338 DIRECT PROPERTIES OF MOLECULES 122 



of its electrical resistance, found the thickness of such a film 

 to be 12 million ths of a millimetre. Buns en, 1 and also 

 Warburg and Ihmory, 2 found magnitudes of the same 

 order by weighing the films of water formed on glass. But 

 long before all these experiments were made Thos. Young, 

 as Lord Rayleigh 3 has pointed out, instituted measure- 

 ments for the determination of the range of the capillary 

 forces, for which he found a 250-millionth of an inch, i.e. 

 0-1 millionth of a millimetre, but by a calculation that is 

 really uncertain. 



Lord Kelvin 4 found just as small a value as these for 

 the volume of the molecules of a liquid by discussing in 

 another way the phenomenon examined by Plateau, viz. 

 the formation of thin films of liquid. He investigated the 

 work required to extend a liquid film, and also the corre- 

 sponding amount of heat, and found from these considera- 

 tions that inconceivable conclusions would follow if we 

 were to assume that several layers of the ultimate atoms of 

 water were to lie over each other in a water film the 

 thickness of which is twenty times less than a millionth of 

 a millimetre. Boltzmann 5 calculated the work which is 

 necessary for a displacement of the particles that are held 

 together by capillary forces, and compared it with the tenacity 

 of the substance in the solid state ; this consideration led to 

 the result that the range of molecular forces in metals lies 

 between 1 and 7 millionths of a millimetre. By similar con- 

 siderations De He en 6 concluded, from a comparison between 

 the tension of a capillary surface and the pressure of the 

 vapour above it, that the radius of the sphere of action for 

 water is about 3 millionths of a millimetre. On the contrary, 

 Gr. Jager 7 obtained the values 0*5 for water and alcohol, 0*4 

 for methyl alcohol, 0'7 for aceton and carbon disulphide, 0'8 

 for ether and chloroform, all expressed in millionths of a milli- 



Wied. Ann. 1885, xxiv. p. 321. 2 Ibid. 1886, xxvii. p. 481. 



Phil Mag. 1890 [5] xxx. p. 474. 4 Proc. Eoy. Soc. 1858, ix. p. 255. 



Wien. Sitzungsber. 1877, Ixxv. Abth. 2, p. 801. 



Bull. Ac. Belg. 1892 [3] xxiii. p. 235 ; Wied. Beibl. 1892, xvi. p. 724. 

 Wien. Sitzungsber. 1891, c. Abth. 2, p. 1233; Wied. Beibl. 1892, xvi. 

 p. 345 ; Winkelmann's Handbuch d. Physik, 1896, ii. Abth. 2, p. 602. 



