168 A NEW SOLAR THEORY. 



photospheric radiation in tho same way as do our atmosphere and its 

 clouds on the radiation from the soil. We are (juite familial" with tlie 

 fact that clear nights are, as a rule, cooler than cloud}" ones, and we 

 explain this phenomenon 1)y the assumption that on clear nights radia- 

 tion from the soil into space goes on more freely than when clouds 

 offer an effective impediment to the dissipation of radiant energy. 



We conclude, then, that the progressive cooling of the star leads to 

 the formation of an absorbing envelope above its photosphere, by 

 which the disproportion ])etween the generatio'n and loss of energy is 

 reduced. But if, under the conditions at the epoch /,, the amount of 

 energy actually radiated into space still exceeds what is produced by 

 contraction, the photosphere will move to a^, still nearer to the centre, 

 and the quantity of al)sorbing matter in the layer <i^a.^ will be further 

 increased. Now, although a.^ enuts the same quantity of energy as did 

 f/^ and <i^ at the former epochs, the total amount of radiation emerging 

 into space must, at the epoch f.^^ be less than it was at t, and t^. Thus 

 the opacity of the cooled atmosphere gradually increases as time goes 

 on, and the total radiation of the star l)ecomes less and less. Since no 

 force is present to interfere with the cooling of the layers a^^ a., . . . , 

 a moment t,^ must eventually be reached at which the photosphere at 

 r/^^, through reflection from all the hwers above it, receives back so 

 much of its radiation that its total expenditure of energy is exactly 

 counterbalanced by the energ}' contributed by the contractile forces. 



This result appears to be of eminent importance, for it shows that 

 even on a star with deficient contraction the exact compensation of the 

 loss of energy ma}" still be possi])le from a certain layer downward. 

 This state, so exceedingly important for the conservation of energy 

 within the star, is brought al)out by the progressive cooling of its 

 superlicial layers, which thereb}" increase their power of absorption, 

 and thus offer a more and more effective check to the radiation from 

 the incandescent layers below. 



Here, now. we are confronted with a question which leads us at 

 once to tiie principal o])ject of this in((uiry: Can the state of thermal 

 e([uilihriiun thus eventually attained l)y the layer a^, be permanent^ 

 Th(^ answer is clearly negati\'e. For when a^, has arrived at this state, 

 none of the layers «j, a,^ . . . a^^.^ outside (t,, have reaclnMl tlu^ same 

 condition. Th(>ir cooling is bound to go on, and consecpuMitly their 

 al)ility to absorb and reflect the heat emanating from the laycnw/^^ nnist 

 still fui'ther increase ev(Mi after the establishnuMit of th(M'mal e(|uili- 

 brium at (/,,. Hut, owing to this increasing amount of reflection 

 toward it, the layer a^-^ will now dissipate e^ (>n less energy than is 

 requii'ed for the maintenance of thermal e(iuilibriuni, and therefore 

 nuist ])ecome overheated. It thus comes to pass that, while the func- 

 tion of the a])s()r])ing envelope is that of reducing as much as possible 

 the waste of imergv from the photosi)hei'ic layers, it is, by the very 



