ATOM. 471 



perties of bodies. But it fails to account for the vibrations of a molecule as 

 revealed by the spectroscope. We may indeed suppose the atom elastic, but 

 this is to endow it with the very property for the explanation of which, as 

 exhibited in aggregate bodies, the atomic constitution was originally assumed. 

 The massive centres of force imagined by Boscovich may have more to recom- 

 mend them to the mathematician, who has no scruple in supposing them to be 

 invested with the power of attracting and repelling according to any law of the 

 distance which it may please him to assign. Such centres of force are no 

 doubt in their own nature indivisible, but then they are also, singly, incapable 

 of vibration. To obtain vibrations we must imagine molecules consisting of 

 many such centres, but, in so doing, the possibility of these centres being 

 separated altogether is again introduced. Besides, it is in questionable scientific 

 taste, after using atoms so freely to get rid of forces acting at sensible dis- 

 tances, to make the whole function of the atoms an action at insensible distances. 



On the other hand, the vortex ring of Helmholtz, imagined as the true 

 form of the atom by Thomson, satisfies more of the conditions than any atom 

 hitherto imagined. In the first place, it is quantitatively permanent, as regards 

 its volume and its strength, two independent quantities. It is also qualita- 

 tively permanent as regards its degree of implication, whether " knottedness " 

 on itself or " linkedness " with other vortex rings. At the same time, it is 

 capable of infinite changes of form, and may execute vibrations of different 

 periods, as we know that molecules do. And the number of essentially dif- 

 ferent implications of vortex rings may be very great without supposing the 

 degree of implication of any of them very high. 



But the greatest recommendation of this theory, from a philosophical point 

 of view, is that its success in explaining phenomena does not depend on the 

 ingenuity with which its contrivers "save appearances," by introducing first one 

 hypothetical force and then another. When the vortex atom is once set in 

 motion, all its properties are absolutely fixed and determined by the laws of 

 motion of the primitive fluid, which are fully expressed in the fundamental 

 equations. The disciple of Lucretius may cut and carve his solid atoms in the 

 hope of getting them to combine into worlds; the follower of Boscovich may 

 imagine new laws of force to meet the requirements of each new phenomenon; 

 but he who dares to plant his feet in the path opened up by Helmholtz and 

 Thomson has no such resources. His primitive fluid has no other properties than 

 inertia, invariable density, and perfect mobility, and the method by which the 



