTES:

[A] Mr. Sorby has drawn my attention to an able and interesting paper by M. Bauer, in Karsten's 'Archiv' for 1846; in which it is announced that cleavage is a tension of the ma.s.s _produced by pressure_. The author refers to the experiments of Mr. Hopkins as bearing upon the question.

[B] See Appendix.