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III. On the Variation of Surface- Tension with Temperature. 

 By Alla.n Ferguson, D.tic.(Lond.), Assistant-Lecturer 

 in Physicsi in the University College of North Wales, 

 Bangor *. 



OVER ranges of, say, twenty or thirty degrees the effect 

 of temperature on surface-tension can be expressed 

 with considerable accuracy by a linear formula of the type 



T = T o (l-a0) (i.) 



For wider ranges such a formula cannot be used with 

 -even approximate accuracy, and formulae of the type 



T = T (l-«6>±/3<9 2 ) (ii.) 



have been used to represent the experimental facts. Such 

 formulae, however, will not bear extrapolation , over even a 

 moderate range of temperature, and if, in particular, they 

 be applied to estimate the critical temperature by calculating 

 the value of 6 for which T vanishes, they invariably give 

 wildly discordant values for 6 C . 



The directly determined values for the critical tempera- 

 tures of liquids are comparatively few in number, and it is 

 a matter of some importance to be able to estimate the critical 

 temperature of a liquid to within a degree or two; for, 

 amongst other things, the problem of tracing relations 

 between surface-tension and chemical constitution has been 

 considerably obscured by a habit of making comparisons at 

 the same temperatures. Schiff f recognized that better 

 results were to be obtained by making comparisons at 

 *' corresponding temperatures," and chose the boiling-point 

 at atmospheric pressure for the purpose of comparison. 

 This, of course, implies that the ratio of the boiling-point to 

 the critical temperature is a constant for all liquids when 

 temperatures are measured on the absolute scale, and, as I 

 have shown in a recent paper J, this is by no means exactly 

 true, as the ratio is markedly affected by constitutive 

 influences. 



It is very necessary? therefore, to have some means o£ 

 estimating the critical temperature of a liquid with fair 

 accuracy, and, as will be shown later, this estimate may b© 



* Communicated by Prof. E. Taylor Jones. 

 t Liebig's Ann. ccxliii. p. 47 (1884). 

 % Phil. Mag. April 1915, p. 599. 



