4 BatemaN, Degrees of Freedom of a Molecule. 



value of ;/ must be taken to be three because the atom 

 does not exhibit any valency undci the given circumstances. 

 It is true that the Mercury atom does exhibit a valency 

 under different circumstances, because Mercury forms 

 chemical compounds, but it does not exhibit a valency 

 when the ratio of its specific heats is calculated, otherwise 

 there would be more than one atom in the molecule. 



We adopt in fact, the view that valency is essentially 

 a relative property, and varies with the circumstances, but 

 that for each element it has a maximum value. 



The rules which are given here are essentially of a 

 hypothetical character, so little is known of the geometrical 

 relations, which two atoms bear to one another, when in 

 a state of combination, that there is no definite information 

 to start from, and we are bound to work backwards from 

 the experimental values. The number of degrees of 

 freedom possessed by a molecule is bound to enter 

 implicitly in the numerical values of constants connected 

 with chemical reactions, and deserves to be estimated 

 just as much as the molecular weight. 



These rules then are suggested as a working hypo- 

 thesis from which to start ; it is claimed that the assump- 

 tions made are quite reasonable, and that the exceptional 

 hypothesis made with regard to Chlorine in order to 

 explain Capstick's results is adhered to throughout. 



In general a relation involving the form and orienta- 

 tion of the groups of electrons contained in two atoms 

 will be called a valency relatiofi, and a relation specifying 

 the distance apart of two atoms, or the nature of the 

 orbit which one describes round the other a distance 

 relation. 



We shall suppose that a simple bond between two 

 atoms is in general equivalent to two valency relations 

 and one distance relation, that is three relations in all, in 



