406 THE CHEMISTRY OF THE SUN. [CHAP. 



Now assume a disturbance. Where is it most likely to arise ? 

 It is most difficult to imagine how a disturbance can arise in 

 the centre of such a globe of gas as we are considering in a 

 state of perfect equilibrium, but on the other hand it is quite 

 easy to see that such a disturbance is very likely to arise among 

 the exterior layers, either from collisions among the solid particles 

 which must exist there mingled with the permanent gases in 

 consequence of the lower temperature of those layers, or from 

 some action from without, such as the planetary action suggested 

 by De la Rue, Stewart and Loewy. Hence : 



2. By the hypothesis, in any given solar area of disturbance 

 of the convection equilibrium a down-rush of cooler materials 

 produced ly gravitation must begin the disturbance. 



Now what are the facts ? There is much evidence to show 

 that a spot is the first disturbance of the photosphere in the 

 region where it is formed. I mean the faculce and the move- 

 ments in the general siirface follow, and do not precede the 

 formation of a spot. On this point I first quote Dr. Peters, 1 

 one of our highest authorities : 



" The spots arise from insensible points, so that the exact moment 

 of their origin cannot be stated ; but they grow very rapidly in the 

 beginning, and almost always in less than a day they arrive at their 

 maximum of size. Then they are stationary, I would say, in the 

 vigorous epoch of their life, with a well-defined penumbra of regular 

 and rather simple shape. So they sustain themselves for ten, twenty, 

 and some even for fifty days." 



I next quote the Rev. S. J. Perry, one of the most constant 

 of modern solar observers : 



" And, to begin with spot-formation, we find almost invariably 

 that large solar spots start life as little dots, frequently in groups, 

 and then grow at once with enormous rapidity. A spot will often 



1 Proceedings of the American Association for the Advancement of Science, 

 vol. ix. 



