LUCRETIUS AND THE ATOMIC THEORY 205 



from Hobbes, and also by John Bernoulli, who further argues 

 that this property may be given by re-entering motion. 



This very idea, first due, we think, to Hobbes, and now 

 proved possible by rigid mathematics, is perhaps the latest con- 

 tribution to our subject. Helmholtz has proved that in a 

 perfect fluid one vortex or whirlpool cannot destroy another, 

 cannot cut through it or divide in any way from the outside 

 so that a ring-shaped vortex, for instance, would be quite inde- 

 structible by other vortices ; by a ring-vortex we do not mean 

 one in which the fluid moves round in a simple circle, but a 

 ring built up of a series of such little circles side by side ; each 

 little circle placed as a circlet of thread tied on a marriage ring 

 would be. Such a ring-vortex as this, once set goirg in a perfect 

 fluid, in which no friction occurs, would go on for ever, if we 

 suppose our fluid endowed with inertia. Our ring- vortex might 

 be stretched, squeezed, even knotted by other similar vortices, 

 but it could never be pierced by them, never destroyed, and 

 would, in all its metamorphoses, retain some of its original 

 characteristics, depending on the velocity of its particles and 

 its magnitude. Sir William Thomson at once pounced on 

 this indestructible vortex as possibly fulfilling the conditions 

 required for a practical atom. Each vortex would be indestruc- 

 tible, since we could never bring to bear on it anything but other 

 like vortices. It would be elastic, in virtue of the motion of its 

 parts only, without any assumption of elasticity in its materials - 

 an idea this hard to grasp, but to be practically felt by anyone 

 who tries to upset a good heavy top. He will find that, as he 

 pushes it over, it resists, and will come upright again, exerting 

 what we may call a kind of elasticity due to motion only. 

 Moreover, Thomson shows that these very vortices have neces- 

 sary modes of vibration, which may correspond to the special 

 waves of light which the chemical atom of each elementary sub- 

 stance is capable of exciting or receiving ; knotted, or even 

 knitted, they would explain cohesion and chemical properties 

 without any supposition of attraction or repulsion between atoms. 

 By their impact they may explain the elasticity of gases in the 

 manner proposed by a later Bernoulli ; by other motions, such 

 as those treated of by Thomson himself and Clerk Maxwell, they 

 may cause magnetism and electricity. Nor is more required for 



