6 Mr. W. Sutherland on the Fundamental 



critical temperature, v the volume of a gramme at that tem- 

 perature, and M the molecular mass, c being a constant the 

 same for all bodies. Now the surface-tensions of a number 

 of solids at their melting-points, or, more accurately, of a 

 number of liquids at their solidifying-points, were measured 

 some time ago by Quincke (Pogg. Ann. cxxxv. p. 138), and 

 quite recently by Traube [Ber. der Deut. Chem. Ges. xxiv. 

 p. 3074). Quincke's data relate to a number of metals and a 

 few salts, and Traube's to a number of salts of Na and K. 

 It is obvious that there must be a certain amount of roughness 

 in the measurements at the high temperatures of the melting- 

 points of these bodies, and there is also an inaccuracy in the 

 equation by which Quincke calculates the surface-tensions 

 from the experimental measurements ; but there is a com- 

 pensating cause at work, and it may be said that both 

 Quincke's and Traube's data give a fairly accurate estimate 

 of the surface-tension at the melting-point, if all the difficulties 

 of the measurements are allowed for. Now in our last equa- 

 tion (5) the surface-tension is supposed to be measured at 

 two-thirds of the absolute critical temperature, though with a 

 different value of c it might be taken at any constant fraction 

 of the critical temperature. Melting-points are hardly likely 

 to be proportional to critical temperatures ; but still, as high 

 melting-points on the whole mean high critical temperatures, 

 there is a rough proportionality between melting-temperature 

 and critical ; so that if we denote the surface-tension at the 

 melting-point by a m and the value of a gramme at ordinary 

 temperatures by 1/p, we can replace the last equation by the 

 approximate form 



l = C f a m {\lp)^jWI^ (6) 



where c' is a constant to be determined. The value of cin (5) 

 is 2 x 5930 when 10 6 dynes is the unit of force ; for c' I have 

 adopted the value 9300. To get values to join on naturally 

 with those tabulated in the " Laws of Molecular Force/' 

 where the unit of force used islO 12 dynes, we can write our 

 present relation in the form 



M 2 Z = 9300xlO- 6 * (M/p) 5 ' 3 (7) 



N.B. — Here and hereafter the unit of force is 10 12 dynes. 



By this equation, then, we can get approximate values of / 

 for v the metals and salts of Quincke's and Traube's experi- 

 ments, and so deduce approximate values of their latent heats 

 of vaporization; but as for thermochemical applications we 

 require the latent heats of a larger number of substances, 



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