1883.] on some of the Questions involved in Solar Physics. 321 



or tbe other, dissociation in obedience to Deville's law must take place 

 irrespective of tbe focal area ; but, inasmucb as tbe single solar ray 

 represented tbe same potential of energy or period of vibration as nume- 

 rous rays associated in a focus, it seemed reasonable tbat it sbould be 

 as capable of dealing with tbe isolated molecule as a mere accumulation 

 of tbe same witbin a limited space, and must tberefore possess tbe 

 same dissociating influence. Proceeding on tbese premises, tbe lecturer 

 bad procured tubes filled witb bigbly attenuated vapours, and bad 

 observed tbat an exposure of tbe tubes to tbe direct solar rays or to 

 tbe arc of a powerful electric ligbt effected its partial or entire dis- 

 sociation ; tbe quantity of matter contained witbin sucb a tube was 

 too sligbt to be amenable to direct cbemical test, but tbe change 

 operated by tbe light could be clearly demonstrated by passing an 

 electric discharge through two similar tubes, one of which had, and 

 the other bad not, been exposed to tbe radiant energy from a source 

 of high potential. If space could be thought filled with such vaj)our, 

 of which there was much evidence in proof, solar rotation would 

 necessarily have the effect of emitting such vapour equatorially by an 

 action of circulation which might be likened to tbat of a blowing 

 fan. When reaching tbe solar photosphere, by virtue of solar 

 gravitation this dissociated vapour would, owing to its increased 

 density, flash into flame, and could thus be made to account in great 

 measure for the maintenance of solar radiation, whilst its continual 

 dissociation in space would account for tbe continuance of solar radia- 

 tion into space without producing any measurable calorific effect. 



Time did not permit him to enter more fully into these subjects, 

 which formed part of his solar hypothesis, his main object on this 

 occasion having been to elucidate the point of cardinal importance to 

 that hypothesis, that of the solar temperature. 



[W. S.] 



I 



Y 2 



