The easy passage of the new rays through carbon and its compounds with oxygen, hydrogen, and nitrogen, is the particular faculty by which they have gained their sudden fame; since it is that to which is due the astonishing transparency to them of all bodies of organic origin. They traverse pineboards hardly enfeebled; almost disregard ebonite, vulcanite, gelatine, leather, jet; and run through printed matter with a facility unknown to the most indolent reviewer, producing their characteristic effects, fluorescent and photographic, after a journey through a book of a thousand pages. Hence a sixpence placed in the midst of a bulky tome becomes fully apparent as an obstacle to their transit. And the heavier the metal, the more conspicuous its interposition. Professor Dewar has in fact ascertained that these gradations of transparency are regulated by atomic weights. The complexity of the molecule is of no consequence in this respect; only the mass of the individual atom tells. Electrical or optical properties, colour, crystalline structure, chemical combinations, count for little or nothing; the atomic principle is dominant throughout. Here we have one of many symptoms that the rays concern themselves with matter only in its finest subdivisions, ignoring it as an extended mass. It should be added that they suffer no deflection in a magnetic field.

What then are these singular emanations? Their indifference to the magnet shows decisively that they are not streams of electrified particles, like the cathode-rays. They may accordingly be set down with confidence as a mode of ethereal vibration. The question inevitably follows: Should they be regarded as a peculiar kind of invisible light'? The title is no longer paradoxical; for the scientific meaning of the word 'light' has of late incalculably widened. Clerk Maxwell's electro-magnetic theory of light, splendidly verified by Hertz, lends coherence and unity to ideas concerning the energies of the material universe. For we now know that the 'luminiferous ether' is possessed of far more versatile powers than that phrase imports. It transmits electrical oscillations some miles long, ultra-violet wavelets, more than 120,000 of which are crowded into a single inch, together with endless intermediate undulations, a small fraction of which serve us for purposes of vision. Yet all belong to the same grand series. They travel with an identical speed of 186,000 miles a second; they obey the same laws of reflection, refraction, and polarisation; all can be extinguished by interference; and all are therefore to be regarded as disturbances of one medium differing only in scale and period. Like the waves of the sea, these various oscilla

tions are of a transversal character; they are such as could be executed in an incompressible, highly elastic medium.

But the code of laws they obey is entirely disregarded by the X-rays. These, so far as authentic experiments have yet shown, can neither be refracted nor regularly reflected. They are accordingly incapable of being concentrated by mirrors or lenses; and this incapacity embarrasses their thorough investigation. Hence, it is hardly to be wondered at that efforts to polarise them, or produce interference-effects, have proved futile. None the less, they have many qualities in common with ultra-violet light. They decompose salts of silver; they excite, in the substances it similarly affects, fluorescence and phosphorescence. An experiment described by Professor Dewar before the Royal Society on February 13, is decisive on this last point. He had previously found that platino-cyanide of ammonium, immersed in liquid air, during a brief exposure to light from the upper part of the spectrum, phosphoresced brilliantly when the frigid fluid was poured off. It now appeared that precisely the same result ensued upon stimulation, under similar conditions, with the X-rays. Their electrical properties have been examined, among others, by Professor J. J. Thomson of Cambridge, and his conclusions are embodied in a Paper named at the head of this article. They are of the utmost theoretical importance. For not only does an electrified body rapidly lose its charge, whether positive or negative, when made the target of a Röntgen-beam, but the effect cannot be prevented by any amount of close-packing with insulating material. Thus, all substances become electrical conductors while transmitting the enigmatical rays—a circumstance implying the production of molecular changes too subtle to be otherwise perceptible. This faculty of causing electrical leakage is so strong that a zinc-plate a quarter of an inch thick is an insufficient screen against its exercise; and it is shared, as regards negative charges, with Lenard's cathode-rays and ultra-violet light. They do not, however, leave the bodies upon which they are thrown in a neutral electric condition. Some, such as gold, copper, and iron, acquire through their influence a positive charge; others, sodium, lead, and zinc, for example, a negative one. This unexplained diversity has been quite lately discovered by Professor G. M. Minchin.*

The hypothesis has accordingly been advocated that the X-rays are ultra-ultra-violet' light-that they consist in excessively minute and excessively rapid transverse vibrations

* The Electrician,' March 27, 1896.

of

*

of the universal ether. It is not absolutely negatived, as Dr. Schuster has pointed out, by their anomalous behaviour. If their wave-lengths are measurable on the molecular scale, they might easily escape refraction, and undergo reflection only of the observed vague and elusive kind. It is suspected, moreover, that they are too little homogeneous for mutual destruction by interference; for they certainly to some extent differ among themselves. Yet their assimilation to ordinary light appears to raise more difficulties than it removes; and the process of explaining these away inspires distrust.

We turn then hopefully to the alternative rationale of the X-rays cautiously advanced by Professor Röntgen himself. Abandoning, in view of their unusual properties, the attempt to force them into the groove of luminous theory, he tried a larger framework for his facts.

'A kind of relationship,' he writes, 'between the new rays and lightrays appears to exist; at least the formation of shadows, fluorescence, and the production of chemical action point in this direction. Now it has been known for a long time that, besides the transverse vibrations which account for the phenomena of light, it is possible that longitudinal vibrations should exist in the ether, and, according to the view of some physicists, they must exist. It is granted that their existence has not yet been made clear, and their properties are not experimentally demonstrated. Should not the new rays be ascribed to longitudinal waves in the ether?'

The suggestion is of great speculative interest. In the textbook theory of light, the qualities attributed to the ether, or transmitting medium, are those of an elastic solid-capable of being strained or distorted, though not of being compressed. But the assumption of incompressibility is only made in the absence of better knowledge; it is by no means essential to the validity of the theory. And, unless that assumption be absolutely true, there must arise, besides the transversal disturbance called light, the laws of which have been wrought into a monument to human genius aere perennius, a longitudinal or condensational disturbance which might be designated ethereal sound. † Lord Kelvin, in his 'Baltimore Lectures' of 1884, spoke confidently of its presence; and he laid before the Royal Society, on Feb. 13 last, the plan of a simple arrangement for producing electrical oscillations of the longitudinal or sonorous type, which should, on Röntgen's hypothesis,

* 'Nature,' Jan. 23, 1896.

† See an important essay on Longitudinal Light,' by G. Jaumann, in 'Annalen der Physik,' Bd. lvii. p. 147, Jan. 1896; translated in the Electrician' for March 13 and 20, 1896.

belong to the class of his X-rays. No experimental results of the kind are, however, at present forthcoming.

If the new photographic agency may be fitly characterised as 'ultra-violet sound' (a term of Professor S. P. Thompson's coinage), the scope of the Würzburg discovery widens indefinitely. In that (unluckily doubtful) case, it affords a clue to the genuine nature of the ethereal basis of the material universe. It gives a holding-ground to thought on the subject which has hitherto been wanting; since the qualities demanded for the transmission of light, although they must in some way correspond to those belonging to the medium filling space, are of a kind entirely transcending experience, while a substance capable of propagating waves of condensation and rarefaction should bear an intelligible resemblance to ordinary matter. Some such definite piece of information has long been vainly sought for as a lever by which to raise knowledge towards the higher plane within view for a score of years. Lord Kelvin anticipates unhesitatingly the establishment of a general theory which shall include light (old and new), old and new knowledge of electricity, and the whole of electro-magnetism'; and the approaching disclosure can be foretold of a more profound correlation of the physical forces than can at present be distinctly apprehended. Towards the attainment of this end, researches concerning the Röntgen rays can scarcely fail to contribute.

A remarkable attribute of the longitudinal waves with which they have been conjecturally associated, is their all but instantaneous transmission. The theoretical velocity of such oscillations exceeds that of light as much as the measured velocity of light exceeds that of sound. A hint seems thus obscurely thrown out of a possible connexion between them and the inscrutable power of gravity. No sensible timeallowance can be made for the action of the central force in the planetary system; otherwise symptoms of instability which have never been observed should be tolerably obvious. Thus a difficulty would be removed by the supposition that the gravitational push or pull was exerted through the intervention of waves of ethereal pressure. But this topic is at present very far from being ripe for discussion.

Amidst the confused accounts of experiments with the X-rays which have thickened the air during the last three months, assertions of their presence in ordinary light could not but attract special notice. Vacuum-tubes, found indispensable by most, were discarded as superfluous by a few operators. Thus, Lord Blythswood obtained unmistakable radiographic

effects