Mr. J. J. E. Durack on Lenard Rays. 29 



to be quite insufficient. The axial colours completely vanish 

 (become white) before even one half of the coronas have 

 been passed. Experiment shows, therefore, conformably 

 with the given interpretation, that the condition of optical 

 saturation (removal of the original white light) is a line of 

 vision quite blocked by water particles all of rigorously the 

 same size. Thus it would seem as if each single water 

 particle colours the area of its projection. 



Now what kind of absorption is this which occurs for 

 particles of such surprisingly large dimensions relatively to 

 the wave-length of light, which occurs moreover in just as 

 marked a degree if the particles are electrical insulators like 

 the precipitates from benzine and petroleum vapours ? For 

 though one cannot regard a water particle captured by a 

 saline nucleus as quite an insulator, there seems to be no 

 electrical conduction possible for the case where a sulphur 

 nucleus condenses benzine vapour. But the discussion of 

 this question is beyond the present purposes. 



Brown University, Providence, U.S.A. ' 



III. Lenard Rays. By J. J. E. Durack, B.A. (Syd.), 

 1851 Exhibition Scholar, Trinity College, Cambridge*. 



THE first experiments here described were undertaken 

 with the object of measuring the velocity of the Lenard 

 Rays as the pressure of air in the discharge-tube is altered. 

 The variation of the energy of the rays, as shown by the 

 intensity of the photographs taken for this purpose, led to an 

 investigation of : — 



(1) The variation of the ionization produced by the rays 

 with the pressure in the discharge-tube ; 



(2) The change in the Lenard-ray current in the same 

 circumstances ; and 



(3) The ionizing power of the Lenard rays, i. e. the 

 number of ions one Lenard-ray ion produces in travelling- 

 unit distance through a gas at some standard pressure. 



These experiments will be described in the above order. 



§ 1. The discharge-tube, together with the apparatus used 

 for determining the velocity, is shown diagrammatically in 

 fig. 1 (p. 30). 



The cathode rays were produced in a discharge-tube 

 similar to that originally used by Lenard. A plane 

 aluminium disk c served as cathode ; the rod of aluminium 

 connected with c was inclosed in a thick- walled glass tube 



* Communicated by Prof. J. J. Thomson. 



