CAPILLARY CONSTANTS 



445 



As mentioned above, the ordinary linear relation only holds 

 over very restricted ranges of temperature, whilst the equation 



T = T o (i-a0-/30 2 ), 



which represents with considerable accuracy the relation of 

 surface-tension to temperature over a fairly wide range, does 

 not give results of even a rough degree of accuracy when extra- 

 polated far beyond the range of observation. In several cases 

 which the writer has examined, it would seem that the equation 



T = T o (i-b0) n 



(vii), 



can be extrapolated over a wide range of temperature so as to 

 give the critical temperature — which is, of course obtained from 



the relation 6 C — -, correct to two or three degrees. 



Thus, the first two columns of Table III below show the 

 relation between the temperature and surface-tension of benzene 

 as given by the experiments of Ramsay and Shields. 



Table III. 



As the table only begins at 8o° C, ioo° was selected as an 

 arbitrary zero, in which case equation (vii) may be written 



T = T„[i - b(0 - 100)]". 



From values of T given in the table and taken between the 

 limits ioo° and 200 the constants b and n were calculated, giving 



T = 18 [1 - -00534 (0 - 100)] 1213 . . (viii). 



The third column in the above table shows the values of T 

 calculated from this equation, and it will be seen that the agree- 

 ment between the observed and calculated values is fairly close. 



