ABSORPTION AND EMISSION. 



61 



ticular case, was enunciated by Kirchoff in 1860. It may be 

 thus expressed: The relation between the power ' of emitting, 

 and the power of absorbing any given ray, is the same for all 

 bodies at the same temperature. This law applies to the 

 calorific, and to the chemical rays, as well as to those of 

 light. 



The relation between the emissive and the absorptive 

 powers of any gas is determined by the difference of tempera- 

 ture of the gas in the two cases. The greater that difference, 

 the more marked is the phenomenon of reversal. 



The law above stated, and which had already been 

 demonstrated, in the case of heat, to be a necessary con- 

 sequence of the conditions of equilibrium of temperature, has 

 been shown by Professor Stokes to be a necessary consequence 

 of the laws of propagation of vibratory motion. The mole- 

 cules of an incandescent gas vibrate in a definite manner, 

 producing rays of definite refrangibility. Hence the ether 

 within a gas will be set in vibration by these same rays, 

 which will thus communicate to it their vis viva, and be 

 absorbed. On the other hand, the waves whose time ^of 

 vibration is different, will not set the ether in motion, and 

 will be transmitted through it without loss. 



(80) We have already adverted to the similarity of the ab- 

 sorptive spectra of the gases, to the phenomena of the solar 

 spectrum ; and we are at once led to the conclusion, that the 

 light of the Sun is that of an incandescent solid, surrounded 

 by an atmosphere of vapours, which absorb certain definite 

 rays. We have now only to compare, by the spectroscope, 

 the position of any of these rays, with those of the bright 

 lines in the spectra of the metallic vapours, and if we 

 find them to be identical, we may conclude with certainty 

 that that vapour exists in the solar atmosphere, and therefore 

 the metal itself in the body of the Sun. This comparison has 



