112 DIRECT PROPERTIES OF MOLECULES 309 



tables, while those marked ' calculated ' are obtained from 

 the chemical formulae. There is in general a fairly good 

 agreement between the two columns, which, as in the 

 former cases, is the less the greater the number of atoms 

 combined in the molecule. But differently from what 

 appears in respect to the three other series, the values of 

 the section as calculated from the chemical formulae are 

 greater than those deduced from the observations, whereas 

 in the other cases it is the observed numbers which mostly 

 are the greater. We shall, therefore, feel inclined to look 

 for the cause of the deviations in the inexactness of the 

 values as deduced from the observations on viscosity or 

 diffusion. 



In the foregoing I have taken account of all the values 

 given by Landolt and Bornstein which are referred to 

 the temperature C. I have left out only those values 

 that are given for very much higher temperatures ; these 

 cannot be brought into agreement with those calculated 

 from the chemical formulae, and are mostly much the 

 greater. From this we may conclude that the section of 

 compound molecules is very variable with the temperature, 

 and, as we might expect, increases considerably as the tem- 

 perature rises. 



We can, consequently, expect agreement between theory 

 and observation only when all the numbers are reduced to 

 the same temperature. And so good an agreement is ex- 

 hibited by the great majority of the values at for gases 

 and vapours that we have to conclude in general that 

 their molecules have a shape that is flat, and not spread out 

 on all sides into space. This view seems to be the most 

 probable, at least for the gaseous state. 



113. Molecular Volumes 



If the molecules were extended in space on all sides 

 they would behave very nearly as if they were spheres; 

 and no further justification would be needed for looking 

 upon the envelopes, which surround them in such wise 



