304 DIRECT PROPERTIES OF MOLECULES 111 



compounds. The rule is therefore not of general validity, 

 although it holds in many cases. 



. Chemical Structure of Molecules 



If the hypothesis were general and exact that the section 

 of the molecule of a chemical compound is equal to the 

 sum of the sections of its atoms, it would allow of but a 

 single interpretation, and thereby permit an interesting peep 

 into the circumstances of arrangement of the atoms. We 

 should not be at liberty to make any other assumption than 

 that the atoms which are bound together into one molecule 

 are all in one plane. 



We ought, of course, to remark that only the average 

 value of the sections can be found from the magnitude of 

 the viscosity of a gas or, more directly, from its mean free 

 path. If now the molecules have a flat shape, then the 

 value of their section, as found by observation, is not iden- 

 tical with the surface-extension of the plane system of 

 molecules. But if we consider in our calculations only the 

 relative and not the absolute measures, we may still be 

 allowed to identify these two sections of the system, the 

 mean and the greatest. For the value of the mean will 

 in this case, in which all others are vanishingly small, be 

 determined almost entirely by the greatest only : hence it 

 seems allowable to extend to the greatest sections the law 

 that holds in many cases for the mean sections. 



In all the cases, therefore, in which the calculated values 

 of Q agree with those observed, we shall be able to suppose 

 the grouping of the atoms within the molecule to be such 

 that all the atoms that are bound together in the molecule 

 lie in one plane. We do not need thereby to assume that 

 they are firmly fixed together in this plane, but we may 

 suppose them to be movable in the plane. The system of 

 atoms, then, that form a molecule appears to us as a small 

 planetary system ; just as all the planets with their satellites 

 move about the central sun in one and the same plane at 

 least approximately and with but unimportant exceptions 

 so the atoms all move in a plane about the centroid of the 



