INTRODUCTORY LECTURE ON EXPERIMENTAL PHYSICS. 255 



But what if these molecules, indestructible as they are, turn out to be not 

 substances themselves, but mere affections of some other substance ? 



According to Sir W. Thomson's theory of Vortex Atoms, the substance of 

 which the molecule consists is a uniformly dense plenum, the properties of which 

 are those of a perfect fluid, the molecule itself being nothing but a certain 

 motion impressed on a portion of this fluid, and this motion is shewn, by a 

 theorem due to Helmholtz, to be as indestructible as we believe a portion of 

 matter to be. 



If a theory of this kind is true, or even if it is conceivable, our idea of 

 matter may have been introduced into our minds through our experience of those 

 systems of vortices which we call bodies, but which are not substances, but 

 motions of a substance ; and yet the idea which we have thus acquired of 

 matter, as a substance possessing inertia, may be truly applicable to that fluid 

 of which the vortices are the motion, but of whose existence, apart from the 

 vortical motion of some of its parts, our experience gives us no evidence 

 whatever. 



It has been asserted that metaphysical speculation is a thing of the past, 

 and that physical science has extirpated it. The discussion of the categories of 

 existence, however, does not appear to be in danger of coming to an end in 

 our time, and the exercise of speculation continues as fascinating to every fresh 

 mind as it was in the days of Thales. 



