26 MOLECULAR WEIGHT AND POLYMERISM 



whilst for polyatomic gases it is less, since the term fa 

 will be greater on account of the kinetic energy of the 

 atomic movements. It is well known that measurements 

 of the velocity of sound gave the expected value 1-67 for 

 mercury vapour, and lower values for polyatomic gases. 



As according to p. 24 the atomic weights so far found 

 are strictly to be regarded as maxima, we have now an 

 indication from thermal reasoning that, at least for 

 mercury, the maximum has not been placed too high. 



Allotropy of the elements. Ozone and oxygen. A third 

 important result of molecular weight measurements of the 

 elements has been to show, on grounds of the molecular- 

 atomistic hypothesis, the existence of the same element in 

 different, so-called allotropic, forms. Oxygen and ozone 

 form the most striking example. Soret proved that ozone, 

 on conversion into oxygen, increased in volume by one-half, 

 so that oxygen being diatomic, ozone is to be regarded as 

 O 3 , the equation 



20 3 = 3 2 



indicating the increase in the number of molecules by one- 

 half, and according to Avogadro, increase of volume in 

 the same ratio. 



Dissociation of the elements at high temperature. The 

 halogens. A very welcome confirmation of the above views 

 on the polyatomicity of the elements is given in the proof 

 of the possibility of decomposing those molecules. Even 

 sulphur, which at low temperatures appears as S 8 , shows an 

 abnormal expansion up to 1,000, leading by then to a 

 density corresponding to S 2 . A further step in this 

 direction was accomplished by Victor Meyer for the 

 halogens, since he showed that iodine, which under 600 

 has a density corresponding to the diatomic molecule I 2 , 

 becomes halved in density above 1,400, with formation of 

 rnonatomic molecules or free atoms. 



It may be added that in this direction lies the possibility, 

 if any, of decomposing the elements themselves : the mona- 

 tomic molecule constitutes the last stage towards the 



