SURFACES OF DISCONTINUITY 595 



which is heat required to get rid of adsorbed molecules on the 

 surface, and bears some resemblance to latent heat. We can 

 best illustrate its nature by a reference once more to MicheH's 

 work on adsorption of gases (Phil. Mag. 3, 895 (1927)). As 

 stated earlier, he showed, if P is the partial pressure of the 

 vapor, that 



r = kP, 



and his results also show that for a given vapor over the water 

 surface the constant k decreases markedly with rise of tem- 

 perature. Thus for pentane at 25°C., A; is 75 X 10-^; at 35°C. 

 it is 35.8 X 10-^; for hexane the decrease is from 106 X 10"^ to 

 55.5 X 10-^ and for heptane from 256 X 10"^ to 115 X 10-^ a 

 rise of ten degrees roughly halving the value of k in each case. 

 This means that a rise of temperature causes desorption, the 

 partial pressure P being kept constant. Thus desorption 

 requires heat and adsorption is accompanied by an evolution of 

 heat. We can, of course, use the well-known Clapeyron rela- 

 tion to obtain this molecular heat of adsorption. Thus from 

 the equation 



d log P„ 



heats of adsorption can be calculated in the same way as latent 

 heats are calculated, where P„ is the partial pressure of the 

 vapor when n mols are adsorbed per unit area and Hn is the heat 

 of adsorption at constant temperature and pressure at the same 

 stage of adsorption. If P„, and P„2 are values of P„ at the tem- 

 peratures ^1 and ti, then as a first approximation 



_ R k t2 (log Pm - log Pn2) 



h — h 



Also, if ki and ki are the values of the constant k for ti and U, 

 kiPni = k^P n2 , and therefore 



_ R h tj (log ^2 - log ki) 

 ti — ti 



