OF THE BRITISH ASSOCIATION. 223 



These, of course, are exceedingly rough estimates, for they are derived from 

 measurements some of which are still confessedly very rough ; but if, at the 

 present time, we can form even a rough plan for arriving at results of this 

 kind, we may hope that, as our means of experimental inquiry become more 

 accurate and more varied, our conception of a molecule will become more definite, 

 so that we may be able at no distant period to estimate its weight with a 

 greater degree of precision. 



A theory, which Sir W. Thomson has founded on Helmholtz's splendid 

 hydrodynamical theorems, seeks for the properties of molecules in the ring- 

 vortices of a uniform, frictionless, incompressible fluid. Such whirling rings may 

 l)e seen when an experienced smoker sends out a dexterous puff of smoke into 

 the still air, but a more evanescent phenomenon it is difficult to conceive. 

 This evanescence is owing to the viscosity of the air ; but Helmholtz has shewn 

 that in a perfect fluid such a whirling ring, if once generated, would go on 

 whirling for ever, would always consist of the very same portion of the fluid 

 which was first set whirling, and could never be cut in two by any natural 

 cause. The generation of a ring-vortex is of course equally beyond the power 

 of natural causes, but once generated, it has the properties of individuality, 

 permanence in quantity, and indestructibility. It is also the recipient of impulse 

 and of energy, which is all we can affirm of matter ; and these . ring- vortices 

 are capable of such varied connexions and knotted self-involutions, that the 

 properties of differently knotted vortices must be as different as those of diffe- 

 rent kinds of molecules can be. 



If a theory of this kind should be found, after conquering the enormous 

 mathematical difficulties of the subject, to represent in any degree the actual 

 properties of molecules, it will stand in a very different scientific position from 

 those theories of molecular action which are formed by investing the molecule 

 with an arbitrary system of central forces invented expressly to account for the 

 observed phenomena. 



In the vortex theory we have nothing arbitrary, no central forces or occult 

 properties of any other kind. We have nothing but matter and motion, and when 

 the vortex is once started its properties are all determined from the original 

 impetus, and no further assumptions are possible. 



Even in the present undeveloped state of the theory, the contemplation 

 of the individuality and indestructibility of a ring-vortex in a perfect fluid 



