I 



Mr Rideal, On the latent heats of vaporisation 297 



across the benzene ring is 10-6 10"^ cm. (the distance between two 

 carbon atoms being 6-2 10~^ cm., hence the diameter is 



a/3 



"^ 2 . 6 . 2 10-8) 



and that the benzene molecule lies flat upon the surface of a liquid. 

 Inserting the values 



Lq = 8000 cal., M = IS, 



Co = 3-187 (observed value 3-20). 



A high value for the chemical constant is found in the case of 

 iodine, where C = 4-0. The molecular weight is 



M = 254, d = 3-96 10-^ cm.* 



L per atom 2146 cal. or per mol. 4292 cal., hence 



V = 4-58 1013, and Co -= 4-09. 



Similar close agreements are to be found in the other cases 

 where the values of L and of d are known. 



The above derivation for the chemical constant leads to the 

 dimensional expression * = Ml-^ t-^, where M, I and t are the re- 

 spective dimensions of mass, length and time. 



If we insert temperature into the dimensional expression we 

 obtain * = Ml-"- 1-^ 0-^. 



This can be compared with the expression derived by Nernstj, 



and with that obtained by SackurJ and by Tetrode §, 



i^iog^'''p^ = Mi-^t--e-\ 



It will be noted that there is a lack of agreement in these expressions 

 as far as the temperature is concerned. Lindemann has developed 

 from dimensional considerations the expression 



, m- k- %-(b'-a')jR 



where h' = ^R approximately (the atomic heat of the gas at 0° K.) 

 and a' is the atomic heat of the soHd at 0° K., which is either zero 

 or a very small quantity. With elevation of the temperature the 



* Rankine, Proc. Roy. Soc. A, 83, 516 (1910); Phil. Mag. 29, 552 (1915). 

 t Gncndlagen des neuen Warmesatzes, HaUe, 1918, p. 138. 

 X Ann. der Physik, 40, 67 (1918). 

 § Ibid., 434, 39, 255 (1912). 



