CHAPTER XX 



MOLECULAR MAGNITUDES 



EVIDENCE as to the reality of the molecular and 

 atomic structure, which we have been discussing, is 

 to be found in the agreement between the values 

 obtained for various molecular magnitudes from en- 

 tirely different experimental determinations. The 

 most important magnitudes are (1) the number per 

 mole under standard conditions of pressure and temper- 

 ature, (2) the mass of each molecule, (3) its diameter, 

 and (4) its kinetic energy of translation. A knowl- 

 edge of these is desirable in order to obtain a fairly 

 complete picture. In addition we shall need to consider 

 later the total energy possessed by a molecule at a 

 given temperature and its partition among the degrees 

 of freedom. 



Many attempts have been made to determine the 

 number of molecules in one mole. The earlier values 

 were obtained indirectly from an estimate of the 

 diameter of a molecule. For example, Lord Rayleigh 

 made use of the fact that the surface tension of a liquid 

 is greatly affected by the presence of impurities. A 

 particle of camphor placed on a water surface jerks 

 about due to variations in the surface tension about 

 it. Oil spread on the water reduces these motions and 

 Rayleigh determined the critical thickness of an oil 



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