216 PROGRESS OF SCIENCE IN THE CENTURY. 



that they give out light of the same wave-length — and 

 that incandescent substances only absorb such rays as 

 they themselves emit. Since, however, incandescent 

 gases possess maxima and minima of light intensity, 

 while solid and liquid substances emit light of every 

 kind when sufficiently heated, the former must also 

 possess a selective absorptive power, and this is not the 

 case in general with the latter. The Fraunhofer lines 

 are thus explained as consequent upon absorptions by 

 incandescent vapours."* 



Applications. — From the coincidence of the two 

 yellow sodium lines in the spectrum of a candle 

 flame with two of Fraunhofer's dark lines in the 

 solar spectrum, Kirchhoff concluded that sodium 

 was present in the sun^s atmosphere; and the same 

 kind of argument was used over and over again. 

 The method is to find in the spectra of terrestrial 

 elements bright lines which exactly coincide with the 

 dark lines in the sun's spectrum. Thus Kirchhoff 

 showed that besides sodium, the sun's atmosphere 

 contained iron, calcium, magnesium, nickel, barium, 

 copper, zinc, and chromium, while others such as 

 gold and silver were similarly shown to be absent. 

 In 1852 Angstrom added hydrogen and others to the 

 list; in 1872-1876 Lockyer increased the number 

 from 14 to 34; in 1887 Trowbridge and Hutchins 

 demonstrated the presence of carbon; in 1891 Row- 

 land detected silicon. The absence of some elements, 

 notably of oxygen, is as remarkable as the presence 

 of others, hut there is, as Lockyer and others have 

 shown, some reason to suspect that elements may be 

 present when they are apparently absent; that is to 

 say they may exist under physical conditions which 



* Ladenburg. History of Chemistry. Trans, by Dobbin, 

 1900, pp. 317 to 318. 



