47* 



iHKm of this fluid is to be traced is pure mathematical analysis. The diffi- 

 culties of this method are enormous, but the glory of surmounting them would 

 be unique. 



There Menu to be little doubt that an encounter between two vortex 

 *,** would be in it* general character similar to those which we have already 

 described. Indeed, the encounter between two smoke rings in air gives a very 

 lively illustration of the elasticity of vortex rings. 



But one of the first, if not the very first desideratum in a complete theory 

 of matter is to explain first, mass, and second, gravitation. To explain mass 

 may seem an absurd achievement. We generally suppose that it is of the essence 

 of matter to be the receptacle of momentum and energy, and even Thomson, 

 in hia definition of his primitive fluid, attributes to it the possession of mass. 

 But according to Thomson, though the primitive fluid is the only true matter, 

 yet that which we call matter is not the primitive fluid itself, but a mode 

 of motion of that primitive fluid. It is the mode of motion which constitutes 

 the vortex rings, and which furnishes us with examples of that permanence and 

 continuity of existence which we are accustomed to attribute to matter itself. 

 The primitive fluid, the only true matter, entirely eludes our perceptions when 

 it is not endued with the mode of motion which converts certain portions of 

 it into vortex rings, and thus renders it molecular. 



In Thomson's theory, therefore, the mass of bodies requires explanation. \\'<- 

 have to explain the inertia of what is only a mode of motion, and inertia is 

 a projx'rty of matter, not of modes of motion. It is true that a vortex ring 

 at any given instant has a definite momentum and a definite energy, but to 

 shew that bodies built up of vortex rings would have such momentum and 

 energy as we know them to have is, in the present state of the theory, a 

 very difficult task. 



It may seem hard to say of an infant theory that it is bound to explain 

 gravitation. Since the time of Newton, the doctrine of gravitation has been 

 admitted and expounded, till it has gradually acquired the character rather of 

 an ultimate fact than of a fact to be explained. 



It seems doubtful whether Lucretius considers gravitation to be an essential 

 property of matter, as he seems to assert in the very remarkable lines 



"Nam si tantundem est in lame glomere, quantum 

 Corporis in plumbo est, tantundem pendere par est : 

 Corpora officium est quoniam premere omnia deorsum." De Rerun. Natura, i. 361. 



