114 DIRECT PROPERTIES OF MOLECULES 317 



In spite of all this, we have no reason to believe that the 

 molecules themselves are larger in a liquid than in the 

 vapour of the same substance. For, to explain the difference 

 of the two states of aggregation, the assumption that the 

 motion of the molecules in the two cases is different seems 

 sufficient. If the molecular motion in liquids were known 

 to us just as much as the motion in gases, we should be in 

 as good a position for liquids as for gases to determine in 

 absolute measure the sum of the sections, or any other 

 corresponding property, of the whole assemblage of mole- 

 cules contained in unit volume, and we could thus by 

 experiment decide the question with certainty whether the 

 difference of the states of aggregation consists only in the 

 motion or also in other properties of the molecules. But so 

 long as we are without a kinetic theory of the liquid state, 

 we cannot in the determination of the extension in space of 

 the molecules bring into the calculation the influence of 

 their motion, as in the case of gases, and we therefore 

 obtain values which are too high. 



116. Possibility of Determining the Size of Gaseous 



Molecules 



We have succeeded, however, in obtaining limiting 

 values, at least, of the sizes of molecules in absolute mea- 

 sure by comparison of the two fluid states of aggrega- 

 tion. Such a calculation was first attempted and made by 

 Loschmidt, 1 and then later by Lord Kelvin 2 (then Sir 

 William Thomson) and by Maxwell 3 in the memoirs 

 already cited. 



These calculations assume the sphere of action to be 

 spherical, and they are based on the relation between the 

 mean free path L and the radius s of the sphere of action, 

 which was discovered by Clausius, and is with Maxwell's 

 theory represented by the formula 



1 Wien. Sitzungsber. 1865, lii. Abth. 2, p. 395. 



2 Nature, 1870, i. p. 551 ; Sillimaris Amer. Journ. 1. pp. 38, 258. 



3 Phil Mag. 1873 [4] xlvi. p. 453 ; Sclent. Papers, ii. 1890, p. 361. 



