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.
The experiments of Roentgen, § 90, seem to show that the discharging effect of X-rays is due to the air through which the rays have passed.
It is certain that the discharge of electrified bodies by light occurs more generally for negatively than for positively charged bodies, § § 99_B_, 99_I_, and 99_S_, that it depends upon the nature, § 97_b_, and density, § 97_a_, of the gas surrounding the body, and also upon the material of the charged body itself. § 98. The discharge would, therefore, seem to be connected with a chemical action, § 153, _near end_, which is promoted by the rays. This seems all the more probable, since it was found, § 98, that the more electro-positive the metal, the longer the wave length that would influence the discharge. In this connection, it is well to note that Tesla found, § 146_a_, that in their power of reflecting (or diffusing X-rays), the different metals stand in the same order as in the electric contact series in air, the most electro-positive being the best reflectors. It would be interesting to know whether connecting the reflecting plate to earth, would, in any way, vary its reflecting power.