292 Mr Rideal, On the latent heats of vaporisation hIi 



from the natural infra red vibration frequency itself. The frequency 

 of the radiation necessary to activate an atom in the solid will 

 correspond to some line in the spectral series, each representing a 

 different degree pf activation of the element, which should be 

 capable of experimental determination in the spectrum. In the 

 attached tables are given a few of the Hnes in the spectrum of the 

 various elements as observed by Rowland, Kayser and others (c/o 

 Landolt-Bornstein Tabellen) and the latent heats calculated there- 

 from (Zj). It will be noted that the line corresponding to the 

 activation frequency of the atom in the sohd is generally fairly 

 widely separated from the other lines and is thus at or near the 

 head of a series. The choice of the particular fundamental hne, 

 however, is somewhat arbitrary and it is consequentlv necessary 

 to find some alternative method for calculation of the latent heats. 

 If we consider that a metal is built up of space lattices of 

 alternating valence electrons and positive nuclei, the activation 

 of a metal atom can be assumed to take place in tM^o distinct steps, 

 the activation of the positive nucleus in the metal and the activa- 

 tion of the valence electron attached to it. The activation fre- 

 quencies of the positive nucleus and the valence electron respec- 

 tively are given by the v^^^^ ^ed and v^^^ve. violet (or the photo 

 electric frequency) frequencies whilst the activation frequency 

 of the atom as a nucleus-electron complex corresponds to the 

 so-called "radiation potential" of the element. It has already 

 been indicated by Haber* that the transfer of a quantum of energy 

 in a sohd results in the activation of two atoms, being virtually a 

 cleavage of a diatomic molecule Hnked by an electron into two 

 active atoms. The latent heat of evaporation will therefore be 

 given by the expression : 



-^ ~ 2^ -^ Irradiation ~ {^infrared + ^- ultra violet}}- 



In the following tables are given the latent heats of evaporation 

 calculated on this basis {L^), the infra red radiation frequency 

 being calculated from Lindemann's melting point formula, the 

 ultra violet frequency from Haber's relationship 



Mv red = *Wr ultraviolet? 



where M and m are the atomic and electronic masses respectively. 

 Where experimental values for iWdiation (or the radiation poten- 

 tial) are not available there have been calculated from the approxi- 

 mate relationship ^radiation = w^jtra violet • In the last column are 



2.3 



given the values of (Zg) calculated from vapour pressure data col- 

 lected by Johnston {Jour. Ind. and Eng. Chem. 9, 876, 1917), 

 Gebhardt (An7i. 40, 438, 1913) and Langmuir (Phys. Rev. 2, 329, 

 1913). to V J , , 



* Ber. Deutsch. phys. ges. 13, 1117 (1911). 



