A New Mode Of Motion. Professor Rontgenls discovery of anew form DI energy affecting sensitive plates reached this cOuatry in a somewhat exaggerated and in- accurate shape. His own paper, written with eonspicuous scientific caution and modera- tion, discloses observation of facts in the bighest degree interesting, which in the 'present state of our knowledge are exceedingly difficuilttoaccountfor upon any consistent theory. W'hat may be called the popular and superficial aspect of his discovery has been seized upon with-avidity. Shadow photographs have suddenly become an article of commerce; no illustrated ~paper is complete without reproductions of pic- tures showing the transparency of the human liand; every one who can command a vacuum tabe and a few sensitive plates is busy repeating .'tho primary exeriment; ladies prattle of the ~new photography, and physicians already dream 'of unheard-of cures by its agency, and the market price of exchaugted tabes-many of them of little valne for the purpose in view-is rapidly rising. The cardinal facts observed by Professor Rontgen are susceptible of plain and easy state- ment. The appearance of a high vacuum tube through whic7h an electric discharge is pas- sing is tolerably familiar. It emits a beau- tiful phosphorescent light varied by brushes of intenser luminosity at the electrodes. If we enclose the whole apparatus in a wooden box, or cover it completely with blackened paper, there is an end of the luminous phenomena so far as the observer is concerned. Professor RBnt- gen, however, followin,g out the beautiful experi- ments by which Lenard showed the power of cathode rays to pass through aluminium, has dis- covered that after the whole of the light is thus cut off some force or energy remains, something which passes freely througi the wooden box, an is capable of causing fluorescence in various sub- stances; notably in platinocyanide of barium, even at distances of a metre or two. The interest- ing question, a question which indefinite repeti- tions of the photographic experiment do nothing to solve, is-What is the unknown force which thus persists after the light of the vacuum tube is completely cut off by an opaque screen7 It is not light in the ordinary sense, for it does not affect the eye. Nor is it light in the more extended sense sometimes given to the word in order to describe vibrations or rays which, though invisible, are obviouslyof the same nature as visible rays. When we have reached the most rapid vibrations that the ear can recognize as a musical tone, we can double or quadruple their rapidity, and describe them as sound which is inaudible merely because the range of our hearing is limited. In the same way we may speak of invisible light, meaning a manifes- tation of the force that gives us the sensa- tion of light, which is either above or below the rapidity of vibration that falls within the range of the human eye. We identify these inaudible or invisible vibrations by physical tests. But Professor R6ntgen's rays are not exactly light, even in this sense. They do not behave as light behaves, whether visible or in- visible. They are not susceptible of refraction,or diffraction, or concentration by alens,or reflection by ordinary reflecting surfaces. That they pass through opaque sabstances is perhaps less im- portant, because we already have examples of selective transparency to light vibrations, and this may be only another, though unusually striking one. It seems probable, however, that these new rays are intercepted by bodies mainly in proportion to their density, in which case the election differs in kind from what we are familiar with. Two things they have in common with ordinary light-they can produce fluorescence, and they can cause chemical changes in the sensitive film on a photographic plate. In the latter respect their action is weak, long ex- posures beingnecessary to obtain the efect. But It must be remembered that the conditions are very unfavourable, because the screens used to shut off the light of the vacuum tube, though permeable by the,new rays, yet offer great re- sistauce to their passage. The rays pass through wood, but not through an indefinite thickness of 0od. The grain of the wood einployed is visible in the photographs, showing varying resistances. So impressed is Profossor Rontgen twith the differences between these X rays, as he -calls themi for the sake of brevity, and ordinary light that hej suggests, though in the most cautious manner, the possibility that they are in fact physically dissimilar, and lie altogether outside of the phenomena covered by the undulatory theor of light. He apparently leans to the belie. that they are due to longitudinal vibra- tions of the ether, instead of to the transversal vibrations which are the fundamental assump- tion of the existing theory of light. In this con- nexion it is interesting to uote that, although vibrations in the line of propagation of the ray have no place in the accepted explanations of the nature of light, the possibility of their existence has henfulIy recognize dby mathematicians. Their existence has even been insisted upon and their cha- racteristics havebeen more or less fully described. That but little attention has been given to them is due simply to the fact that they have not been experimentally encountered; but, should it turn out upon further investigation that Professor Ront-en's X rays are really due to condensa- tional waves in the ether as distinguished from distortional .waves, the requisite modification of accepted equations can be made with great facility. Lord Kelvin, in his Baltimore lectures in 1884, referring to condensational waves in the ether, said :-_ We ignore this condensational wave in the theory of light. We are sure that its energy is very small in comparison with the energy of the luminiferous vibrations we are dealing with. But to say that it is absolutely null would be an assumption that we have no right to make." A little later in the same course Lord Kelvin spoke even more definitely, for, after saying that if condensational waves exist their energy must be exceed- ingly small, he added :-" But that there are such waves I believe, and I believe that the velocity of propagation of electrostatic force is the lnknown condensational velocity that we are speaking of." It wouldssem possible from Professor Rontgen's experiments that the condensational waves in which Lord Kelvin believed, although he had not found them, may be found if looked for in the proper place and manner. In the same way room is easily found for them in the electro-magnetic theory of light of Clerk Maxwell, although, in the absence of experimental proof of their existence, he confined his equations to the case of transverse vibrations. Longitudinal vibrations must have a velocity of propagation enormously greater than that of transversal vibrations; hence it is easy to con- ceive that they may have no effect upon the human eye. At the same time it would be rash in the extreme to assume that they are incapable of transformation into other vibrations, or of setting up other vibrations, of which the human eye can take cognizance. Thus, the question of absorbing interest for physicists is at this moment perfectly open. Pro- tessor-Ri5ntgen may be dealing either with trans- versal waves of exceedingly short period, or with longitudinal vibrations; either with something we already know, carried as it were to a higher power, orwithsomethingessentiallydifferent from averything hitherto contemplated in our specula- tions. Sir George Stokes leans to the belief that the new phenomena may be explained without re- course to condensational waves in the ether; and it is obvious that upon general scientific prin- ciples a new cause cannot be admitted until known causes are roved to be incapable of pro- ducing the effect.- But at this moment no suffi- cient explanation is forthcoming upon either hypothesis, or perhaps it is better to say that no conclusive argument can be framed for accept- ing either. If the X rays are simply ultra- violet rays of shorter wave-length than any we have yet recognized, we might expect them to follow the analogy of shortening waves by being more highly refrangible than the ultra-violet. They are not refrangible at all; but. then, as Professor J. J. Thomson has pointed out, this result would follow were the period so excessively short as to be comparabie to the dimensions of a molecule. We can, there- fore, conclude nothing with certainty from non-refrangibility. Again, the X rays ex- cite fluorescence,-which is recognised as due to the degradation of short waves into longer oues.. This is co=antible 'with the theory that they are transverse vibrations of very short period, but it does not exclude the hypothesis that we are dealing with .longitudinal vibrations, which, for anythingwe know, mayequallyhavothe power of setting up transverse vibrations affecting the human eye. Bolzma'in endeavours to har- monize the new facts with what we know about fluorescence by assuming that excessively short waves are in this case degraded into longer ones, which however are still short as compared with ultra-violet light. Air is exceedingly opaque to transverse waves-of very short period, but these X rays are operative after traversing as much as a mUtre of air. This would seem to show that thev cannot be ultra-violet ravs; but, on the other hand, we have plenty of examples of selective absorption, and air may absorb short waves up to a certain point, yet be perfectly trans- pIarent to waves shorter still. These X rays apparently cannot be polarized. They seem to pass with equal facility through two Plates of tournmaline, whether their axes be parallel or at right angles. This is a very important observation, since the phenomena of polarization reallv form the corner-stone of the theory that light depends upon vibrations transverse to the direction of propagation. Yet even here there is the disturbing possibility that analogy may not liold when we come to deal with transverse vibrations of a period comparable to molecular dimensions. Much careful experiment will be required, not only to resolve the doubts now felt as to the nature of the force in question, but also as a preliminary to settle the precise conditions in which it manifests itself. It has not been given to every one to obtain Professor RiGntgen's effects by a repetition of his experimeuts as de- scribed. Professor J. J. Thomson says that the essential condition is a very high vacuum ; but, even with very complete exhaustion and more powerful electric stimulation than he finds neces- sary, the desired results are not invariably attained. More exact knowledge is evidently desirable merely to form a sure experimental basis for speculation ; while various check experi- ments upon phosphorescence and cathode rays will probably have to be undertaken by any one desiring to elucidate the question. In the mean- time physioists are face to face with at least a possibility that they may be caled upon to revise their theories bO as to account for such a novel and disturbing element as a form of ether waves possessed of infinite velocity, and related in ways yet unknown to ordinary light. A INEW MODE OF MOTIOX.