Referring to an article by the writer which appeared some months ago (The Electrician, Feb. 7), under a title akin to the above, in which the present state of our knowledge concerning the radiation experimented on by Lenard and discovered as such by Roentgen was summarised, and an account given of the various hypotheses which had suggested themselves, it may be not inappropriate to state the present aspect of the matter, now that there has been further experimental progress.
The remarkable discovery of MM. Henry, Niewenglowski, and Becquerel that salts of zinc, of calcium, and especially of uranium exposed to strong light acquire the power of emitting both then and afterwards, an invisible radiation which can penetrate aluminium and act on a photographic plate, has greatly strengthened the position of those philosophers who maintained that the X-rays were of the nature of ultra-ultraviolet light; and it has done this in the following way:--
The Becquerel rays are capable of some amount of polarisation, and hence are certainly transverse disturbances, like light; they can also be reflected and refracted to a small extent, whereas the X-rays can be hardly at all reflected and not at all appreciably refracted. Neither kind can be deflected by a magnet, not even in a vacuum according to the experiments of M. Lagrange; and an assertion that the X-rays could be magnetically deflected after passage through an electrified plate has not been substantiated by careful experiments, made by the writer among others. Taking all these things together, and looking at them in the light of a notable dispersion theory of Von Helmholtz, to which Prof. J. .J. Thomson called attention in the Rede Lecture at Cambridge this year (June 10), it has become almost certain that the X-rays are simply an extraordinary extension of the spectrum--far beyond the ordinary ultra-violet--and that the Becquerel rays are a less extreme extension in the same direction.
As a matter of scientific history it may be worth recording that in an article on Roentgen's discovery, published in the Revue Generale des Sciences for January 30th, Prof. Poincare hazarded the suggestion, "that all bodies which fluoresce strongly enough may perhaps emit X-rays in addition to ordinary light, no matter how the fluorescence is caused.'' He goes on to say that although this is not very probable yet that it is possible, and should be easy to verify; and that, if true, the X-rays would no longer be producible by electrical means alone. In attempting the verification of this surmise, M. Charles Henry found and published on February 10th that sulphide of zinc emitted something which could affect a photographic plate after penetrating black paper, or even a sheet of aluminium 6mm. thick; and M. Niewenglowski, February 17th, found the same thing for calcic sulphide. Then M. Becquerel, February 24th, repeating Niewenglowski's experiments, discovered the remarkably persistent ray-emitting power of the double sulphate of uranium and potassium. Moreover, it is noteworthy that, at a meeting of the French Physical Society held on February 7th, M. Raveau called attention to the fact that several existing theories of dispersion led to the value unity for the index of refraction of substances for very short waves, and hence argued that it was quite possible for the non-refrangible X-rays to be a variety of ordinary transverse ether waves of extremely short period.
To us at the present time the dispersion theory of Helmholtz is by far the most interesting, because it was worked out entirely on the basis of the electromagnetic theory of light. It is contained in Vol. XLVIII. of Wiedmann's Annalen. Helmholtz there shows, on electromagnetic principles, that ethereal radiation of smaller and smaller wave lengths should become more and more refrangible, by matter in the molecular form, up to a certain maximum; and this, of course, is ordinary dispersion; but that for waves which are shorter still, the refrangibility--i.e., the refractive index of substances for such very short waves--should rapidly, indeed almost suddenly, drop nearly or quite to zero, thus doubling the spectrum back upon itself, and giving an anomalous dispersion so great that the rays might be bent by a prism in the wrong direction (the direction beloved of examination candidates) for a certain size of wave. This state of things would be accompanied by extreme opacity, or absorption of the vibrations by the material molecules. If, however, waves existed of a kind still smaller, then the opacity would become less obtrusive; the refractivity would likewise remain very small--either positive or negative, perhaps--but probably negative; and ultimately, for extremely small waves of atomic dimensions, the refractivity (mu - 1) would become nothing and the opacity very small.
In a general way it may be said that material atoms act as if they loaded the ether, so that coarse ether waves large enough to affect some dozens or some hundreds of molecules in a row, such as are the waves of visible light, would by reason of this loading be retarded, and therefore both reflected and refracted. All very coarse waves would be refracted about the same amount, but for smaller waves a new phenomenon would appear; as they got smaller the period of the waves might synchronise with some of the periods of atomic vibration, such vibration as enables atoms to emit light, and whenever that occurred a violent absorption might be expected, owing to the syntonic response or sympathetic resonance between the matter and the ether. This would have the effect at first of retarding the waves rather more, and of giving the well-known effects of ordinary dispersion, or the sorting out of waves roughly according to size, which we get in the prismatic spectrum. Or if the syntony is strongly marked, fluorescent and phosphorescent effects are to be expected from the jangled atoms; and if for this or any other reason absorption is rapid, the dispersion will be what is called "anomalous," which in this connection--indeed in all possible connections--only means unexpectedly complicated.
Push the matter further, however; assume the existence of waves smaller still, so small that they cease to evoke any vibratory response from the material atoms among which they now make their way; the ether of the interstices can hardly be appreciably loaded by the great blocks of immovable substance which now represent the appearance of the atoms, and accordingly retardation and refraction abruptly disappear together, and true absorption also nearly ceases.
To waves penetrating ethereal interstices, matter, even conducting matter, is fairly transparent; for ordinary notions of conductivity do not apply to these intermolecular spaces; electric displacements no longer excite necessary conduction currents, even in bodies which in the gross are conductors, and accordingly there is little or no dissipation of energy, and any obstruction that exists to the passage of light of this kind is of the ground-glass or turbid-medium type, a certain percentage of the energy being scattered at each obstacle in all directions, instead of being able to excite the material vibrations which we know as heat.
This is a very bare account of the matter, but it may suffice to indicate the sort of view which is now coming to be almost universally held regarding the nature of these no longer quite x rays. The proof is not complete, and will not be till their length has been measured, but in all probability they are ordinary transverse ethereal waves, moving with the customary velocity of light, of various grades of wave length down to 10-8cm. in length, vibrating therefore some trillions of times in a second (a trillion being 1018); and by the aid of this highest type of X-ray we may hope in the future to gain some diffractional insight into the actual structure and appearance of the material molecules among which they go.
In all probability they are excited by Hertz vibrations in the atoms themselves. Ordinary light may be due to mechanical or acoustic atomic vibrations; but this X kind of light is more likely due to electric vibrations, i.e., to surgings of the atomic charges. A globe of steel vibrating mechanically might excite ether waves a hundred thousand times the sphere in size, if it could excite them at all; but, vibrating electrically, its radiated ethereal waves would be not much bigger than the sphere itself. In other words, its vibration frequency would be multiplied nearly a hundred-thousand fold. The mechanical vibration of an atom may emit ordinary light. Its electrical vibration may quite possibly emit X-rays.
There is not lacking indirect evidence to show that what we call atomic weight is approximately proportional to atomic bulk, i.e., that the heaviest atoms are the biggest atoms, and that the actual substance of all matter may be much more nearly of one uniform density than is commonly supposed. Grant this hypothesis, and it is plain why platinum or other dense material appears to be the easiest substance in which to excite the necessary electric atomic oscillations, by the impact of charged and excessively rapidly moving gaseous particles. It also suggests that the gas with the most rapidly moving atoms, viz., hydrogen, may be the best substance for the vacuum bulbs to contain; for these would impart their charges to the large platinum atoms in the most sudden manner. It would also be plain why dense bodies should be more turbid than rare, and it is not unnatural for the turbidity to be largely a matter of atomic weight, i.e., bulk, than anything else, because the ethereal interstices required for the passage of the waves would in such substances be considerably filled up.
Calculate the speed of a hydrogen atom in a vacuum tube between two electrodes, kept oppositely electrified with a difference of potential corresponding to a two-inch spark between flat plates, i.e., 150,000 volts. The atomic monad charge is 10-11 electrostatic unit, so the force acting on an atom is 0.3 millionths of a dyne, over a range of, say, 5 cm. The mass of the atom is 10 -26 gramme; so its acceleration is 3 x 1019 C.G.S. units, and hence the speed that may be got up in 5 cm. of free path approaches very near the velocity of light, say 1.5 x 1010 cm. per second. If there are collisions the speed will be reduced, hence it is probably desirable that the gas should be pure, and the necessity for high vacuum is obvious. Too high a vacuum reduces the number of impinging molecules, and so weakens the intensity, but it permits the emission of the most penetrating rays, i.e., those with the greatest frequency number, or highest up in the spectrum.
Unless a charge is imparted to a molecule with something approaching the above estimated rapidity, it would not be likely to have electrical oscillations excited in it; just as it is only possible to excite Hertz vibrations in ordinary small pieces of matter by some very rapid means of communicating the electricity; otherwise the disturbed electric equilibrium restores itself in a dead-beat manner.
It is likely that the amount of energy thus consumed in the production of Roentgen radiation is extremely feeble, and that the rays themselves are of low intensity. All that they do is consistent with the supposition that their activity depends more upon synchronism than upon violence,--which is indeed the case with ultra-violet radiation of every kind.