Brown. — The Maintenance of the Sun's Heat. 397 



but both of them place a limit to the solar life. In the case of 

 the meteoric theory, it cannot be supposed that the supply of 

 meteors is inexhaustible ; we must look forward to the time 

 when every stone wandering in the planetary spaces shall have 

 fallen iuto the sun, and when, therefore, the supply of heat 

 shall cease, a time to be followed at no great distance by the 

 dying away of the solar light. Since a mass of matter cannot 

 go on contracting for ever, it is evident that the shrinkage 

 theory, like the meteoric, cannot invest the sun with the attri- 

 bute of permanence. In this respect they both fail to commend 

 themselves to the mind, as has been said by Sir W. Siemens : 

 " If either of these hypotheses could be proved, we should only 

 have the satisfaction of knowing that the solar waste of energy 

 by dissipation into space was not dependent entirely upon loss 

 of its sensible heat, but that its existence as a luminary would 

 be prolonged by calling into requisition a limited, though may 

 be large, store of energy in the form of separated matter. The 

 true solution of the problem will be furnished by a theory, 

 according to which the radiant energy which is now supposed 

 to be dissipated into space and irrecoverably lost to our solar 

 system, could be arrested and brought back in another form to 

 the sun itself, there to continue the work of solar radiation." 



In accordance with this idea, Sir W. Siemens propounded a 

 theory regarding the conservation of the sun's heat, which I will 

 endeavour to explain to you. In order to understand this theory 

 we must suppose that the planetary system is immersed in a 

 rarified atmosphere, consisting mainly of hydrogen, marsh gas, 

 carbonic oxide, water vapour, etc. ; that this is no unreasonable 

 assumption is made clear to us by the fact that it has been 

 proved by Maxwell, Clausius, and Thomson that it is impossible 

 to assign a limit to a gaseous atmosphere in space. The nature 

 of this interplanetary atmosphere is, moreover, made known to 

 us by the meteorolites which frequently find their way to the 

 earth ; these meteorolites contain gases hidden in their pores, 

 which, being other than oxygen or nitrogen, must, one would 

 think, have been derived from the interplanetary spaces. These 

 gases are those just enumerated. Further proof, if any be 

 needed, of the existence of gaseous matter in interstellar space, 

 is furnished by spectrum analysis, which tells us that the nucleus 

 of a comet contains carbon, hydrogen, nitrogen, and probably 

 oxygen. 



Having arrived at a conception of an interplanetary atmo- 

 sphere, we have next to think of the action of the sun upon it. 

 Let us first investigate the action of any revolving body upon 

 the gaseous medium in which it is placed. (Exp., wheel and 

 candles.) 



We thus see that the sun must act like a great fan, projecting 

 the gases from its equator, and drawing them in at its poles, 



