Laws of Molecular Force. 11 



at § of the absolute critical temperature, which is near the 

 ordinary boiling-point, v is the volume of unit mass at the 

 same temperature, and M the ordinary molecular mass 

 (weight) ; so that with the megamegadyne (10 12 dynes) as 

 the unit of force, we have 



M 2 Z=2x5930xl0- 6 «(M//))i . . . (1) 



In the case of melted solids a has been measured at the 

 melting-point only ; and as the absolute melting-point of 

 solids is not a constant fraction of their absolute boiling- 

 points, we shall have to be content with comparatively rough 

 anproximations to the values of M% if in the above equation 

 (1) we use the value of a at the melting-point instead of that 

 at \ of the critical temperature. In the case of mercury the 

 surface-tension at the melting-point has been found by 

 Quincke to be 58' 8 grammes weight per metre; and I have 

 estimated that at the boiling-point, if mercury behaves as an 

 ordinary liquid, the surface-tension would be 42*6. Then, to 

 keep the values of M 2 Z in a rough way more comparable with 

 those hitherto discussed, we will for all the melted solids take 

 a at § of the critical temperature as roughly given by 

 42'6/58*8 times its value at the melting-point ; denoting 

 which by a m , we have, when we likewise allow for the differ- 

 ence between p at the melting-point and at § of the critical 

 temperature by a factor 1 # 09, the equation 



M 2 /=2 x 5930 x 10 " 6 x -723 x l-09* m (M/p)* 



= 9346xlO- 6 « TO (M/ / 3)i ... (2) 



To preserve continuity in the work this equation will be 

 applied first to compounds only, and later on to the metals. 

 It is to Quincke that we owe the first measurements of the 

 surface-tensions of a number of elements and compounds at 

 their melting-points (Pogg. Ann. cxxxv. & cxxxviii.); and 

 there are some more recent measurements for a number of 

 compounds by Traube (Ber. der Deut. chem. Ges. xxiv. 

 p. 3074) . The values of the surface-tension given by Quincke 

 in his second paper are brought into better agreement with 

 those of his first and with Traube's when multiplied by 1*4, 

 and accordingly these values multiplied by 1*4 are entered 

 in brackets in the following Table as values of a m , and the 

 numbers derived from them are put in brackets also. The 

 density at 15° C will be used in place of that at the melting- 

 point, because too few values at the melting-point are known, 

 and the difference between the two is really immaterial in 

 comparison with other unavoidable roughnesses in these cal- 

 culations. The formula by which Quincke calculated a m from 



