123 DIRECT PROPERTIES OF MOLECULES 347 



in round numbers, so that 



According to this calculation the centres of two mole- 

 cules of water-vapour at 0, if they undergo direct collision 

 when moving with their mean kinetic energy, approach to 

 a distance of f/o, which is only two-thirds of their separation 

 when they are in the liquid state at maximum density at 4. 

 In order to compress water so strongly, a pressure of 20,000 

 atmospheres must be employed. 



Though several points in this calculation may have only 

 a very doubtful justification, yet from the result we learn 

 that the molecular forces which are developed on the 

 encounter of two molecules possess a very considerable 

 intensity. 



From the value of the molecular energy we can at once 

 form an estimate of the energy of the motion of the atoms 

 within the molecule ; for the ratio in which these two : 

 energies stand to each other is ( 53) determined by the 

 ratio of the two specific heats. The forces, therefore, to 

 which the atoms are subject must also be in general very 

 great. 



The only exception is that of the case when the mole- 

 cule is monatomic. In this case we might assume that 

 there is not interior motion at all in the molecule, since it y 

 is simple and consists of but one atom. But this assump- 

 tion cannot be right, for monatomic gases and vapours 

 mercury vapour, for instance can radiate light the spectrum 

 of which consists of a series of bright lines, and therefore 

 internal motions must be present in a gaseous molecule that 

 contains but one atom just as in a polyatomic molecule. 



Eilhard Wi ode in ami 1 has cleared up this apparent 

 contradiction by measuring the energy necessary to make 

 a vapour luminous. He compared the light radiated by 

 sodium vapour with that coming from a platinum wire 

 made to glow by the passage of an electric current ; from 

 the resistance of the wire and the strength of the current 

 he could determine the luminous energy in heat units, and 

 1 Wicd. Ann. 1889, xxxvii. pp. 241, 248. 



