48 Vapour Pressure of Zinc, Cadmium, and Mercury. 



The angle of slope of the vapour pressure curve for 



mercury is slightly less than half that of the slopes of the 



zinc and cadmium curves, which are very similar. The 



dio 

 values of Jp at the melting-points of the three elements are : 



Zn 4'41 . 10" 3 mm., 



Cd 3-70 . lO" 3 mm., 



Hg 2-81 .lO" 7 mm.; 



while at a point of equal pressure 2'0 . 10~ 6 mm., 



Zn 1*25 . 10" 7 mm. at the temperature 502° absolute, 

 Cd 1-39. lO" 7 mm. „ „ 436° 



Hg 2-81 . 10" 7 mm. „ „ 234 c 



lO 



At the melting-point the vapour pressures of zinc and 

 cadmium appear to be very nearly equal (Zn 1'13 . lO -1 mm. ; 

 Cd 1'0 . 10 -1 mm.). The physical constants for these three 

 elements are remarkable in that zinc and cadmium closely 

 resemble each other, while the constants for mercury have 

 throughout a very much smaller value. 



It should be possible to link the constants obtained from 

 the vapour pressure formula corrected for the real change 

 of specific heats with the tensile strength and other elastic 

 constants, and so obtain data for the measurement of the 

 forces of cohesion and of the law of attraction and repulsion 

 between atoms. This work was in hand when other more 

 pressing duties forced it to be placed aside nearly two 

 years ago. 



It was not intended to publish the results before finishing 

 the above calculation, but it is now thought advisable to 

 publish the experimental results as they stand. I am 

 indebted to Professor Strutt and Professor Callendar for 

 ha vino- allowed me to proceed with this work at the imperial 

 College, South Kensington. 



August 1915. 



