96 Mr. C. S. Hastings's Theory of 



bright lines in the spectrum of any flame to which daylight 

 had access ; for in this case the conditions demanded by the 

 first principle are fully met, the sun being the origin of the 

 daylight. That we do not see absorption-lines is due, then, 

 alone to the lack of necessary brilliancy iu the daylight. 



Thus much premised, we can frame a theory which explains 

 all the observed phenomena exhibited by the spectroscope, and 

 is also rendered highly probable by the revelations of the 

 telescope. 



As is well known, the solar surface, when examined with a 

 powerful telescope of large aperture, presents a granulated ap- 

 pearance, the granules in general subtending an angle of a 

 fraction of a second only. Probably this appearance is better 

 known to the majority of astronomers by means of Professor 

 Langley's admirable drawings * rather than by personal obser- 

 vation. These granules I regard as marking the locus of cur- 

 rents directed generally from the centre of the sun. About these 

 currents are necessarily currents in an opposite direction which 

 serve to maintain a general equilibrium in the distribution of 

 mass. Let us consider the action of such an ascending current. 

 Starting from a low level at a temperature which we may regard 

 as above the vaporizing-point of all elements contained in it, 

 as it rises to higher levels it cools, partly by radiation, more 

 by expansion, until finally the temperature falls to the boiling- 

 point of one or more of the substances present. Here such 

 substances are precipitated in the form of a cloud of fine 

 particles which are carried on suspended in the current. The 

 change of state marked by the precipitation is accompanied 

 by a sudden increase in radiating-power; hence these particles 

 rapidly lose a portion of their heat and become relatively dark, 

 to remain so until they are returned to lower levels by the 

 currents in a reverse direction. 



In this theory, it will be observed, there is nothing which 

 does violence to our accepted notions of the solar constitution. 

 Indeed it differs chiefly from that of Faye in localizing the 

 phenomena of precipitation, instead of regarding it as proper 

 to all portions of the photosphere, and (what is quite as im- 

 portant) in supposing' the precipitation confined to one or two 

 elements only. I shall attempt to define these elements 

 further on. 



In our theory, then, the granules are those portions of up- 

 ward currents where precipitation is most active, while the 

 darker portions between these bodies are where the cooler 

 products of this change with accompanying vapours are sink- 

 ing to lower levels. 



* Silliman's Journal, vol. vii. 1874, and vol. ix. 1875, plates. 



