10 



Mr. G. J. Stoney on the Physical 



[Recess, 



as the earlier observers supposed ; hence at least the outer layer of the 

 photosphere is mobile. It is accordingly either a gas, a liquid, or a cloud. 

 The nature of its spectrum forbids our admitting it to be a layer of gas * of 

 moderate depth ; and if the layer of gas were so profound as to be opake, 

 it would radiate the maximum amount of light belonging to its tempera- 

 ture at great depths, and so obliterate the mottled appearance which exists. 



Again, an opaque liquid would be luminous only at its surface, which we 

 have found to be inadmissible. Nor is an ocean of transparent liquid suf- 

 ficient. Little light gets through 20 metres of sea-water, and probably a 

 few hundred metres of the most transparent liquid would be practically 

 opaque. This trifling depth therefore would render the incandescent 

 ocean luminous to almost the full extent which is possible for a body of its 

 temperature. Such an ocean, therefore, if tranquil at the surface, would 

 reduce the whole sun's disk to an uninterrupted gradation t of brightness. 

 If, to account for the granulation, we suppose the ocean to be every here 

 and there fretted with storms, the foam, being endowed with the property 

 of scattering light abundantly, would no doubt be a bad emitter, and would 

 therefore form dusky spots; but these spots would be most conspicuous at the 

 centre of the disk. We must therefore reject the hypothesis of a transparent 

 ocean. The hypothesis of a cloud, then, is the only one which remains. 



20. Of clouds, there are two well-marked varieties — clouds precipitated 

 from a state of vapour, like the clouds in our atmosphere, and clouds of 

 fixed solids or fixed liquids, such as smoke, a cloud of dust, the mud in 

 turbid water, oil in an emulsion. The sun attracts with so much more 

 force than the earth that everything on his surface presses down with a 

 force twenty-eight times as great as that with which it would press down- 

 wards on the earth's surface. From this, and from the amazing extent of 

 his outer atmosphere, it is natural to suppose that the pressure in its lower 

 strata must be enormous. This must occasion the lower strata to be very 

 dense, unless the effect of the pressure be counteracted by the terrific heat. 

 On the other hand, the average density of the whole sun being only about 

 one-fourth of that of the earth, the solids and liquids on his surface are pro- 

 bably much less dense than with us. If it should happen that the lower 

 strata of the atmosphere were more dense than some of the solid or liquid 

 substances on the sun, these latter would rise until they reached that part 

 of the atmosphere which is of the same density as themselves, and would 

 float there ; and if in a state like dust, they would doubtless be maintained 

 in violent agitation by currents of convection, those on the outside being 

 most cooled by radiation and sinking, to be replaced by others from the 



* [i. e. a layer of gas whose spectrum is interrupted. But if the luminous matter of 

 candle-flames be gaseous, such a gas is uot excluded by this consideration. A gas of 

 this kind, however, would be in a considerable degree opaque, and behave on the sun 

 like the cloud of dust which is disposed of in § 20. —September 1868.] 



t The surface, if sufficiently undisturbed, would act as a mirror near the margin of 

 the disk ; and accordingly the light emitted by it would in proportion fall off. 



