102 Theory of the Constitution of the Sun. 



As these currents move upward, they are cooled, mainly by 

 expansion, until a certain element (probably of the carbon 

 group) is precipitated. This precipitation (restricted, from the 

 nature of the action) forms the well-known granules. There 

 is nothing which has come under my observation which would 

 indicate a columnar form in these granules under ordinary cir- 

 cumstances. 



The precipitated material rapidly cools, on account of its 

 great radiating-power, and forms a fog or smoke, which settles 

 slowly through the spaces between the granules till revolati- 

 lized below. It is this smoke which produces the general 

 absorption at the limb and the " rice-grain " structure of the 

 photosphere. 



When any disturbance tends to increase a downward con- 

 vection current, there is a rush of vapours at the outer surface 

 of the photosphere toward this point. These horizontal cur- 

 rents or winds carry with them the cooled products of preci- 

 pitation, which, accumulating above, dissolve slowly below in 

 sinking. This body of " smoke " forms the solar spot. 



The upward convection currents in the region of the spots 

 are bent horizontally by the centripetal winds. Yielding their 

 heat now by the relatively slow process of radiation, the loci 

 of precipitation are much elongated, thus giving the region 

 immediately surrounding a spot the characteristic radial struc- 

 ture of the penumbra. 



This conception of the nature of the penumbra implies a 

 ready interpretation of a remarkable phenomenon amply 

 attested by the most skilful observers, and, as far as my know- 

 ledge goes, wholly unexplained — namely, the brightening of 

 the inner edge of the penumbra in every well-developed spot*. 



This interpretation is perhaps most readily imparted by a 

 comparison of the hot convection currents in the two cases. 

 When the convection current is rising vertically, the medium 

 is cooled by expansion until the precipitation-temperature is 

 reached, when all the conductible material appears suddenly, 

 save as it is somewhat retarded by the heat liberated in the 

 act. Immediately afterward the particles become relatively 

 dark by radiation. In the horizontal current a very different 

 condition of things obtains. Here the medium does not cool 

 dynamically by expansion, but only by radiation — hence, since 

 the radiation of the solid particles is enormously greater than 

 that of the supporting gas, practically by that of the particles 



* Relating to this phenomenon, see important observations by Professor 

 Langley, Silliman's Journal, vol. ix. (1875) p. 194 ; also Le Soleil, par Le 

 P. A. Secchi, Paris, 1875, chap, iv. p. 80, and particularly fig. 46, p. 90, 

 with explanatory text. 



