ON THE ORIGIN OF THE PLANETARY SYSTEM. 157 



of our atmosphere, and hydrogen, an element in water, 

 which indeed is formed by its combustion. Both have 

 been found in the irresolvable nebulae, and, from the 

 inalterability of their shape, these must be masses 

 of enormous dimensions and at an enormous distance. 

 For this reason Sir W. Herschel considered that they 

 did not belong to the system of our fixed stars, but 

 were representatives of the manner in which other 

 systems manifested themselves. 



Spectrum analysis has further taught us more 

 about the sun, by which he is brought nearer to us, as it 

 were, than could formerly have seemed possible. You 

 know that the sun is an enormous sphere, whose 

 diameter is 112 times as great as that of the earth. 

 We may consider what we see on its surface as a layer 

 of incandescent vapour, which, to judge from the 

 appearances of the sun-spots, has a depth of about 

 500 miles. This layer of vapour, which is continually 

 radiating heat on the outside, and is certainly cooler 

 than the inner masses of the sun, is, however, hotter 

 than all our terrestrial flames hotter even than the 

 incandescent carbon points of the electrical arc, which 

 represent the highest temperature attainable by terres- 

 trial means. This can be deduced with certainty from 

 Kirchhoff's law of the radiation of opaque bodies, from 

 the greater luminous intensity of the sun. The older 

 assumption, that the sun is a dark cool body, sur- 



