Vapour Pressure of Zinc, Cadmium, and Mercury. 33 



contact as transmitted by an electrical transmission line 

 ■containing inductance and capacity. Thm similarity is very 

 striking, especially if we add the delayed record of a second 

 line of' high resistance, constituting the after-image. 



The postulation of inductance and capacity for the trans- 

 mitting media in vision is, of course, a considerable step in 

 advance over the simple type of diffusivity which is sufficient 

 to handle the brief exposure and intermittent phenomena. 

 The oscillatory character of the image as described by 

 Bidwell may just as well be ascribed to an oscillatory 

 photo-chemical reaction, of which several are known. At 

 the same time the very great light thrown on these transient 

 visunl effects by treating them as physical transmission 

 phenomena will, I think, entitle visual " diffusivity " to a 

 place of some importance in future theories of vision, 



Physical Laboratory, 



The United Gas Improvement Co., 



Philadelphia, Pa. 



III. The Vapour Pressure of Zinc, Cadmium, and Mercury. 

 By A. 0*. Egerton *. 



IN a quest for a method to determine the vapour pressures 

 of various solid elements, it was attempted to employ a 

 process similar to that which has been used by L. Pfaundler 

 (Ann. d. Phys. (o) lxiii. p. 36, 1897) and others, but the 

 results did not promise to lead to a satisfactory general 

 method. Subsequently Professor Nernst drew my attention 

 to Knudsen's f work on the vapour pressure of mercury, and 

 it has been found possible to adapt his method to the deter- 

 mination of the vapour pressures of other elements. 



The following is an account of the determinations of the 

 vapour pressure of zinc, cadmium, and mercury. It is hoped 

 to continue the investigations for other elements and, by 

 comparing the results obtained for different elements, to 

 gain a measure of the forces between the atoms. The results 

 for these elements already show that that hope is well founded. 

 In 1909 Knudsen published an account of the determina- 

 tion of the vapour pressure of mercury by a method which 

 depended on his investigations of the kinetic flow of molecules 

 through small apertures and tubes. The apparatus consisted 

 of a glass tube, separated into two compartments by a thin 



* Communicated by Prof. H. L. Callendar, F.R.S. 

 t Ann. d. Phys. (4) xxix. p. 179 (1909). 



Phil. Mag. S. 6. Vol. 33. No. 193. Jan. 1917. D 



