OP ARTS AND SCIENCES. 149 



ments found in abundance above it, and those in general of low vapor 

 density. 



3d. The element is not a rare one. Of these guides the last is 

 perhaps of the least value. 



The substances which apparently meet all these conditions are car- 

 bon and silicon : nor is it easy to name any other which will. Accept- 

 ing for a moment as an hypothesis that the light coming from the sun 

 is radiated by solid or liquid particles of carbon just at the point of 

 vaporization, let us see if the facts of observation fulfil the implied 

 conditions. 



As a first consequence, we see that the temperature and light of the 

 photosphere are defined as those of solid carbon at the point of volatili- 

 zation. In the electric arc there is a very small area of the positive 

 carbon pole at this high temperature. Though this area is in a very 

 disadvantageous position for observation, and can consequently have 

 but a disproportionately small share in producing the total effect, the 

 splendor of the electric light might almost tempt us to believe the 

 guess a valid one. Another consequence implied, however, — namely, 

 that the spectral lines proper to simple carbon are absent in the solar 

 spectrum, — is doubtless better adapted as a crucial test of the hypoth- 

 esis than a study of the electric light. There has been evidence 

 recently offered that carbon lines are present in the solar spectrum. 

 Granting this, we perceive that the photosphere contains solid or liquid 

 particles hotter than carbon vapor, and consequently not carbon, 



I am then inclined to suspect that the photospheric material may be 

 silicon, which, though denser in the gaseous state than carbon, is not 

 improbably more abundant. There is also good reason to suppose 

 that carbon is precipitated at a higher level ; and the analogous but 

 less common element boron may add a minor effect. 



In the explanations which I shall give of the remaining phenomena, 

 it may serve to fix the ideas, to think of the granules which charac- 

 terize the sun's photosphere as clouds of a substance like precipitated 

 silicon. At any rate, we are sure that the substance in question, so 

 far as we know it, has properties similar to those of the carbon group, 



I have given plausible explanations of all the phenomena included 

 specially in my own observations. It remains to discuss the others, 

 briefly mentioned above. 



The substance precipitated cools very rapidly, as it is an excellent 

 radiator separated from space only by extremely diathermous media. 

 It forms then a smoke-like envelope, which ought to exert just such a 

 general absorption as that observed at the limb of the sun. It is thin 



