Latent Heat, and Compressibility. 31 



Now — T^T . ^ is the " work of expansion " or the heat that 

 ft J 



must be withdrawn from a body in compressing it iso- 

 thermally by unit volume. In other words, we should 

 expect to find that at 0° C, the work of expansion of a 

 liquid should be roughly equal to the latent heat per unit 

 volume. That this is the case was discovered empirically by 

 Lewis [Zeit. Phys. Chem. lxxviii. p. 24 (1911)). 



Surface Energy and Compressibility. 



In conclusion, it may be pointed out that since, as shown 



above. -——eXxS (in mechanical units), we have, from 



(iii.) : 



1 _ _e.6p 

 ft" d ' 



Q 4 1 



Since d is approximately the same for the common -organic 

 liquids (about 4 5xl0 -8 cm.) and e at 0° 0. is not very 

 different from —3, we may expect to find the product ftp 

 approximately constant at 0° 0. and equal to 



4*5xi0~ 8 OK 1A _ 8 



-^ =-25x10 8 . 



lo 



The mean value for ftp from the data used above is *252 X 10 -8 

 at 0° 0. It is, of course, well known that the product ftp is 

 by no means constant at other temperatures ; this follows at 

 once from (xi.), since e varies with temperature and dif- 

 ferently for different liquids. It is to be expected that at 

 higher temperatures, where the ratio e has become —2, 

 ftp would again become approximately constant at the value 



4*5 



— - x 10 _8 = , 39 x 10 ~ 8 . The empirical relations of Richards 



J- j6 



and Matthews (Zeit. Phys. Chem. lxi. p. 449) and particu- 

 larly Tyrer {Zeit. Phys. Chem. lxxxvii. \\. 169), who found 



ft 

 respectively that the expressions ftp*' 3 and T1 ., are approxi- 

 mately constant over the ordinary temperature ranges. 



