fills most of the tube. A dark intervening space, however, always remains. As rarefaction proceeds, the column shrinks, and the glow spreads; until, at last, both become nearly extinct. The electrical circulation does not therefore cease; but a wonderful change occurs in the manner of its maintenance. Instead of being conducted, the discharge seems to be actually carried by streams of what Mr. Crookes termed radiant matter.' These invisible emissions are highly susceptible to magnetic influence; they can be intercepted by metallic or other screens ; and they are capable according to some authorities — of doing real mechanical work by turning the vanes of the radiometer, a sort of tiny windmill sometimes inserted into the exhausted tube. Repelled from the negative pole at an estimated average velocity of a couple of kilometres per second, these negatively electrified molecular torrents form the wellknown cathode-rays,' not the least remarkable property of which is their power of exciting fluorescence in the substances struck by them. Under their bombardment' the walls of their glass-prison become resplendent with greenish light; a ruby placed in their path blazes up in crimson radiance; sulphide of calcium glows violet blue; and other bodies become vividly and variously luminous.

We have called the vacuum-tube their 'prison'; but their incarceration is no longer so strict as it used to be. The late Heinrich Hertz of Bonn-immortalised by his discovery of electric waves-ascertained the permeability by them of thin metallic films; † and his pupil, Dr. Philipp Lenard, procured their escape through an aluminium window for the purpose of testing their qualities in the open.‡ These proved to be somewhat surprising. Gases of whatever description acted upon them as turbid media, stopping them by vague diffusion, as milky water stops light. Under magnetic influence, they formed, as it were, a spectrum, in consequence of the unequal deflection of individual rays. Cathodic beams are then of heterogeneous composition, like pencils of white light. Slight photographic effects were derived from them; they were found to possess the faculties of dispersing negative electric charges, and of penetrating, to some slight extent, opaque solids. The questions, however, raised by these experiments were of too abstruse a nature for easy popular apprehension. There was nothing to show in connexion with them, and very much to

* Fluorescence is an evanescent kind of phosphorescence. It vanishes with the removal of the exciting cause, while phosphorescence is more or less lasting. Wiedemann's 'Annalen der Physik,' Bd. xlv. p. 28, 1892. Ibid., Bd. li. p. 225; Bd. lii. p. 23, 1894.

learn ;

learn; so that discussions regarding them were limited to experts. But public indifference was changed into a flare of excitement by the news of some extraordinary observations made at a Bavarian University.

Wilhelm Conrad Röntgen is by origin Dutch, by education Swiss, by long residence German. He has had a strictly professorial career. The vicissitudes of his life have consisted in the frequent exchange of one lecturership for another. From the University of Zürich he followed Kundt, whose favourite student he was, to the University of Strassburg; thence he removed to Würtemberg, returned in 1876 to Strassburg, proceeded later to Giessen, and accepted in 1888 the Chair of Physics, which he now occupies, at Würzburg. Still in the prime of life (he was born in 1845), he has enjoyed, for a quarter of a century, a high scientific reputation, although of the purely technical kind that seemed little likely to bloom out into world-wide fame. His mind had, however, been trained by intent study of the recondite forces of nature to neglect no hint as to their workings; and he was accordingly prepared to grasp the unlooked-for opportunity which came in his way last autumn.

He was experimenting with a Crookes's tube made luminousby electricity, but closely enveloped in black paper. The room too was darkened, and absolutely nothing could be seen, until a sheet of paper coated with platino-cyanide of barium—a highly fluorescent substance--was brought near the tube. A curious effect then became apparent. Instantaneously, as if smitten with a ray of bright sunshine, the paper lit up; and its adventitious lustre persisted at a distance of two metres from the shielded source of energy. This was the first manifestation of the X-rays,' as their discoverer not unfitly designated them. They form indeed one of the unknown quantities in a particularly complex set of equations.

exercise was attended by most peculiar effects. Keeping his hand for some minutes between the exhausted bulb and a sensitive plate enclosed in a wooden box, and developing as usual, Professor Röntgen was confronted with a perfect representation of four skeleton fingers! The skin, veins, and muscles were so transparent as to have hardly left a trace, while the relatively opaque bones stood out alone, denuded of their customary paste and cover.' Hamlet's aspiration was realized. The 'too too solid flesh' had virtually resolved itself into a dew' at the magic touch of the Röntgen rays.

For the art thus remarkably initiated, the word 'photography' is plainly a misnomer. Mr. Justice Wills proposes to substitute for it 'scotography'; the term 'radiography'* appears to us in many ways preferable; and we trust, at any rate, that the cacophonous 'rontography' will be permitted to die without benefit of clergy. The methods of the old and the new photography are essentially different. In the former, an optical image of some object is thrown, in a dark chamber, upon a prepared plate, and there chemically fixed. In the latter, the object is simply interposed between the fount of influence and the plate, which is carefully protected from daylight by screens of material impervious to it, although penetrable with ease by the invisible agency. The resulting impressions are silhouettes or transparency-pictures; they originate from true shadows of the things portrayed, the structure of which is given in terms of the opacity to the X-rays of their various parts. And this opacity is of so abnormal a kind that the process brings about startling inversions of visibility, the commonly unseen being laid bare, while the encasing substances which alone show to the eye are rendered inconspicuous or evanescent. The familiar surface-photography has, in short, obtained as an ally a wonderful art of organic portraiture, already promoted and availed of with unexampled zeal, although it would have been pronounced six months ago, alike by the learned and the unlearned, a fantastic impossibility.

Its productions strike us with inexhaustible wonder. We have scarcely yet got inured to the notorious skeleton-hand holding up a signet-ring poised round the vanished bulk of one of its fingers; nor to the oft-displayed coins, scarcely veiled by their leathern receptacle, or to arrays of instruments, ineffectually concealed in wooden cases; still less to the feathered creatures, stripped to the bone, and piteously displaying their

Suggested by Professor Goodspeed, of the University of Pennsylvania. The objection that radiography' is a mongrel of Greek and Latin is perhaps a trifle pedantic.

anatomy

anatomy without the shelter of a quill. Plumage indeed of every kind is non-resistent to Röntgen's rays; and that the skin of a fish is scarcely less transparent than the element in which it moves, will be seen at a glance by our readers. It was taken by Mr. A. A. Campbell Swinton with a focustube' invented at King's College, by means of which the shadows thrown on the sensitive plates gain greatly in sharpness. The companion-pictures are likewise specimens of his best work; and they have been admirably reproduced by the Swan Electric Engraving Company. The embryonic character of the skeleton of the two days' old puppy-dog will be noticed; also the poor frog's broken leg, the discomfort attendant on which must have considerably detracted from the 'marshy joys' of his life. The spine and ankle are masterpieces; the dissecting faculty of the X-rays could not be more perfectly displayed; and the pair of spectacles, in its shagreen case, illustrates both the extraordinary permeability by them of animal-hides, and the marked difference in opacity between steel and glass. Among other curiosities of portraiture, we hear of pigeons and sparrows, mice and rabbits, anatomised in despite of fur and feathers; and of a crayfish, taken by Mr. W. A. D. Rudge, the shell of which proved to be of about the transparency of aluminium.

The surgical uses of Professor Röntgen's marvellous discovery are innumerable; and fresh instances of its successful application furnish themes for weekly comment in the 'Lancet' and other specialist papers. The veil of flesh may now at will be withdrawn from the bony structure beneath, the diseases and malformations of which can be studied in autographic representations. Calcareous deposits in the various organs of the body, ossifications, intruded foreign substances, stand out in relief, indicating with certainty the seat of disorder, guiding the hand of the operator, and rendering exploratory processes superfluous. Suitable patients are at a premium. A woman who has absorbed a needle, a man harbouring a projectile, is a persona grata at every Surgical Institute in the Old and New Worlds. Had Aspromonte been fought in these tempi leggiadri, Garibaldi's ankle would have been quit of its bullet in fewer hours than the weeks of its actual lodgment.

The profit derivable to medical diagnosis from the novel method, although less direct and immediate than its surgical utility, is sure before long to accrue. But the delicate modifications of exposure and development, by means of which the internal organs will be brought, it is hoped, within the range of radiographic inspection, must be worked out tentatively, by

the

the aid of instruction from daily experience. An important step has, however, already been taken by the substitution, to a certain limited extent, of the human retina for the sensitive plate. Professor Salvioni's 'cryptoscope,' described before the Medico-Chirurgical Academy of Bologna, February 6, 1896, consists of a tube, completely closed at one end by a disc of black cardboard overlaid with fluorescent sulphide of calcium, for which Edison substitutes the tungstate of the same metal. Under the influence of the X-rays, the diaphragm thus prepared becomes brilliantly luminous, and the shadows of interposed objects, more or less dense according to the degree of their transparency, are readily seen by an eye applied to the open end of the observing cylinder. We can, in this way, count at a glance the bones in our own hand, or inspect the contents of our neighbour's pocket. Hence instant discernment of the 'invisible' must henceforth be reckoned among the resources of civilization. So handy an instrument for examining the interior of the body must, before long, find its way into the hands of every physician. Nor is it impossible that, by its employment, much information may be gathered regarding the nature of vital processes. The circulation of the blood may be watched, the throbbings of the heart, the progress of digestion, the elaboration of secretions. Even a bony rampart will not perhaps avail to protect from scrutiny the organs behind it. For M. Charles Henry has discovered that a coating of sulphide of zinc diminishes or abolishes opacity to Röntgen's rays; and he hazards the forecast that, by painting the breastbone with this fluorescent salt, a view right through it of the heart and lungs may be afforded. Nor do we dare to pronounce him over-sanguine. The practical performances of radiography warrant high hopes for its future. M. Brouardel of Paris has induced it to display the contents of infernal machines; volumes innocent of aspect have in the same way been shown by MM. Girard and Bordas to be crammed with explosives and projectiles; and thus, the peril of forcing open suspicious parcels can be evaded by merely exposing them to emissions from a vacuum-bulb. The similar detection of flaws in pieces of metal promises to afford a safeguard against accidents to machinery, or with ordnance; † a great advance in dentistry, through the same agency, is announced as imminent; and it supplies an easy and unfailing test for the genuineness of diamonds. But the versatility of its powers can hardly yet be appreciated.

*Comptes Rendus,' Feb. 10, 1896.

† Nature,' Feb. 21, 1896.

The