1868.] 



Constitution of the Sun and Stan 



41 



clouds of the photosphere, we must remember that there may exist other 

 substances in the sun or in some other stars capable of giving rise to clouds. 

 If there be materials of sufficiently low vapour-density, and in a sufficient 

 degree more volatile than carbon, though not volatile enough to stand the 

 cold of the height to which their vapour- density would otherwise lift them, 

 they will be precipitated in cloud. Or gases in the solar atmosphere which 

 are kept asunder by the temperatures of its lower strata, may be able to 

 combine in the cooler regions above. If the new body be a solid or liquid, 

 it will constitute a cloud. Even if it be gaseous, it will in general have 

 other spectral lines than those of any lower-lying gas in the atmosphere, 

 and will therefore be subjected to the direct radiations of the photosphere ; 

 it will accordingly become intensely heated, and in many respects behave 

 like a cloud. Its density, too, will in most cases be greater than that of 

 either of its constituents. And, finally, a gas which in the lower parts of 

 the sun's atmosphere emits only rays of a spectrum of the second order, 

 may in the upper regions find itself under circumstances to produce a 

 spectrum of the first order. If this should happen, the gas in its new 

 condition would be exposed to the full heat of the photosphere, and 

 would conduct itself like a cloud. 



70. From the exceeding transparency of the solar clouds, they are 

 entirely without that abundance of internal reflections and refractions 

 which are what give to a cloud of steam dense enough to be opake, or 

 a sheet of paper, or a piece of white marble, their lustre when illumi- 

 nated. It is accordingly by their inherent splendour given to them by 

 their being made intensely hot by the photosphere, not by borrowed 

 light, that they shine. A cloud of dark opake materials is therefore, 

 cceteris paribus, the brightest. Those which Mr. De la Rue found im- 

 pressed on the photographs, though not visible to the eye, must have been 

 of substances transparent in regard to most visible vibrations, but opake for 

 some of higher refrangibility. 



71. It is very likely that there maybe substances in the sun's atmo- 

 sphere, or in those of some of the stars, which reach a height at 

 which they are unable to remain in the state of gas by reason of the sur- 

 rounding cold, or to assume permanently the form of cloud because of the 

 heat of the photosphere to which they would thereupon immediately become 

 exposed. In such cases there will be a struggle between the two condi- 

 tions, the vapour continually condensing and redissolving until it has by 

 this process imported much heat into its neighbourhood. Wherever such a 

 state of things exists, it must inevitably have the effect of raising some of 

 the isothermal surfaces above the position in the atmosphere they would 

 otherwise occupy. Similar consequences would ensue if two gases became 

 so cool that they could no longer continue uncombined, and were so heated 

 through their new spectral lines the instant they united that the new sub- 

 stance was at once resolved back into its constituents ; or where a gas 

 reaches a situation too cold for its existence in the state in which it sends 



