34 PHYSICAL PROPERTIES AND COMPOSITION 



either of properties which are independent of surroundings, 

 such as mass, or under such circumstances that the influence 

 of surroundings is excluded, as by the sufficient separation 

 of the components which occurs in mixtures of solids, and 

 on extreme dilution. In a chemical compound, since this 

 separation is excluded in the case of the atoms, and is only 

 possible with regard to the molecules, strict additivity in 

 the relations of volume is not to be expected, and actually 

 is not found ; still the character of a mixture is retained 

 so far as more or less approximate additive relations exist, 

 and the deviation may in part be referred to the relative 

 influence of the atoms, i.e. to constitutive influences, in 

 a more or less definite manner. 



Since the data which allow of judging of additive rela- 

 tions at the absolute zero are scanty we will first make use 

 of the critical volume for this purpose, since according to 

 p. 30, that is about four times the volume at the absolute 

 zero. 



2. The Critical Volume. 



The most direct way of testing additive relations is 

 obviously to determine the critical volumes of the elements 

 and compare with those of compounds, as e. g. 



16 

 Oxygen ... D k = 0-65 V k = ^ = 25 



Nitrogen . . D k = 0-37 V k = =-. 38 



Nitrous oxide . D k = 0-41 V k = = 107 (2 xs8 + 25 = 101). 



0*41 



For want of material we are reduced to the indirect way, 

 and so find that isomers have nearly the same density, and 

 therefore molecular volume, whilst the same difference of 

 composition is associated with nearly the same difference 

 in molecular volume : hence conclusions may be drawn as 

 to the volumes of the elements, as appears from the follow- 

 ing data : 



