52 SCIENCE PROGRESS 



forces determining equilibrium may be of a somewhat different 

 nature. The work of Rankine, which will now be discussed, 

 indicates that the assumption is justifiable, and that one can pre- 

 dict the distance between the centres of electro-negative elements 

 forming a molecule by using the data represented by Fig. i . 



The length of the free path of a molecule in a gas depends on 

 the size of the target which the molecules present to each other 

 when approaching. From the viscosity of the gas we can cal- 

 culate the size of this target, the attraction of the molecules 

 being allowed for by means of the Sutherland correction. Pre- 

 viously, in the absence of any knowledge as to its constitution, 

 the molecule has been considered as a small spherical body and 

 its diameter calculated on this assumption. Rankine {Proc. 

 Roy. Soc, 98 A, 693, pp. 360-74, January 1921) has reconsidered 

 this question in the light of the evidence as to molecular con- 

 stitution afforded by crystal structure. 



We may illustrate this by taking the case of the gases argon 



Fig. 2. 



and chlorine. Argon is a monatomic gas, and in this case it is 

 almost certainly justifiable to consider the atoms as spherical 

 in shape. The diameter of the atom, calculated from the 

 viscosity of argon, comes to be 2-87 x 10 ~* cm. Chlorine 

 occupies a position just before argon in the periodic table. 

 The chlorine molecule is formed by the sharing of a pair of 

 electrons by two chlorine atoms. If the data given in the graph 

 of Fig. I are correct, we would expect the centres of these two 

 chlorine atoms to be at a distance of 2-05 x iQ-^cm. apart. 

 Rankine, therefore, considers the chlorine molecule to be com- 

 posed of two atoms each identical with argon as regards their 

 outer structure, except at the point where they come in contact. 

 Here the structures have interpenetrated to a certain extent, 

 since the distance between the centres is less than the diameter 

 of either. The molecule, from the point of view of the Kinetic 

 Theory, may be regarded as the dumb-bell shape structure 

 shown in Fig. 2. 



