Electric Discharge in Gases. 87 



motion in the kathode-rays consequently transverse. The ab- 

 sorption of the kathode-rays is not, however, to be conceived 

 as interference in the usual sense of the word, but rather as 

 a transference of motion to other particles already moving 

 in the same direction. 



(2) Let us now turn to the property possessed by the 

 kathode-rays, that they cannot traverse any larger layers of 

 matter, i. e. to the absorption of the kathode-rays. 



We find that every solid body, whether conductor or insu- 

 lator, is absolutely opaque to the kathode-rays even in the 

 thinnest films. Hertz (loc. cit.) has shown that sodium or 

 mercury vapour introduced into the path of the kathode-rays 

 opposed an absolute hindrance to their further progress; and 

 he supposes that the non-appearance of the kathode-rays at 

 higher pressures depends upon their absorption by the gas 

 itself. 



An absorption of the ultra-violet rays has been proved deci- 

 sively by Cornu*; no experiments with hydrogen are yet at 

 our disposal; but an increased absorption in the ultra-violet 

 is probable on spectroscopic grounds. 



It is also a priori probable that a general absorption of 

 the rays of light takes place so much the more easily the 

 smaller their oscillation-period, or the greater the number 

 of vibrations performed during the time that the ray of 

 light advances through a given distance. There will be an 

 absorption if certain relationships exist between the oscil- 

 lations of the aether envelopes of the molecules and those of 

 the free asther in respect to period and direction of oscilla- 

 tion; it is clear that with similar conditions of the body, such 

 will occur the more frequently the greater the number of 

 gethereal oscillations per unit length. The strength of the 

 absorption must, cceteris paribus, be proportional to the pro- 

 duct of the number of sethereal oscillations per unit-length 

 and the number of molecules in the unit volume. Hence the 

 number of bodies which yield a continuous absorption-spec- 

 trum in the ultra-violet is much greater than the number of 

 those which absorb both the visible rays and also the ultra- 

 violet rays. 



(3) With the absorption of the rays is closely connected the 

 facility with which they produce phosphorescence and fluores- 

 cence f , and with which they initiate chemical change. 



It is this fluorescent or phosphorescent luminosity of a gas 



* C. E. lxxxviii. p. 1285 (1879) ; Beibl. iv. p. 40 (1880). 



t The opinion of Herr Goldstein that the ends of the kathode-rays, 

 when they strike upon a solid wall, become covered with a layer of ultra- 

 violet light does not appear to me to be very probable. 



