ELECTROSTATICS AKD MAONLTISM. 



deb: "The explanation of all phenomena of electro-magnetic attraction or 

 Ision, and of electro-magnetic induction, is to be looked for simply in tin- 

 inertia and pressure of the matter of which the motions constitute heat. Whether 

 thin matter ia or ia not electricity, whether it is a continuous fluid interpt-r 

 meating the spaces between molecular nuclei, or is itself molecularly grouped; 

 or whether all matter is continuous, and molecular heterogeneousness consists in 

 finite vortical or other relative motions of contiguous parts of a body; it is 

 impossible to decide, and perhaps in vain to speculate, in the present state of 



The date of these remarks is 1856. In 1861 and 1862 appeared Maxwells 

 "theory of molecular vortices applied to magnetism, electricity, &c." which may 

 be considered as a development of Thomson's idea in a form which, though 

 rough and clumsy compared with the realities of nature, may have served its 

 turn as a provisional hypothesis. 



The concluding sections of the book before us are devoted to illustration! 

 of magnetic force derived from the motion of a perfect fluid. They are not 

 put forward as explanations of magnetic force, for in fact the forces are of 

 the opposite kind to those of magnets. They belong more properly to that 

 remarkable extension of the science of hydrokinetics which was begun by 

 Helmholtz and so ably followed up by Thomson himself. 



The conception of a perfectly homogeneous, incompressible frictionless fluid 

 ia as essential a part of pure dynamics as that of a circle is of pure geometry. 

 It is true that the motions of ordinary fluids are very imperfect illustration! 

 of those of the perfect fluid. But it is equally true that most of the objects 

 which we are pleased to call circles are very imperfect representations of a true 

 circle. 



Neither a perfect fluid nor a perfect circle can be formed from the materials 

 which we deal with, for they are assemblages of molecules, and therefore not 

 homogeneous except when regarded roughly in large masses. The perfect circle 

 is truly continuous and the perfect fluid is truly homogeneous. 



It follows, however, from the investigations of Helmholtz and Thomson that 

 if a motion of the kind called rotational is once set up in the fluid, the portion 

 of the fluid to which this motion is communicated, retains for ever, during all 

 its wanderings through the fluid mass, the character of the motion thus impressed 

 on it. 



This vortex then, as Helmholtz calls it, be it large or small, possesses that 



