LAW OF MOLECULAR FORCE. 39 



We have reasons now, however, to believe that this too has 

 been overcome by the ingenuity and patient experimenting of Sir 

 Howard Grubb, of Dublin, to whom the manufacture of several 

 of the instruments destined for this undertaking has been 

 entrusted, among which will be those for Sydney and Melbourne. 



In closing, pei'mit me to express the hope that if not the next, 

 at the next following gathering of this Association both 1 and my 

 friend Mr. Russell may have a good account to give of the 

 commencement and progress of the Australian share of the 

 photographic charts of the southern heavens. 



The following papers were read : — 



l._ON THE LAW OF MOLECULAR FORCE. 



By William Sutherland, M.A., B.Sc. 



The object of this paper is to apply the hypothesis of a molecular 

 force, varying inversely as the fourth power of the distance, 

 already discussed by me in certain of its aspects in the 

 Philosophical Magazine for August, 1886, and July and August, 

 1887, to the subject of capillarity, with special reference to the 

 recent beautiful discoveries of Eotvos and Robert Schiff in that 

 subject. The results obtained are : — First, the following law of 



ni' 

 the parameter A, in the expression — j for the force between two 



similar molecules of mass, m at distance, r apart. In compounds 

 containing C, O, and H, the molecules may be considered to have a 

 volume to which each atom of H contributes an amount very 

 small in comparison with that contributed by an atom of O or C, 

 while an atom of O, when singly bound to another atom, con- 

 tributes an amount equal to that of two C atoms, and when 

 doubly bound, to that of three carbon atoms. The volume of 

 such a molecule can then be expressed in terms of that of a 

 carbon atom, and the jmrameter A varies inversely as the surface 

 of the molecule. • 



By the volume of a molecule is not meant what is usually 

 called the molecular volume, which is really the domain of the 

 molecule, but the actual volume of the molecule. 



Second, this law, that the rate of change of the translatory 

 kinetic energy of nearly all liquid molecules with temperature is 

 the same when measured at low constant pressure. 



Let a denote the surface tension of a liquid, and v its molecular 

 domain (usually called molecular volume) measured by m//j 

 where m is the molecular weight and p the density of the liquid. 



