The next supposition naturally is, that the phenomena are due to streams of particles. It has been suggested that the rays may be streams of _material_ particles, but this theory cannot be maintained in view of the fact that the rays proceed, without hindrance, through the highest vacuum. § § 72_b_ and 133, _near end_. Neither is it consistent with the high velocity of propagation. Molecules of gas could not be propelled _through air_ with any such velocity or to any such distance as X-rays are propagated. Tesla has claimed § 139, that the residual gases are driven out through the glass of the vacuum bulb by the high potential that he employs. This has not been confirmed by other experimenters. It has been observed that the vacuum may be greatly improved by working the bulb, § 121, that is, sending the discharge through it, but experimenters generally have found that heating the bulb impairs the vacuum and restores the original condition. The gases, were, therefore, occluded during the electrical discharge, to be again set free by heating the bulb. § 139_b_. The rays may be ether streams, perhaps in the form of moving vortices, but of such streams we have no independent knowledge, and can only determine by mathematical analysis, what their characteristics should be. They would not suffer refraction, and would not produce interference nor diffraction phenomena. Whether they would _do_ what the X-rays do, go through the flesh and not through bone, through wood and not through metal, excite fluorescence, or affect the photographic plate, cannot be said. There is evidence that there are at least two kinds of X-rays, § 152, differing in penetrating power, though perhaps not differing in other respects. [Illustration: FROM SCIAGRAPH OF NORMAL KNEE-JOINT, FLEXED. _Phot. Times_, July, 96. Copyright, 1896, by William Beverley Harison, Publisher of “X-ray” Pictures, New York. ] X-rays have their origin only in electrical discharges in high vacua. They are absent from sun-light and from light of the electric arc, and other sources of artificial illumination, § 136. Proceeding from the bombarded spot, they are not deflected by a magnet, except in an evacuated observing tube, as proved by Lenard, § 72_a_, and show no evidence of carrying an electric charge like cathode rays, § 61_b_, p. 47. On the contrary, they will discharge either a negatively or positively charged body in their path. The evidence seems conclusive (Chap. VIII.) that the ultra-violet rays from an illuminating source also discharge charged conductors. In this respect, therefore, there is a similarity between the X-rays and ultra-violet light. The action of the waves of light upon a cell formed of selenium lowers the resistance of the latter and herein is circumstantial evidence at least, concerning the similarity of the properties of X-rays and light, because the former are also found to increase the conducting power of selenium. § 171.