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XXXIV. The Temperature of Sun-spots. Reply to Prof. 

 Wiedemann by Professors Liveing and Dewab*. 



PROFESSOR WIEDEMANN'S Note on our paper is 

 very much to the point and deserves full discussion ; 

 but, admitting the truth of the principles on which he rests 

 his argument, the conditions of the problem of solar spots do 

 not seem to us so simple that those principles can be directly 

 applied to it. We are not dealing with the radiation of a 

 single body at different temperatures, but with the sum of the 

 radiations and absorptions of a great thickness of more or less 

 transparent material. 



If we suppose the photosphere to consist of a cloud of liquid 

 or solid particles giving a continuous spectrum, most of the 

 radiation we get from those parts of the sun where there is no 

 spot will be derived from this cloud, and we shall get scarcely 

 any rays from the interior of the solar mass : the cloud will be 

 comparatively opaque, its absorption corresponding generally 

 to its emission. A further cooling of this cloud cannot well 

 produce the phenomena of spots which are characterized by a 

 selective absorption suggestive of a gaseous absorbent. Any 

 cooling of the photospheric cloud, which must be surrounded 

 by saturated vapour of its solid or liquid particles, would pro- 

 duce more solid or liquid with more continuous absorption. 

 On the other hand, if the photospheric cloud were raised in 

 temperature, so that the solid or liquid particles were to a 

 considerable extent vaporized, we should then get radiation 

 from the interior of the sun's mass, but sifted through the 

 vaporized photospheric matter. The total radiation would, 

 it is conceded, be greater now than before, but it would be 

 made up of the radiation of the vaporized photosphere 

 together with that of the sun beneath it. As regards the 

 former part the radiating matter would be a gas instead of 

 being solid or liquid, it would have changed its physical state 

 and therewith the character of its spectrum. The radiation 

 from the interior of the sun is added ; but this arises from 

 matter which may be different chemically, probably is different 

 as to physical state, from the matter of the photospheric cloud. 

 The argument of Prof. Wiedemann requires a continuity in 

 the nature and in the state of the radiating matter ; that is to 

 say, it must be the same matter first and last, and it must not 

 change from solid to gas. Neither condition is exactly ful- 

 filled in the case before us. Let us suppose, for ihe sake of 

 definiteness, that the photosphere consists of particles of silicon, 



* Communicated by the Authors. 



