THE PROGRESS OF PHYSICS. 167 



point without extension in space; it has not the 

 property of impenetrability, for two atoms can, it 

 is supposed, exist at the same point." * A similar 

 view was held by Faraday. 



Heterogeneousness. — (3) In his Recent Advances 

 (1876, p. 288), Prof. P. G. Tait described '' a third 

 notion — that the matter of any body, wdiere it does 

 not possess pores, like those, for instance, of a sponge 

 (which obviously does not occupy the whole of the 

 space which its outline fills), fills space continu- 

 ously, but with extraordinary heterogeneousness." 

 If the moon were built up of irregular stones and 

 mortar, it would seem homogeneous to us (at a dis- 

 tance of 250,000 miles), so the drop of water (re- 

 moved as it were to a distance by its minuteness) 

 may only be apparently homogeneous. 



Vortex Atoms, — (4) A more fertile theory, sug- 

 gested in 1867, is that of Lord Kelvin — " that what 

 we call matter may really be one only the rotating por- 

 tions of something which fills the whole of space; 

 that is to say, vortex-motion of an everywhere present 

 fluid." t 



The beautiful circular vortex-rings which can be 

 so readily made with tobacco or other smoke in air, 

 and with a little ingenuity in water, have very inter- 

 esting properties (first mathematically deduced by 

 Helmholtz). Thus a vortex ring cannot be cut; " it 

 simply moves away from or wriggles round the knife, 

 and, in this sense, it is literally an atom." J It moves 

 through the air of the room as if it were an independ- 

 ent solid body ; one will pass through another and al- 

 low that other to pass through it; and it obviously 

 has an extraordinary power of persistence. 



* Glazebrook. James Clerk Maxwell and Modern Physics, 

 1896, p. 108. 

 t Recent Advances, p. 20. $ Recent Advances, p. 297. 



