Art. VIII. — Presidential Address.
[Delivered to the Wellington Philosophical Society, 8th July, 1896.]
In accordance with the established custom of this society, that each of its newly-elected presidents should open the year's proceedings by delivering an address, I propose to bring under your notice this evening some of the special subjects that are now engaging the attention of scientific men in Europe and the United States.
Amongst these none has excited a greater degree of general interest, since the initiation of the “Challenger” expedition, than the explorations about to be undertaken in the antarctic regions, most of the secrets of which have hitherto remained as in a sealed book. The south polar land, which lies entirely within the antarctic circle, may, for the purposes of practical discussion, be treated as occupying a space equal to the whole area between the 70th parallel and the south pole; and the enterprising men who are about to engage in the proposed explorations will, therefore, have to deal with an unknown region of enormous dimensions. The surrounding ocean, within the limits of the antarctic circle, is at all seasons of the year more or less encumbered with icebergs and pack-ice, whilst the apparent coast-line, for nearly the whole of its known extent, presents a perpendicular and unbroken
wall of glacier-ice, perpetually fed from the snows which fall oh the land within it, and varying in height from 150ft. to 200ft. As you are no doubt aware, many navigators of note, including Cook, Wilkes, Briscoe, D'Urville, Bellingshausen, and Ross, made attempts, prior to the year 1844, to examine these regions, of whom Ross alone penetrated the ice-pack, and after the most arduous exertions succeeded in reaching a point within two or three degrees from the 80th parallel, from which he saw the great active volcano named by him Mount Erebus. From the point thus reached he ran eastward for about two hundred miles along a perpendicular ice-wall, and then returned in a diagonal line, from the neighbourhood of the 162nd meridian west of Greenwich, to a point on the 180th meridian where it intersects the 70th parallel. Little has been added since Ross's voyages to our knowledge of the position or extent of the south polar land, but, although the accounts of the several voyages are in themselves of much interest as evidences of the indomitable energy of the navigators engaged on them, and as matters of geographical information, their value, from a scientific point of view, is small when compared with the many more important results which may be expected from the operations of those who are about to engage in the proposed expeditions. In the hope of obtaining these higher results, all the scientific bodies in Europe and the United States have long concurred in urging the necessity of replacing, by active and skilled explorations, the practically total neglect into which the examination of the antarctic regions has fallen since the above-mentioned expeditions; for with the exception of the data brought home by Ross, and those more recently obtained by the “Challenger,” little, if anything, has been done to increase our stock of knowledge in any department of science in connection with those regions which can bear satisfactory comparison with the wonderful results that have followed from work done within the arctic area. There is no doubt, however, that the difficulties experienced by the few who have since attempted to explore the antarctic seas, and the gloomy accounts which they, one and all, gave of long and wearisome working through, the ice-pack, and of the apparently interminable and impenetrable wall of glacier-ice which barred all efforts at land-exploration, were well calculated to excite exaggerated ideas of the obstacles to success presented both by the sea and shore aspects of the area in question. But recent experiences have, to a considerable extent, modified these gloomy views, and the fact that Captain Larsen, of the Norwegian steamship “Jasen,” was not only able, with comparatively little difficulty, to sail along the eastern coast of Graham's Land, and to ascertain its correct position for upwards of two hundred miles, but was also able
to land twice in positions from which, had he possessed the appliances ordinarily used in arctic explorations, he might have made excursions over the glaciers that he saw flowing amongst still active volcanos, sufficiently indicates that no greater difficulty will in all probability be found in penetrating inland than has usually been experienced in exploring many parts of the arctic land areas. It will be remembered, too, that the Swedish whale-ship “Antarctic” recently visited the coast of Victoria Land in search of whales, following Ross's track to the vicinity of Mount Erebus, thus successfully fighting its way through the ice-pack to the ice-free sea of Ross. From thence she returned to the northward; and it is important to note that the return voyage through the pack-ice, made, it is true, in the latter part of summer, occupied only six days, as against the thirty eight days which were required for the southward course in the earlier part of the season, the pack during her return through it having been found to be loose and easily penetrated. It was in this ship that Mr. Borchgrevink (a Swedish naturalist) obtained a passage under circumstances which have appeared, in some detail, in the Victorian newspapers, and he has told us that no difficulty was found in landing, first on Possession Island, on which Ross had raised the British flag upwards of fifty years before, and next at Cape Adare, on the mainland. Those who have read Mr. Borchgrevink's narrative of this adventure will remember his expressions of delight at having gathered a lichen, which he assumed to have been the first specimen of terrestrial vegetable life that had ever been discovered on the antarctic continent, and those of his regret at his having been unable to extend his researches, owing to the fact that the ship was engaged in a commercial adventure only.
These later accounts have led practical navigators and explorers to conclude that, with such bases of operation as are afforded by the eastern colonies of Australia and by New Zealand, those who are enterprising enough to engage in the arduous work of antarctic exploration will be able, as successfully as our great arctic explorers have done, to overcome all obstacles, and to hope that their labours will be accomplished without having to deplore any serious loss of life.
But it is not merely in the domain of ordinary geographical knowledge that the world would gain by persistent and scientific explorations of these regions. The wonderful results obtained by the “Challenger” staff have demonstrated how much may be done to extend our knowledge in many important branches of science by well-conducted researches even in regions which lie within the limits of ordinary traffic and observation. How much more, then, may be expected from
similarly-conducted explorations of practically new fields of inquiry, not only in the special departments of science which came within the scope of the “Challenger” expedition, but also in others of equal if not of greater practical importance. It is well known that the solution of some of the most intricate problems in various branches of scientific inquiry is at present impossible, in the absence of data that can only be obtained by means of successful observations in circum-polar regions, and more especially within the antarctic area. Many of these problems have lately been discussed in papers submitted to the greater scientific societies in England. For example, we are told on the highest authority that it is hopeless to strive with any prospect of success at the advancement of the theory of the earth's magnetism in the complete absence of data from the Southern Hemisphere beyond the 40th parallel. Those afforded by the magnetic survey made by Sir James Ross are no longer of any use for that purpose, for it is well known that such changes have taken place, since his time, in the magnetic elements south of that parallel that nothing less than a new and complete survey can supply the materials for properly revising the current theory of the revolution of the magnetic poles. The knowledge thus desired is of the greatest importance for the practical requirements of navigation. Magnetic maps, based on numerous direct observations, were calculated and drawn by Gauss upwards of fifty years ago, and some of the most eminent investigators in this branch of scientific inquiry have, ever since his time, been engaged in attempts at reconstructing them so as to make them available for the whole globe; but all their efforts to do so have been obstructed by the one insuperable obstacle—namely, want of knowledge of the necessary elements within an area of not less than three thousand five hundred miles in every direction from the south pole, a want which cannot be supplied by any amount of mathematical speculation. It is true that the Bureau des Longitudes of France is at present engaged in the construction of new magnetic maps, but it does not appear that their labours have as yet extended far, or proved very successful. It would seem that, in order to obtain a satisfactory general magnetic map, observations should be made over all regions as nearly as possible at the same time, and for this purpose observatories, with a competent staff and a supply of similar instruments for each, would be required at a considerable number of places. With such aids it will be possible to make practically simultaneous observations, and it may be hoped that all the great maritime nations will unite in a general effort to obtain them. Six of the expeditions appointed by the Bureau have already started to their appointed stations,
but I cannot find that any of these is charged with this duty within high southern latitudes. Great results are, however, expected by the Bureau from their present effort, and the maritime world is to be congratulated upon the important initiative thus taken up by the French Government.
A similar difficulty exists as regards modern investigations into the shape of our globe. It will be in your recollection that, during the last year's proceedings of this society, General Schaw read a paper pointing out that these investigations, like those last alluded to, are completely blocked for want of data from the Southern Hemisphere. Hitherto we have been content to look upon the earth as having the form of a ball, flattened at the poles, and to treat this flattening as following regular meridional curves, extending north and south from every point on the equator. It has, however, been demonstrated that this is by no means a correct view, but rather that our globe presents considerable irregularities of shape, both local and general, especially towards the poles, and that pendulum swingings made at various places in polar regions are absolutely necessary before reliable measurements of the earth's diameters can be made, correct diameter measurements being, in effect, the necessary bases of all measurements in astronomy. Pendulum swingings are now the recognised means for determining the extent of local deviations from the ideal form above alluded to, and very precise methods have lately been elaborated for utilising such swingings, which are found to afford rapid and exact means for the purpose in view. It is stated that not more than seven pendulum swingings have yet been made beyond the 50th parallel of south latitude, none of which were made within the antarctic circle, and hitherto all efforts at determining the exact shape of the earth have failed for want of them.
In 1867 the late Mr. Proctor drew attention to the permanent low barometer of the south temperate zone, and pointed out, in part explanation, that the centre of gravity for the solid portions of the earth, lay somewhat to the south of the centre of figure. He stated that this explanation had long been received as accounting for two remarkable geographical features,—namely, the prevalence of water over the Southern Hemisphere, and the configuration of nearly all the peninsulas over the whole globe. He stated, moreover, that, in his view of its causes, it was immaterial whether or not those portions of the antarctic regions which had not then been explored were occupied chiefly by land, or whether the unexplored north polar regions were or were not chiefly occupied by a north polar ocean. But, although the existence of the low barometer in the temperate parts of the Southern Hemisphere has been long known, no generally-received explanation of its
cause has been put forward. We may hope, however, that this problem will also be solved as one of the results of the proposed expeditions.
It is also well known that the crust of the earth is not perfectly rigid, and pendulum swingings taken from time to time will aid in determining whether, if at all, the great masses of ice resting on the outer limits of the antarctic land, either with or without the accumulations of drift resulting from glacial erosion and deposited along the true shoreline, have caused subsidence of the areas on which they rest. Geologists are at present divided in opinion on this question, for the determination of which even a limited number of pendulum experiments made in Graham and Victoria Lands will be of more value than almost any amount of observation elsewhere.
Besides these matters, there is a probability, and, at all events, much expectation, that the proposed investigations will throw light upon important points relating to the origin and distribution of animals and plants in Tertiary times. It is abundantly clear that such questions cannot be fully solved whilst we remain in ignorance of the physical conditions, past and present, of the antarctic continent. In papers which I read before this society in 1877, I pointed out that, until the surface of our globe had cooled down to such an extent as to admit of water resting upon it at a temperature not inconsistent with life, no life in any of the forms known to us could have arisen at all. I also pointed out that astronomers and physicists were agreed that long before such a degree of cooling had taken place the earth must have revolved round the sun in its present orbit; and I further pointed out that, for long after any part of its polar surfaces had cooled down to the extent required for the existence and maintenance of life, the surface heat must have remained too great in equatorial regions to admit of this. If these views be correct, then it must be assumed that the polar regions were the first to present the necessary conditions for the development and maintenance of living organisms, and that these, and their modified forms, must gradually have spread towards the equator when and as the intervening surfaces became suited by temperature and otherwise to receive them. I still hold the same views, and the more strongly because, since I wrote the papers referred to, much greater authorities than I can pretend to be upon such questions have expressed similar ones. Whether, however, they have any foundation or not, it is certain, looking to the enormous extent and peculiar form of the antarctic continent, that we may reasonably look forward to some light being thrown upon the nature of the fauna and flora that occupied it during past times, by the
researches which will doubtless be shortly attempted in this direction.
There appears also to be ground for supposing that the greater portion of the interior of the antarctic continent will be found to be free from snow, at all events during the summer, or, in other words, that the area of precipitation of the snow which gives rise to the coastal ice cannot extend very far inland. There are, unquestionably, meteorological grounds for this assumption, which, however, can only be verified or disproved by actual exploration.
In connection with this branch of my address, I may be permitted to quote the following passages from a paper written by that veteran in the advocacy of north polar explorations, Mr. Clements Markham, in the year 1885, which may, with equal appropriateness, be used with reference to the proposed expeditions to the south polar regions:—
“Voyages of discovery have been, since the dawn of modern times, one of the chief causes of England's power and greatness. The material wealth which they have been the means of pouring into her lap is incalculable. For this alone they will ever be a leading feature in the history of a mighty commercial nation; for this alone they have been fitted out by many a merchant adventurer; and for this they have been incessantly urged upon the attention of many successive Governments. But it is not on account of the commercial advantages that have been derived from the labours of the explorer that those labours are to be most prized, seeing that it is not to wealth alone that England owes her greatness. Exploring adventures by sea and land have done as much to increase the store of knowledge as any other kind of research. They have led the way to the creation of that colonial empire which has spread the Anglo-Saxon dominion far and wide over the earth. They have fostered the spirit of enterprise, and formed the nursery for the best of our seamen. They have been a school for our best officers, educating them in that calm self-reliance which the presence of danger alone can give. They have been most important agents of civilisation, creating a brotherly feeling of sympathy between the nations in time of peace, and giving one bright side even to the horrors of war, for, by the courtesy of international law, a scientific expedition is respected by all civilised nations. Let it once be known that an expedition of discovery will add to the sum of human knowledge, that it will lead to valuable scientific results, and that there is no chance of the men who compose it being overtaken by a catastrophe such as that which befel Sir John Franklin's people, and it ought to receive cordial support from public opinion. All men may not fully appreciate the value of
scientific researches, but no true Englishman can underestimate the importance of fostering the spirit of enterprise in his countrymen, or fail to desire that the race of men, from Cabot to McClintock, which has been formed by expeditions of discovery should be continued.”
The noble views thus expressed lead me to suggest a hope that, when an adequate conception of the many advantages that must accrue to the world at large from the aid which properly-equipped antarctic expeditions may be expected to afford in solving the problems I have alluded to has been brought home to the Governments and people of the Australasian Colonies, they will extend to the enterprising men who are about to engage in those expeditions the like countenance, sympathy, and material assistance which are certain to be afforded to them, sooner or later, by the Government and people of our Mother-country.
Discovery of the Röntgen Rays.
Passing from the foregoing subject, I propose now to refer to a most remarkable event in the history of physical and chemical science that has lately occurred, and has excited extraordinary and universal interest. I allude to the discovery, by Professor Röntgen of Berlin, of the peculiar properties possessed by certain electric rays, associated with the well-known kathode rays, to which he has given the specific name of “X rays.” On looking into the circumstances which led to this discovery, we find that in 1893 an account was published describing the researches in electro-magnetism made by the late Professor Hertz, of Berlin, with a view to an experimental demonstration of electro-magnetic waves. These researches formed the subject of an address read by Lord Kelvin at the opening of the winter session of the Royal Society early in 1894, in which he sketched the new horizons that were being opened out by experimenters in England and elsewhere in their further researches on the lines indicated by Hertz, and pointed out that in the results of these further researches lay his hope of obtaining additional knowledge of the relations between what is termed the ether of space and ponderable matter, and the part played by each in the transmission of electrical energy. Since the publication of Hertz's work it is no longer contended that electric waves can, any more than any other form of energy, pass through space without affecting the intervening medium; and, although it has not yet been demonstrated what form of strain would be produced upon the ether by these waves, it is conjectured that its molecules, if, indeed, the ether consists of molecules, are set into vibration across their line of propagation. Amongst the instruments used by Hertz in connection with these investigations
were the vacuum tubes invented by Geissler. These tubes were made of glass, the two ends presenting dilatations into which platinum wires were fused, the tubes being closed when as much as was then possible of the atmospheric air within them had been exhausted. When the wires of one of these tubes were then connected with the poles of an induction apparatus a beautiful stream of light traversed its interior, the colour of which varied with the nature of the contents of the tube; and many of you have, no doubt, seen the exquisite luminous effects resulting from the passage of an electrical discharge from one wire to the other in such a tube. In this connection, too, I have to mention that there are numbers of fluid and solid bodies which become self-luminous under the influence of particular rays of light shown in the spectrum, a peculiarity first noticed in a form of spar called fluor-spar or calcium-fluoride, from whence the phenomenon has been named fluorescence; and it has been found that this property is exhibited by reason of the fluor-spar absorbing a portion of the light-rays directed upon it. But the only rays which produce this effect are the violet and ultra-violet rays of the spectrum, the latter of which are wholly invisible to the human eye except when passed through a glass or quartz prism, and when the bright part of the spectrum has been carefully shut off. But their existence at once becomes apparent when, as already noted, they are projected upon some fluorescent body. Experiment, moreover, has shown that the highly-refrangible rays which possess in the greatest degree the power of exciting fluorescence are contained in large proportion in the light emitted by a Geissler tube containing rarified nitrogen—a point to be specially noted in connection with the experiments made by Röntgen to which I am about to allude, and with the recent discovery of the gas named argon, to which I intend to call your attention further on. The means thus placed at the disposal of experimenters, and the study of the phenomenon produced by the use of them, soon opened up a wide range of discovery in the region of electrical science. Mr. Cromwell Fleetwood Varley, so long ago as 1871, pointed out that the luminous cloud which appeared in a Geissler tube as soon as the current had attained a certain intensity was composed of attenuated particles of matter projected from the negative pole in all directions, and he showed that an electro-magnet acted upon such a stream, gathering it into an arch and attracting it. He found also that the stream thus formed was intercepted when he placed a thin plate of talc in its way, and that, whilst a luminous cloud was formed on the side of the plate bombarded by it, what he described as a “shadow,” but which really was a space protected
from the bombardment, was apparent behind it. The material character of the electrical discharge, and the actual transport of matter by electricity, were thus demonstrated by Varley, but his experiments and their results remained for many years quite unnoticed.
Eventually, however, the matter was taken up by Crookes. After studying the mechanical work of light rendered evident by the radiometer, he devoted his attention to the phenomena indicated by Varley, and, utilising the enormous progress in mechanical science which had taken place since the latter had made his experiments, he obtained such an exhaustion of the Geissler tube as to leave in it only a few millionth parts of the air which it had originally contained. With this more perfect instrument of research he soon accumulated a vast array of important facts. He demonstrated that the electrical excitation of the negative terminal of the tube produced a molecular disturbance which affected the surface of the terminal, and that on this disturbance being communicated to the ratified gas in the tube a real torrent of material particles, which he treated as molecules of the residuary gas within the tube, rebounded from the surface of the negative pole in a direction normal to that surface. He also determined the velocity with which these molecules moved, which, of course, varied with the intensity of the current, and found that they did so at a speed varying from one to two miles in a second. But the most interesting observation which he made was that the torrent was composed of particles so material in character that a magnet exercised a powerful effect upon them, curving their trajectories in the same manner as gravitation acts upon a bullet fired from a gun. He also noted that the phosphorescent glow of the tube did not emanate from the particles themselves, but was produced by their impact upon the surface of the glass, to which they evidently imparted sufficient energy not only to render it luminous, but also to raise its temperature. From these effects he justly concluded that the particles thrown off from the negative pole and striking the glass were truly material. These experiments have been repeated and verified by a large number of observers in the first rank of electrical science, who have also come to the conclusion that particles of matter are projected by electricity at great speed from the negative terminal of a vacuum tube, and that this effect, coupled with their behaviour towards the magnet, and their sensitiveness to the approach of any conductor, affords a positive demonstration that the component particles of gas in the tube had been electrified by the discharge. In these remarks I have advisedly used the word “particle” in connection with the stream projected from the negative pole of the vacuum tube. Unfortunately, confusion
is often created by the indiscriminate use of the terms “atom” and “molecule.” The word “atom” is properly applicable only to matter in its ultimate condition of divisibility, and the word “molecule” to a particle of matter formed by the chemical combination of two or more atoms. The number of possible combinations is practically infinite, and I have long thought that the lines in the spectrum indicate the more or less complex nature of such combinations, in the case of each of the substances experimented upon, those which give the fewest lines being less complex in molecular structure than those which present many lines. Professor J. J. Thomson, in a paper published in the Philosophical Magazine in October, 1893, points out that the electrification of a gas is not a mere mechanical process. In his view it is a chemical or quasichemical one, which goes on within its molecules. He conceives that part at least of the materials must be split up into atoms before electricity can be carried, for he shows that new combinations are formed when the dissociated atoms part with electricity. He maintains that a molecule of a gas, quâ molecule, cannot be electrified.
Two important questions at once arose from these demonstrations,—namely, What is an electrified particle? What progress is being made in physical research when we find that electrified particles are substituted, by electrical excitation, for the body of which they had been the component parts? These questions have not yet been wholly solved, but Crookes's experiments show clearly enough that the medium for the transmission of electricity must consist of ponderable matter in some form or other. Lord Kelvin, in commenting on these experiments and demonstrations, remarks that they had led him to the conclusion that the molecule contains both what are called ether and ponderable matter; though, as already observed, I think it much more probable that the supposed ether is neither more nor less than the ultimate form of matter, and that the molecule of any of the so-called simple bodies of chemistry is composed of atoms of some one homogeneous substance in a special condition of combination.
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It was whilst engaged in similar researches, following the lines of discovery propounded by Hertz, that Röntgen noticed the peculiar nature and some of the properties of the dark radiations to which he gave the name of X rays. Now, when we speak of these rays as “dark radiations,” we are using the term “dark” merely with reference to the limited visual capacity of the human eye, which, as is well known, can only perceive light-waves that are not shorter than 1/63000 of an inch and not longer than twice that length, and it is by reason of this limitation in the capacity of the human eye that the ultra-violet rays at the violet end of the spectrum
make no sensible impression upon the retina. It is more than probable that in this we differ from many of the lower animals, especially amongst those which roam by night; but upon this point I do not pretend to be in a position to offer more than the suggestion. These ultra-violet rays, however, are those which most strongly, if not exclusively, affect the sensitive photographic plate, and therefore photography by means of that peculiar anomaly invisible light—viewing its effect from the standpoint of human vision only—is no new thing. What is entirely new in the X rays, and the points in which they differ completely from the Hertz electric waves (which, as already shown, possess all the properties of, and are capable of being reflected, broken, and polarised in the same way as, ordinary light) are that they have an incomparably smaller speed, are with difficulty reflected, are incapable of being refracted or polarised, and otherwise differ in so many respects from the latter as altogether to upset the current ideas about light. It has, indeed, been suggested that they belong to the borderland between light and electricity discovered by Hertz, and only those who have watched the progress of the researches made by experimenters following the suggestions of Hertz, and especially those made by Professor Lenard, could possibly have foreseen the existence of radiations having such singular properties.
Shortly before Hertz's death he noticed that, when the streams of apparently luminous matter already alluded to were projected against a thin plate of matter opaque to ordinary light, the light produced appeared to pass through it. For the purposes of his experiments in the same direction Professor Lenard transferred the kathode rays from the tube in which they were generated into another tube where he could use them in a variety of ways. This transfer was effected through a window composed of leaf aluminium, which would have completely intercepted rays of ordinary light, but through which the kathode rays passed at once into the next tube, a strong smell of ozone being developed during their passage. He afterwards found that a large proportion of the rays which passed through the window were invisible, for when the whole were thrown upon a paper screen covered with fluorescent matter this matter began to glow precisely as it would have done under an ordinary beam of sunlight or of electric arc light; but his experiments showed also that in many other respects their behaviour was quite different from that of ordinary light. To ascertain the nature of this difference he passed them through a variety of gases, liquids, and solids, his operations being thus summarised by a writer from whose work I have derived much assistance in preparing this part of my address:—
“When Lenard made the kathode rays pass through different gases, liquids, and solids their behaviour proved quite different from that of ordinary light. Various substances are, we all know, not equally transparent to sunlight, but their different degrees of transparency depend upon their inner structure or their chemical composition, not upon their density. Glass has a greater density than paper, but it is transparent to ordinary light, while paper is not.”
“With the kathode rays it was quite the reverse. Paper was more transparent to them than glass, and aluminium, which is slightly less dense than mica, was more transparent than mica; as to the denser metals, such as gold and silver, they were quite opaque for the kathode rays even in very thin leaves. The same was noticed with all gases: their transparency, too, depended entirely upon their density. At the ordinary atmospheric pressure the kathode rays ceased to act upon the phosphorescent paper at a distance of a little over 2in., but in rarefied air they travelled a distance of 6ft. without being absorbed; and when Lenard experimented upon gases of different densities, such as oxygen and hydrogen, he found that it was sufficient to rarify oxygen to one-sixteenth part of its usual density to render the two gases equally transparent.”
“In short, the absorption of the kathode rays proved to be in direct proportion to the density of the medium which they passed through. ‘Like inertia and gravity,’ Lenard wrote in December, 1895, ‘the kathode rays depend in their absorption upon the mass of the matter they traverse. They do not behave like light, but like a cannon-ball, which is arrested in its course by the density of the heap of earth which it has to pierce.’ Moreover, while usual luminous vibrations would take no heed of a magnet placed near their path, the kathode rays explored by Lenard were deflected by a magnet from their ordinary rectilinear directions. And yet—such is, at least, Lenard's opinion—the magnet acted not upon the rays themselves, but upon the medium they passed through; and what seemed still more incomprehensible was that the action of the magnet depended upon the way in which the kathode rays were generated—the more the air was rarefied in the vacuum tube where they took origin the greater was the magnetic deflection.”
“At every step the physicist thus met with some new problem which he could by no means explain under the now current theory of luminous radiations. And finally, as if it were to establish one more affinity between these extraordinary rays and common light, Lenard discovered that” when a photographic plate was brought near to the aluminium ‘window’ the silver salts of the plate were decomposed by
the invisible rays. One step more—a simple piece of wire placed between the ‘window’ and the plate—and Lenard would have obtained a shadow photograph similar to those obtained a few weeks later by Röntgen.”
It is certainly singular that, even by accident, Lenard should not, during the course of those experiments, have discovered the peculiar property which some at least of the invisible rays, the existence of which he had ascertained, possessed, of throwing the shadow of any substance which absorbed them upon the fluorescent screen. This was the result of the further step taken by Röntgen.
It seems that his researches were being carried on upon somewhat different methods from those adopted by Lenard, and the experiment which led to the discovery of the rays in question, as distinguished from the other kathode rays, is thus described: “Having made a Crookes's or Lenard's tube glow in the way already mentioned, he surrounded it with a close-fitting shield of blank paper, and when the light from the glow was thus effectually intercepted, and the room completely darkened, he found that a paper covered on one side with barium platino-cyanide lit up with brilliant fluorescence when brought into the neighbourhood of the tube, precisely as if a ray of sun or arc light had been thrown upon it. It at once became evident to him that the effect produced was due to the presence of rays differing from those which were intercepted by the black paper in which the tube was wrapped, and what was especially remarkable was that they made the fluorescent screen shine, even at a distance of 6ft. from the covered tube.” A full account of his experiments, translated from the German, appeared in Nature, of the 23rd January, 1896, and contains a statement of many of the facts that have been published in the ordinary Press, showing the force with which these dark rays penetrate solids. Boards, books, blocks of ebonite, and other substances quite opaque to the rays of the sun or other light-rays, proved to be as transparent to the X rays as glass is to the visible rays. A plate of aluminium half an inch thick was penetrated by them, and they appear to be only effectually resisted by comparatively considerable thicknesses of the heavier metals. When, however, a human hand was interposed between the darkened tube and the screen a shadow was seen, showing the bones darkly, with only faint outlines of the surrounding soft tissues; but, amidst all the recorded experiments with these rays, I have searched in vain for the mention of any attempt to determine whether they are, and, if so, to what extent, intercepted by the combinations of orange and yellow glass used to prevent the action of the ordinary actinic rays upon a sensitized photographic plate. But whatever interest attaches to Röntgen's discovery
from the power which the X rays afford of exploring the human body, its chief importance for theoretical science lies elsewhere. Unlike the electric waves of Hertz, they are not capable of either refraction or polarization, and, although they appear to have something in common with the invisible ultraviolet rays of the spectrum, they differ from them in most respects, and especially in their electric effects. It appears that they do not emanate from the kathode itself, but originate from the glass of the tube where it is struck by the kathode, rays, and, whilst these are capable of being deflected by the magnet, the X rays take no notice of it, and pursue their course, in spite of any interposed medium, in perfectly straight lines. What, then, are these rays? For the present the ordinary enquirer must be content with the knowledge we have of their power of piercing substances impervious to all ordinary rays of light, and of recording a shadow upon a photographic film. Whether or not the results of the experiments already made are to be accepted as affording evidence that waves of compression and rarefaction are produced in the so-called ether by the passage of light through it, must remain open until the learned physicists who are now engaged in investigating the structure and movements of the ultimate particles of matter have had time to deal with the application of this new discovery to the solution of the problems involved. Röntger himself concludes the paper referred to in these words: “Should not the new rays be ascribed to longitudinal waves in the ether? I must confess that I have in the course of this research made myself more and more familiar with this thought, and venture to put the opinion forward, whilst I am quite conscious that the hypothesis advanced still requires a more solid foundation.”*
Discovery of Argon.
It might well have been thought impossible that any of the constant elements of atmospheric air, which has so frequently been analysed and examined by chemists and physicists in the foremost rank of science, could long have escaped detection, but, singularly enough, it was not until last year that the nitrogen of which it is so largely composed, amounting, in effect, to nearly four-fiths of its volume, was conclusively proved to be associated with an unknown chemical element. The compound character of atmospheric nitrogen had, however, long been suggested, and even to some extent demonstrated, by the older chemists, for we find that Berzelius, a contemporary
[Footnote] * Since this part of my address was written I have received the April number of Nature, in which readers will find papers of the highest interest by Professors J. J. Thomson and Oliver Lodge, on the subject of the Röntgen rays, but the contents do not materially affect what I have ventured to bring before the society.
of Davy, satisfied that it was a compound body, was under, the impression that it was associated with an inflammable, base combined with oxygen, for which he proposed the name Nitricon. But he is said to have distrusted or abandoned this hypothesis in consequence of experiments made by Davy, who also believed that atmospheric nitrogen was a compound body, of which oxygen formed an element, and endeavoured, but in vain, to detach the latter by means of the vapour of potassium. But his experiments led only to the negative result that the divellent power of potassium was insufficient to overcome the affinity by which oxygen was held in combination with the other elementary matter associated with simple nitrogen. Mr. David Low, of Edinburgh, who published an important treatise on the “Simple Bodies of Chemistry,” in 1856, also treated atmospheric nitrogen as a compound substance, and mentioned that, from its known characters, the same opinion had long been entertained, but that, as all attempts to decompose it had failed, chemists had been content to acquiesce in regard to it, in their own maxim, that it must be regarded as simple because they had not been able to prove it compound, whilst he points out that all the known circumstances ought to have led to the juster conclusion that it should be treated as compound, although chemists had not been able to prove it so by means of the agents which they had employed to dissociate its elements. He also mentioned that, although it had theretofore resisted all attempts to dissociate it directly from the substance with which it was evidently compounded, there were reasons for believing that in many unheeded experiments in the laboratory its compound nature had manifested itself, especially in cases in which it was impossible otherwise to account for its presence.
But this point has now been set at rest by recent investigations made by Lord Rayleigh and Professor Ramsay, which have not only resulted in conclusively establishing the compound nature of atmospheric nitrogen, but also in showing that the substance with which it is associated is a gas previously unknown, to which they have given the name of argon. But the question, What is argon? still remains to be solved. So far as present researches into its chemical character have been carried, it is found to possess properties of so peculiar a description as to raise questions of paramount importance for chemistry.
It is well known that, prior to this discovery, Lord Rayleigh had been for many years engaged in inquiries as to the densities of several of the gases, and that he had found discrepancies in many of the results obtained which could only be accounted for in one of two ways,—namely, either by the occurrence of unavoidable errors in experiment or by the
assumption that some of the supposed simple bodies were in reality compound. These discrepancies were especially obvious in the ease of atmospheric nitrogen, for when obtained from that source by any of the methods usually employed for the purpose, it was invariably found to be heavier in an appreciable degree than when obtained from any other compound of which it formed a part. This would doubtless have been noticed by Berzelius, or Davy, or Faraday, who had also experimented on atmospheric nitrogen, had any of them compared its relative densities when obtained from that and other sources, such, for example, as ammonia, and yet the fact that nitrogen entered into the composition of ammonia was one of those which led some of the older chemists to infer its compound character. It would certainly be strange if Lord Rayleigh was unacquainted with these older speculations, although, so far as my reading goes, I have seen no reference to them in the published accounts of his experiments. The difference actually found by him between the weight of atmospheric nitrogen and that chemically obtained from its compounds was that its average weight in the former was 1.2572 grammes, whilst in the latter it was only 1.2505, a difference of ·0067 grammes. Many suggestions were offered to explain this discrepancy, all of which, however, were based upon the supposition that atmospheric nitrogen was the purer of the two, and that the nitrogen chemically prepared must still contain some lighter gas. But upon proper test experiments being applied in connection with these suggestions they were found to be untenable, and it soon became clear that the supposed position must be reversed, and that atmospheric nitrogen must have in combination some heavier gas previously unknown. For some time after this discovery the test of separating argon from the atmospheric compound proved to be a very difficult one. Nitrogen, chemically speaking, is an inert substance, by which is understood that it is very difficult to force it into combination with other substances, and it soon became obvious that argon could only be obtained as a residue after removing from any given quantity of atmospheric air all its other constituents. This was effected in several ways, all of which were slow and wearisome. It has, however, been justly remarked that “chemical bodies must be taken as we find them, and that those amongst them which, even in the hands of the best experimentalists, yield only to methods outside of ordinary chemical routine are precisely those whose investigation leads most to the extension of chemical knowledge.”
The chemical nature and properties of the new substance have as yet been only partially ascertained, the chief obstacle
to investigations into this branch of inquiry arising from the difficulty of obtaining it in sufficient quantity for experiment; and, in fact, what is known is chiefly negative. Mr. Crookes submitted it to spectrum analysis, the result of which led him to suppose that it consisted of a mixture of two gases; whilst the experiments of Olszewski into its temperature of liquefaction and its critical temperature and pressure seemed also to indicate that it is a compound substance, but that the mixture could only contain a very small proportion of another gas.
Its leading physical properties were more easily ascertained. It was found to be colourless and inodorous, to have a density of about twenty times that of hydrogen, although it is probable that this may be exceeded. It is more soluble in water than oxygen or hydrogen, and therefore it is not improbable that in drinking unboiled water we imbibe a proportionately larger quantity than we inhale in breathing. It requires a very low temperature for liquefaction, Professor Dewar having ascertained that it liquefies at 305° Fahr. below zero, and is converted into opaque ice at 310°. These physical properties, however, have as yet afforded little aid in determining its chemical nature; but, as this question is being investigated by some of the greatest chemists of the day, and, amongst others, by Mendeleeff, Berthelot, and Professors Dewar and Ramsay, we may expect to receive, within a reasonable time, full information in respect to it. In the meantime it is supposed to be a tri-atomic form of nitrogen, as ozone is a bi-atomic form of oxygen; and many circumstances already known—for example, its concurrent appearance in nature with nitrogen, the difficulty of separating them, their common inertness—exaggerated in argon—their common lines in the spectra, their double spectra, and the outer resemblance of their benzine compounds as shown in Berthelot's experiments —are said to lend strength to this hypothesis.
The announcement recently made by Professor Ramsay that he had discovered that argon is contained in a mineral called clevéite, is likely to lead to a rapid increase in our knowledge of its chemical character and properties. A bright-yellow line in the spectrum of the sun's chromosphere had long been observed with interest, and was generally ascribed to an element unknown on the earth, but widely spread on the sun, from which circumstance it had been called helium. Now, this element was lately captured by Crookes in a glass tube in the laboratory, quite unexpectedly, in the course of investigations which Professor Ramsay was making with a view to extracting and analysing the gas contained in clevéite, said to be nitrogen. He communicated his discovery to Professor Cleve, of Upsala (in whose honour the mineral had
been named by Nordenskjold), who at once extracted the New gas, which was submitted for spectroscopic examination to Thalen, one of the best spectroscopists of the day, who confirmed Crookes's statement, but found no trace of argon. Ultimately, however, Professor Ramsay, while boiling clevéite in weak sulphuric acid, not only obtained helium, but also argon, devoid of the gas which is usually found associated with atmospheric argon, and which may be the cause of the high density of the latter. Thus several new forms of gas have already been discovered, whilst more are apparently in view. From these facts it appears certain that the means now available for producing argon in a pure condition, coupled with the further discoveries made in the search for it, are sure to launch both chemistry and physics into a new domain of philosophical inquiry, which will not only materially widen our knowledge of facts and our theoretical views in chemistry, but will probably also lead to some more definite conceptions of the nature and structure of matter.