Surface- Tension with Temperature. 45 



very small, and as far as can be seen from the figures in 

 Table I. is quite irregular. As errors in setting the ruler 

 have a marked effect on the value of n, it is quite possible 

 that, had the experimental results been treated by some 

 more exact method, the variations in n would show regu- 

 larities not here apparent. But the labour of such calcula- 

 tions would have been out of all proportion to the slight 

 gain in accuracy, and for our purposes — the accurate repre- 

 sentation of the temperature-variation of surface-tension, 

 and the calculation of the critical temperature from surface- 

 tension observations carried out at ordinary temperatures — 

 it is sufficient to note that the variation in n is very slight, 

 and that the mean value of n is about 1*21. 



As far as b is concerned, the discussion practically resolves 

 itself into a consideration of the effect of constitution on the 

 critical temperature. Table I. shows clearly that in the case 

 of the esters R .000 R x f> diminishes — i. e., the critical tempe- 

 rature increases — with increase of R or R x (the other radicle 

 meanwhile being supposed constant). Also the value of b 

 for an iso-compound is greater than the value for the normal 

 ester — i. <?., the critical temperature is lower. 



Again, R and R : are not mutually interchangeable. For 

 example, we have 



C 2 H 5 .COOOH 3 .... 6 = -003891 

 CH 3 .C00C 2 H 5 .... b = -003984 



and 



3 H 7 .OOOCH 3 , . . . & = -003559 

 CH 3 .COOC 8 H 7 .... b--= -003623 



In each case the transference of the more complex radicle 

 to the carboxyl group increases the value of b ; the effect is, 

 however, much less pronounced in the second case than the 

 first, and probably decreases with increasing complexity of 

 R and R 1# 



If we denote by 7 the ratio of the absolute critical tem- 

 perature to the absolute boiling-point at normal pressure, I 

 nave elsewhere shown that for the normal paraffins * 



yn g =7i, 



where g and h are constants, and n the number of carbon 

 atoms in the molecule. The experimentally observed values 



* Phil. Mag-. April 1915, p. 602. 



