OF ARTS AND SCIENCES. 145 



comineut ; the second, thougli not less evident, is less familiar because 

 less important. As we shall make use of it, however, it may be well 

 to enforce it by reference to common experience. Were it not true, it 

 would be impossible to see 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 in the daylight. 



Thus much premised, we can frame a theory which explains all the 

 observed phenomena exhibited by the spectroscope, and is also ren- 

 dered 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 appearance, the 

 granules in general subtending an angle of a fraction of a second only. 

 Probably this appearance is better known to the majority of astrono- 

 mers by means of Professor Langley's admirable drawings,* rather 

 than by personal observation. These granules I regard as marking 

 the locus of currents 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 cur- 

 rent. 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 vio- 

 lence to our accepted notions of the solar constitution. Indeed, it 

 differs chiefly from that of Faye in localizing the phenomena of pre- 

 cipitation, instead of regarding it as proper to all portions of the pho- 

 tosphere ; and, what is quite as important, in supposing the precipitation 

 confined to one or two elements only. I shall attempt to define these 

 elements farther on. 



* Am. Jour. Sci., Vol. VH., 1874, and Vol. IX., 1875, Plates. 



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