EADIATION. 175 



bability the actual nature, form, and motion of those 

 molecules. He shows that an atom is in all probability 

 nothing more nor less than a vortex ring. You have all 

 seen smoke rings which have been formed from the mouth 

 by tobacco smoke, or which are often seen coming from the 

 mouth of a cannon a ring of smoke passing through the 

 air with considerable velocity, and this ring having con- 

 volutions from the exterior of the ring into the interior 

 round and round. Such a smoke ring is the fundamental 

 idea of a vortex atom as denned by Sir William Thomson. 

 These results may seem to be too hypothetical, but on a 

 clear examination of the grounds which have led him to 

 adopt these views, there is very little doubt that if not the 

 true explanation of the nature of a molecule it is something 

 very close to it. 



Furthermore as to the existence of the ether, we have 

 several independent proofs. The illustrious astronomer 

 Encke found that the comet which bears his name was 

 retarded year by year. 



Each time this comet appeared in its elliptic orbit round 

 the sun, it was found to have a shorter period, and to be 

 revolving quicker round the sun. Now we know from the 

 law of gravity, that if it be revolving quicker round the 

 sim, it must be closer to the sun, and the only force which 

 Encke could conceive of to bring the comet close to the 

 sun was the resistance of an ether to its motion which 

 increases the force which the sun exercises upon it in 

 relation to its momentum forwards. Various astronomers 

 have calculated the effects of this, and the general 

 impression is that there is a resisting medium acting upon 

 this comet, but that the exact law of resistance which was 

 assumed by Encke is perhaps not exactly correct. 



But we have furthermore some most remarkable experi- 

 ments by Professors Tait and Balfour Stewart, which appear 

 to point most clearly to the existence of an ether. Here 

 we have the remarkable apparatus which was employedby 

 them. It consists of a receiver which can be exhausted by 

 means of a powerful air-pump. Within this receiver there 

 is a disc which is capable of rapid rotation, and mechanism 

 is applied to it by means of which this rotation can be 

 given. A rod acts upon this mechanism and passes down 

 through a barometer tube, so that it can be easily turned 



