1868.] 



Constitution of the Sun and Stars. 



5 



nimum which would only be possible under the condition that the luminous 

 stratum was an absolute screen stopping- every ray of heat coming from 

 beyond it, and also reducing the entire of the intermingled atmosphere of 

 fixed gases quite to its own low temperature. The former of these con- 

 ditions, especially, seems improbable when we bear in mind that the film 

 in which cloud can form must be so transparent as to admit of the abundant 

 radiation towards the sky which we have found to be essential. We shall 

 be able to treat this subject further on with more precision ; but, in the 

 meantime, we are clearly entitled provisionally to regard the film of 

 clouds as colder than the regions on either side of it. 



10. Let us now consider more attentively the thermal conditions of the 

 photosphere and of the subjacent regions. In doing this it is necessary 

 to distinguish that part of the condensed vapour from which there is 

 so abundant a radiation outwards as would enable vapour in that region 

 to pass into cloud, from such other parts of the condensed vapours as are 

 too much screened from the sky to allow any more cloud to form. It will 

 accordingly be convenient henceforth to restrict the word cloud to the 

 former, and to use such words as mist or rain when we have occasion to 

 speak of condensed vapour in a lower situation. Now, in the first place, 

 if from any cause a part of the vapour fitted to produce luminous clouds 

 rose above the general level and became detached, it would form a cloud, 

 which by its own weight, and by the coolness it would impart to the fixed 

 gases interspersed through it, would gradually settle down till it became 

 merged in the general luminous stratum. This behaviour would be hastened 

 by a sudden change of the density of the solar atmosphere, which, as we 

 shall find hereafter, takes place at the boundary of the photosphere. 



11. The clouds, though of a thickness small when compared with the 

 enormous extent of the atmosphere of the sun, may nevertheless be of 

 considerable depth ; but they can in no place be of such a density and 

 thickness as to be opaque, since no part of the stratum can come into 

 existence from which there is not a sufficiently free radiation towards parts 

 already cooled down or towards the open sky. This, then, will put a 

 limit to the density of the clouds. If, from any cause, heat is supplied 

 unequally to different parts of the stratum, the density of the clouds 

 must be correspondingly unequal, inasmuch as, in the more heated 

 regions, even the lowest part of the stratum, which is the worst-situated, 

 must be sufficiently exposed to the sky to enable it, under these adverse 

 circumstances, to maintain the low temperature which is essential to the 

 formation of cloud. The clouds will accordingly be rarest where most 

 heated. As, then, the clouds are translucent in all parts, and in some parts 

 more so than in others, it becomes of importance to study the intensity 

 of the heat and light which reach us from beyond them. 



12. The clouds must either brood like a fog over the surface of the 

 subjacent ocean, or they are separated from it by an interval. I will deal 

 with the latter hypothesis first. Assuming, then, that there is such an in- 



