412 THE CHEMISTRY OF THE SUN. [CHAP. 



The last act in the history of a spot is its invasion by the 

 faculse. These remain long after the spot has entirely closed 

 up, and in this connection it is important to remark that new 

 spots very often break out in the old place. These of course, 

 unlike the first spot in that position, will appear to be preceded 

 by faculae. 



We have then the following generic grading of the effects 

 produced by the descending material, in the order of intensity : 



(1) Pores. 



(2) Veiled spots. 



(3) Small quiet spots with penumbra? and umbrae. 



(4) Large areas containing several spots, giving evidence of 

 violent disturbance. 



4. By the hypothesis this falling material is dissociated in its 

 descent before or when it reaches the photosphere ; the particles 

 which descend sparsely and gently will be vaporized gently, and 

 those which descend violently and in great masses will be exploded 

 violently. 



We have before seen that the down-rush is of course pro- 

 duced by gravitation. The velocity generated in the fall gives 

 us kinetic energy in the moving gas, and as this velocity (which, 

 neglecting friction, must reach many miles a second) is checked 

 by the surrounding gases, the kinetic energy becomes heat. 

 This heat produces sudden expansions, and the central initial 

 down-rush is now surrounded by up-rushes. 



These effects will differ in degree, depending upon the amount 

 of descending material and the height of fall, and we shall get 

 different appearances due to these various degrees. 



On this point the history is quite complete. 



(1) Domes. All over the surface are seen domes of faculae, 

 either separate, or combined in masses of greater or less import- 

 ance ; and when the definition is quite perfect it is noted that 



