442 P. A. Secchi on KirchhofFs Theory of Solar Spots. 



that the interior of the solar globe must be as bright as is the 

 photosphere, Kirchhoff insists greatly on the principle that all 

 substances become luminous at the same temperature. It ap- 

 pears to us that two very different things are here confounded — 

 the invariability, for all bodies, of the temperature at which lumi- 

 nous rays, or such as affect the eye, begin to be emitted, and the 

 equal luminosity of these bodies at the same temperature. We 

 may admit the truth of the first of these propositions, and at 

 the same time utterly deny that of the second. In furnaces we 

 have gases and solids of very different luminosities ; and is not 

 the strongest known flame, that of the oxyhydrogen lamp, one 

 of the least luminous ? The conclusion, therefore, is altogether 

 untenable that the parts which form the solar nucleus must be 

 as luminous, as the photosphere. From this, of course, it does 

 not follow, either that the so-called nucleus must be solid, or 

 that its temperature must be much lower, but merely that it is 

 less luminous. It may be either liquid or gaseous ; but in it 

 that specific vivid action which characterizes the photosphere 

 does not exist. As Loret well observes, the analogy with other 

 planets indicates to us that the denser parts must be accu- 

 mulated in the lower strata, and the lighter at the surface; 

 and amongst the latter are the gases and more subtle materials 

 from whose modifications the solar light results. Thus there is 

 no absurdity whatever involved in the assumption that, under- 

 neath the highly incandescent stratum of the photosphere, there 

 may be another, equally hot, possibly, but less luminous, which 

 is rendered visible whenever the more incandescent stratum of 

 the photosphere is itself rent asunder. 



But careful consideration will show that it is impossible to 

 admit an absolute identity of temperature in the various parts 

 of the sun. In fact the incessant work which is performed in 

 it, and the continual emission of heat, implies that one part 

 must be actually in a state of chemical alteration, whilst another 

 part must be on the point of entering that state. The first may 

 be the photosphere, and the second the less luminous nucleus, just 

 as is the case in an ordinary fire. Nor must we omit to observe 

 that if the argument according to which all parts of the sun 

 must have the same temperature be valid, it must apply with 

 equal force to our furnaces also. We do not here compare the 

 sun with a furnace in which wood burns ; we assert merely that 

 the work itself which is there performed, in order to preserve 

 solar activity, involves the assumption of more and of less in- 

 tense parts; otherwise we should be compelled to regard the 

 sun as a merely incandescent body, whose light, as W. Thomson 

 has shown, must necessarily be extinguished in some millions of 

 years. 



