450 SCIENTIFIC PAPERS OF SIR WILLIAM SIEMENS. 



able that it should be as capable of dealing with the isolated 

 molecule as a mere accumulation of the same within a limited 

 space, and must therefore possess the same dissociating influence. 

 Proceeding on these premisses, the lecturer had procured tubes 

 filled with highly attenuated vapours, and had observed that an 

 exposure of the tubes to the direct solar rays or to the arc of a 

 powerful electric light effected its partial or entire dissociation ; 

 the quantity of matter contained within such a tube was too 

 slight to be amenable to direct chemical test, but the change 

 operated by the light could be clearly demonstrated by passing an 

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

 and the other had not, been exposed to the radiant energy from a 

 source of high potential. If space could be thought filled with 

 such vapour, 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 

 that of a blowing fan. When reaching the 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 the 

 continuance of solar radiation into space without producing any 

 measurable calorific effect. 



Time did not permit him to enter more fully on 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. 



