صور الصفحة
PDF
النشر الإلكتروني

On Hypobromous Acid. By Prof. H. E. ROSCOE.

Professor Roscoe communicated to the Section the results of an investigation upon the lowest oxide of bromine, hypobromous acid, which had been made in the laboratory of Owens College, Manchester, by Mr. William Dancer. Balard in 1826 mentions the formation of a colourless bleaching salt formed by the action of bromine upon the alkalies, and since that date many chemists have indicated the presence of such a body, but it has not been prepared in a pure state or analysed. Mr. Dancer has succeeded in preparing the aqueous acid in a pure state, and has examined its chief properties and determined its composition. If bromine-water and nitrate-of-silver solution be brought together, one-half the bromine is precipitated as bromide of silver, whilst the other half remains in solution as hypobromous acid (BrO HO). The aqueous acid may be obtained by distillation at 30° C. in vacuo, but decomposes into bromine and oxygen at 100° C. The aqueous acid may likewise be prepared by shaking bromine-water together with oxide of mercury, and distilling in vacuo; in this case half the bromine forms the bleaching compound. Hypobromous acid unites with the alkalies, and forms salts analogous in smell and bleaching properties to the corresponding hypochlorites. Owing to the ease with which this compound splits up into bromine and oxygen, it was found impossible to prepare the hypobromous anhydride by any of the methods used for the isolation of the corresponding chlorine compound.

Description of a rapid Dry-Collodion Process. By T. SUTTON.

A rapid dry-collodion process, by which dry plates can be prepared as sensitive as with wet collodion, has more than any other problem interested photographers. By the wet process, the negative has to be finished on the spot. The rapidity of this dry process depends upon the effect of bromine in dry collodion. In the Daguerreotype process a silver plate iodized is extremely insensitive, but when submitted to the fumes of bromine it is increased a hundredfold. In the wet, but not in this process, nitrate of silver is required, which is the element of instability. In preparing, therefore, rapid dry-collodion plates, bromo-iodized collodion must be used. But the image produced thus is extremely thin and superficial; it is therefore necessary to apply to the film a coating of some organic substance, in order to darken parts of the negative. Many substances have been tried for this purpose, but none produce so good an effect as gum-arabic. The paper concluded with the operations required for this process.

GEOLOGY.

Address to the Geological Section by J. BEETE JUKES, M.A., F.R.S. It is now thirty-two years ago since I first, when a "freshman" of this University, attended the geological lectures of Professor Sedgwick. I had previously had access to a cabinet of fossils, and had been accustomed to seek for specimens in my schoolboy rambles on the hills near Dudley. It may be imagined, therefore, with what interest I listened to the "winged words" of the Woodwardian Professor, which used day after day to delight an audience composed of all ranks of the University.

Geology and its kindred sciences did not then, indeed, form any part of our regular course of university studies, and many of the college tutors were so far from encouraging our attention to them, that they rather discountenanced it, considering them as at best useless and probably even dangerous pursuits. With such a man as Professor Sedgwick, however, in the Woodwardian chair, whose wit and humour delighted, while his eloquence aroused and informed his hearers, the love of the science and the knowledge of it could not fail to extend from one year to another. The natural sciences are now considered as worthy of study, by those who have a taste for them, both in themselves and as a means of mental training and disci

pline. In my time, however, no other branches of learning were recognized than classics and mathematics, and I have with some shame to confess that I displayed but a "truant disposition" with respect to them, and too often hurried from the tutor's lecture-room to the river or the field, to enable me to add much to the scanty stores of knowledge I had brought up with me. Had it not been, then, for the teaching of Professor Sedgwick in Geology, my time might have been altogether wasted. But it was not only in the lecture-room that I learnt from him. With that kindness of heart and geniality of disposition which make him as much loved as his powers cause him to be admired, he was good enough to step down from his high place as a Professor of the University, and to take some notice of the young undergraduate whom he saw lingering over the trays of specimens when the lecture was over, to inquire his name, and to invite him to his table. He subsequently allowed me to accompany him on some excursions in different parts of England, and gave me some of those practical lessons in the field, which, as you know, teach more in three days than can be learnt in months or years in the museum or the lectureroom. I look back upon these circumstances as those which gave direction to the whole course of my life, and as the origin of a paternal friendship with which Professor Sedgwick has honoured me for so many years, and which it has been my chief pride to endeavour to deserve. I hope, Ladies and Gentlemen, I may be pardoned for these few personal allusions; but amid all the gratification which I must necessarily feel at the honour which has now fallen upon me, that, namely, of being called upon to preside, within the walls of my own Alma Mater, over the Geological Section of the British Association, it was impossible for me to neglect the opportunity of acknowledging the debt of gratitude I owe to one of the ruling spirits of both bodies, and of avowing that my chief claim to occupy this chair is that I am an old pupil of Professor Sedgwick.

That

One of the most obvious difficulties in the way of any person who now undertakes to preside over this Section is the thought of the contrast that will necessarily arise in the minds of many of you between him and his predecessors. I am now occupying the seat that has been filled by Sedgwick, Buckland, Lyell, Murchison, Hopkins, De la Beche, Forbes, and so many other illustrious men, may well cause me to doubt my own capability of fulfilling its duties. One lesson I must certainly learn, and that is, to endeavour to make up for other deficiencies by attention and assiduity, and, above all, not to take such an advantage of the position, as to bring anything of my own before your notice, to the hindrance of others who may have something to produce that may be more worthy of it. At the end, then, of this Address, which I will endeavour to make as brief as possible, I shall consider my own mouth as almost closed for the remainder of the meeting, and shall endeavour so far to imitate the Speaker of the House of Commons as to say as little as possible.

I propose to take for my subject the external features of the earth's surface. The principal business of Geology is to acquire as accurate a knowledge as we can of the internal structure of the crust of the earth, and to learn as much as possible of all the operations by which that structure was originally formed, or by which it has been subsequently modified. The crust of the earth has always been receiving accessions to its composition, both from within and from without. In like manner it has always been subject to modifying influences proceeding both from within and from without. It is obvious that the external influences act directly upon the actual surface of the time being. It is equally obvious that the internal influences can only reach that surface by penetrating through the thickness of the crust.. If, therefore, we ask by what means the present surface of the earth, or, to bring the problem within more narrow limits, by what means the present surface of any of our dry lands, has been produced, we should naturally conclude that it owes its form to the external influences that have been brought to bear directly upon it, rather than to the indirect action of those deep-seated agencies, which can only reach it through an unknown thickness of solid rock.

I believe this conclusion to be a true one. It is, however, by no means the idea which is commonly entertained, even by many geologists, while those who are not geologists are always inclined to refer all the more striking features of the surface

of the earth to the direct action of convulsive force proceeding from the interior, rather than to their true source in the gentle, gradual, silent influence of the "weather," continued through an indefinite period of past time.

I have heard even educated men speak of the correspondence in the chalk cliffs of the opposite sides of the Straits of Dover, as evidence in favour of the notion that England had been separated from France by the tearing open of those straits by what they called some "great convulsion of nature." There is hardly a description to be found in any book, of any deep and narrow valley or mountain gorge-especially if the precipices on each side of it show entering and re-entering angles, and rocks that were obviously once continuous across the gap,-but what its formation is unhesitatingly attributed to this vague imaginary force, a "convulsion of nature." Nay, I have even heard the existence of broad valleys over an anticlinal arch, such, for instance, as the valley of the Weald, attributed to the effect of the gaping of the rocks at the surface, consequent on the upward flexure of the beds. Mythical powers of disturbance are called into existence with as bold a personification as the Bia and Kpáros of the poet, and with even less warrant for their existence.

It seems to me, therefore, that the time is come when geologists should study a little more closely this problem of the mode of production of the surface of the land, and determine exactly the method of the formation of those variations in its outline which we call mountains, hills, table-lands, cliffs, precipices, ravines, glens, valleys, and plains.

Few men, perhaps, ever pause to inquire into the origin of a great plain; nevertheless the question may well be put, and it is one which deserves an answer. Some plains are doubtless the result of original formation. They are level and flat, because the beds beneath the surface are horizontal. Even these, however, have very rarely a surface formed simply by the last-deposited beds. The actual surface is one that has been formed by the erosion and removal of more or less of the uppermost beds, and the production of undulations formed by the act of cutting down into the beds below. This erosion or denudation has even in many such cases gone to the length of entirely removing a much greater thickness than we should at first suspect, the present surface being one that has been laid bare by that removal.

In all cases where the beds below the surface are not strictly horizontal, or do not accurately coincide as to their "lie "with the form of the surface, it is obvious that the plain must be one of denudation.

Suppose we take the great plain on which we now are, and which stretches from Cambridge far into Lincolnshire. The hills which rise from it towards the east are formed by the escarpment of the Chalk, the beds of which terminate abruptly at that escarpment, and allow the clays which lie beneath the Chalk to come up to the surface and spread beneath the plain. The hills rising to the west of the plain, on the other hand, are formed of the Oolites, the beds of which lie below these clays and rise gently from beneath the plain, and themselves terminate in an escarpment still further west.

There can be no reasonable doubt that the whole thickness of the Chalk and the beds below it once spread many miles to the westward of their present boundaries. The little chalk-capped monticule of the Castle Hill, at the western end of the town of Cambridge, and the hills near Madingley show that the Chalk was once continuous that far, at all events, from the Gogmagogs; and, had still higher ground been left by the denudation still further west, that would in like manner have been capped by the bottom beds of the Chalk.

The hill on which Ely stands is, I believe, an outlier of the Lower Greensand, the general mass of which crops out some miles to the eastward; and other hills rising from the plain will in like manner be found to have their summits capped by beds, apparently horizontal, but in reality dipping at a very gentle angle to the eastward, so as to ultimately cut the surface of the plain in that direction and then sink beneath it. All such outliers are clear proof that the beds formerly extended over the intervening spaces, and show us that the rocks now left in the ground are only a portion of those that were originally deposited.

The great plain of the Fens, then, is one of denudation, its surface being one that

is now bare in consequence of the removal from above it* of a thickness of many hundred feet of Chalk, and of other beds below the Chalk. But this reasoning may be carried out with respect to the whole of the flat lands of England and the British Islands. The great central plain of Ireland, for instance, stretching from Dublin Bay to Galway Bay, with an average elevation of less than 300 feet above the sea, has immediately beneath it abruptly undulating beds of Carboniferous limestone, rising up at all angles, and dipping in all directions. The most level parts of the surface sometimes cut horizontally across the most contorted and highly inclined beds. The small isolated hills scattered here and there about the plain are formed sometimes of beds of Old Red Sandstone that rise up from beneath the bottom of the Limestone, and sometimes of beds of Coal-measures which rest upon the top of it. It is here abundantly evident, then, that the internal forces of disturbance which have bent the beds from their original horizontality into so many curves, and broken them by so many dislocations, had nothing at all to do with the production of the present surface, which has been formed across all these bent and broken beds after the disturbances had ceased.

But, in fact, the very first glance at a geological map of a flat country, if there be two or more colours on it representing conformable groups of stratified rocks, is just as good a proof of this vast denudation as the most elaborate reasoning. The last-deposited group of beds would of course conceal all those beneath it; it would be represented by one uniform colour. Let the internal forces bend, or tilt, or break it in any fashion you like, they cannot of themselves remove a particle of it. It will still lie over all those on which it was originally deposited, and the map would show the one colour only, unless we go the length of supposing that a piece of the crust of the earth could be tossed over like a pancake, and laid down again with its bottom upwards.

I have taken the case of a plain in the first instance, because it is obvious that if we arrive at the conclusion that many plains are low and level because mountainous masses of rock have been removed from above their present surface, it will be easy for us to recognize the proofs of denudation in the hills and mountains, on whose flanks the obvious marks of it are still left.

A little reflection will show us that the outcrop of a bed is always a proof of denudation, for the present surface cannot possibly be the original termination, not only of that particular bed, but of all the beds above it. When then a succession of beds crop out rapidly one after another, as they always do in all hill-ranges and mountain-chains, we cannot escape from the conclusion that the existing surface has been formed by the removal of the former extension of the beds. This is the inevitable conclusion, whether the surface be horizontal and the beds below it inclined, or the beds be horizontal and the surface inclined, or the surface slope one way and the beds dip another, or there be any kind of discordance between the "lie" of the beds and the form of the surface of the ground. The only possible escape from this conclusion would be in the case where a succession of beds had been deposited on a slope, and had never been covered by any other deposit. This, however, is a case that could only occur in very recently formed rocks, and cannot apply to the outcrop of beds on the flanks of hills or mountains, where the surface of the ground itself has a high inclination.

In such situations the only escape from the conclusion that the surface was formed by denudation would be, proof that the undulations of the surface were exactly followed by the undulation of the beds below it, and, in fact, that the very same bed was everywhere found to be the one immediately below the surface.

If we except Volcanos or "Mountains of Ejection," all other hills and mountains are either caused by the removal of the rocks which once surrounded them, or have suffered from the removal of those that once spread over them. The first kind of hills have simply been left high, while the surrounding ground has been worn down to a low level about them. In the second kind, the rocks composing them have, indeed, been thrust up from beneath by internal force to a much greater elevation than those same rocks have in the surrounding area, and their height is due entirely In this general statement the few feet of peat, or the little banks of drift gravel and sand which have been subsequently deposited on or grown over the plain, are, of course, disregarded.

to that upward tilting, vast masses of once superincumbent beds having been removed from above them. These hills are high, not in consequence of, but in spite of denudation. I have elsewhere proposed to call the first kind "hills of circumdenudation," and the second "hills of uptilting." To the latter class belong all the great mountain-chains of the world, and most of the smaller ones.

It may be taken as an invariable rule, that, as we approach all mountain-chains formed by uptilting, the beds rise towards them, and end successively at the surface; lower and lower beds still rising up, until the lowest of all appear in the heart of the mountains, where they are often reared up into the loftiest peaks. True as is this general statement, it is only generally true. The great groups of rocks thus rise successively one from beneath another; but this general rise is often complicated by numerous folds and reduplications, by great longitudinal fractures, or by complex flexures.

The geological axis of a mountain-chain runs along the line where the lowest group of beds rises to the surface. The geographical axis may be said to run along that dominant crest which forms the watershed of the chain. But it by no means follows that these two axes are coincident, that the lowest group of beds is always confined to the line of watershed, or even that the loftiest peaks and summits rise from that crest. The geological axes are dependent solely on the internal forces of elevation; if, therefore, the geographical axes do not coincide with them, it shows at once that they are independent of those forces; in other words, that the great external features have not been caused by the direct action of internal movement. The position of the geological axes of mountain-chains has, however, been often erroneously placed, from the tendency to refer them to any great masses of granite or other plutonic rocks that may show themselves,-a reference which is more often erroneous than correct.

All mountain-chains of uptilting tell the same story, that if the internal forces of disturbance and elevation had acted alone, without any external action of denudation, and if they had acted without it to the same extent which they have with it (supposing that possible), the mountain-chains would have been many times more lofty than they are. I say "supposing that possible," because it appears to me that the elevation of the lowest rocks might never have proceeded to the same extent, if the internal force had not been gradually relieved of some of the external weight which it had to lift. However that may be, we see now that the lowest beds which appear at the surface, about the geological axis of a mountain-chain, dip on either hand beneath an ever-increasing thickness of superincumbent rock, as we recede from the axis. All the rocks which have been affected by the same action of disturbing force must have stretched unbroken across the disturbed district, before the disturbance commenced; for the lowest rocks appear at the surface now, not in consequence of the flexure or fracture of those that were above them, but in consequence of their removal. That removal could not have taken place prior to the internal disturbance, unless we assume the existence of a deep hole or trough of erosion along the space where the mountain-chain was subsequently thrust upwards. The removal of the bent or broken beds, then, must have taken place either during the action of disturbance or subsequently to its termination. In either case it was an external action, the result, in fact, of moving water, which slowly wore away and carried off so many square miles or, as in some cases, so many hundreds or thousands of square miles of rock, so many thousands of feet in thickness. The internal forces operated simply by lifting up the rocks to within the region of the denuding influence, and they have only produced that indirect effect upon the features of the surface which results from their having brought up to different levels, and placed in various positions, masses of rock of varions hardness and constitution, on which the forces of erosion and transport have had a corresponding variety of effect, when they reached them.

I believe that all our uptilted mountain-chains have thus grown by a very slow and gradual growth, the internal force thrusting upwards what the external agencies always tended to wear down.

The investigation of the nature and effects of the mechanical forces that have acted on the crust of the earth from the interior has been undertaken by many eminent philosophers, by none with more acuteness and profundity than by our pre

« السابقةمتابعة »