38 



Mr. G. J. Stoney on the Physical 



[Recess, 



possesses the properties which are the conditions for the formation of cloud, 

 although it is probable that carbon is, of such substances, that one which 

 has by far the lowest vapour-density. It is, at all events, presumable that 

 among such abundant elements as nitrogen, oxygen, silicium, and alumi- 

 nium, or such of their compounds of low vapour-density as can exist in the 

 sun, there may be some which, like carbon, are solid or liquid at the tem- 

 peratures and pressures of the greatest heights to which they would, if 

 gaseous, rise. And if the atmosphere of the sun extend to any great dis- 

 tance below the photosphere, there must be in the sun such a substance 

 to account for the dusky background we see in the penumbrse and umbrae 

 of spots. There must in this case be a second layer of clouds, formed not 

 far beyond the photosphere, in the comparatively short space through 

 which the temperature augments rapidly between the luminous clouds and 

 the central parts of the sun. These clouds must, moreover, be of some 

 transparent material to possess in a sufficient degree that property of scat- 

 tering light which would render them as devoid of emissive power as we 

 see them to be. For the same reason we must conclude that the sooty 

 shower from above cannot reach them, as it would inevitably soil them, so 

 as to deprive them of these essential qualities. We learn from this, that the 

 point at which carbon boils must fall within the short interval between the 

 two layers of clouds. This is not at all unlikely, inasmuch as the advance 

 downwards of the inverse flame, of which mention has been so often made, 

 would probably be arrested only by its close approach, either to the bottom 

 of the atmosphere, or to the situation in which carbon boils, so as to be 

 entirely dissipated in vapour. And the second layer of clouds would quickly 

 follow, since its position depends on that taken up by the carbon clouds, 

 as it must lie within the layer of rapidly varying temperature immediately 

 under them. If this hypothesis, then, be the true account of what takes 

 place on the sun, the penumbrse of spots are caused by our seeing the 

 clouds beneath through a gauze-like film of carbon cloud which has ceased 

 to send down rain ; and the umbrae of spots are formed when a very shal- 

 low saucer-like depression of the photosphere has carried a part of its 

 outer surface so far that it has reached the region in which carbon will 

 boil. Here the filmy cloud of carbon, which nowhere else can entirely 

 disappear, will be completely dissolved away. 



65. In this branch of our enquiry we are often obliged to deal with hy- 

 pothetical matter, and cannot, in such cases look for conclusions which 

 command our assent. We must be satisfied if we may hope that they will 

 prove of use in guiding future investigations. Nevertheless, I am disposed 

 to think that we should give the preference, as a provisional hypothesis, to 

 the supposition of a layer of cloud lying under the photosphere, rather 

 than to the only other alternative which seems in any considerable de- 

 gree admissible, namely, a highly reflecting ocean. It is perhaps, on the 

 whole, and in our present state of ignorance, encumbered with fewer dif- 

 ficulties. 



