THE SUN. 



75 



really surfaces deprived of light, for the most intense artificial light which can 

 be produced, such, for example, as that of a piece of quick-lime exposed to the 

 action of the compound blow-pipe, when seen projected on the sun's disk, 

 appears as dark as the spots themselves ; an effect which must be ascribed 

 to the infinitely superior splendor of the sun's light. All that can be legiti- 

 mately inferred respecting the spots, then, is, not that they are destitute of 

 light, but that they are incomparably less brilliant than the general surface of 

 the sun. 



The thickness of the luminous coating which covers the sun, was attempted 

 to be measured by Sir William Herschel, by means of observations made on 

 the spots, and the result of his inquiry was that its depth varied from two to 

 three thousand miles. The under and non-luminous stratum, by reflecting a 

 considerable portion of the rays which fall upon it from the luminous stratum 

 above, not only increases the light which the luminous stratum disperses through 

 space, but serves as a canopy to screen the solid body of the sun from the 

 overpowering effects of the light and heat of the superior stratum. Herschel 

 even supposed that the density of the lower stratum might be such as to main- 

 tain a temperature on the actual surface of the solid globe of the sun not higher 

 than that upon our earth. However this may be, there seems to be little doubt 

 that the actual temperature at the visible surface of the sun, that is to say, 

 upon its luminous coating, must be much more elevated than any artificial heat 

 we are able to produce. 



According to Sir John Herschel, we have various indications of this. 



First, from the law of the decrease of radiant heat and light, which being in 

 the inverse proportion of the squares of the distances, it follows that the heat 

 received on a given area exposed at the distance of the earth, and on an equal 

 area at the visible surface of the sun, must be in the proportion of the apparent 

 magnitude of the sun to the whole extent of the firmament, that is, in the pro- 

 portion of about one to three hundred thousand. A far less intensity of solar 

 radiation collected in the focus of a burning-glass, is sufficient to evaporate 

 gold or platinum. 



Secondly, from the facility with which the sun's heat passes through glass, 

 a property possessed by artificial heat in a very small degree, and always in 

 the direct proportion of its intensity. 



Thirdly, from the fact that the most vivid flames and intense artificial light 

 appear, as we have already stated, only as black spots when held between the 

 disk of the sun and the eye. 



The idea that the heat of the sun arises from any process analogous to that 

 of common combustion, seems to be beset with insuperable difficulties. How 

 can we suppose the inexhaustible supply of the materials necessary to sup- 

 port so enormous and interminable a conflagration? There are two other 

 sources of heat which may be imagined, that are not subject to the same dif- 

 ficulty. Bodies submitted to friction evolve heat without any change in the 

 condition of their constituent parts. Also when a galvanic current is trans- 

 mitted through certain conducting substances, they become heated with more 

 ) or less intensity and sometimes to such a degree as to emit light of the most 

 intense brilliancy, and yet in this process they suffer no other physical change 

 than that of temperature. It is therefore possible to suppose either of these 

 causes, but especially the latter, to be in constant operation on the sun, with 

 sufficient energy to educe the light and heat which it affords. 



The actual physical character of the luminous matter which coats the sun 

 had not been ascertained until a recent period. According to the report of 

 the astronomical lectures of Arago, lately delivered in Paris, it would seem 

 that that philosopher has succeeded in solving this problem. As we have not 



