200 THE NEW KNOWLEDGE. 



4. A spectrum consisting of a relatively very ^mall num* 

 ber of lines which are intensified in the spark. 



We see, thus, that the spectrum of an element depends 

 upon the temperature to which it is subjected, and if we 

 ask ourselves the meaning of the change which the spectrum 

 undergoes, it is difficult to imagine any other efficient cause 

 than elemental dissociation with increasing temperature to 

 account for it. But other explanations, more or less spe- 

 cious, may be assigned, and as the dissociation, if it exists, 

 exists for the briefest possible time under the immediate in- 

 fluence of the arc or spark, it is impossible to prove it. 

 Help, however, is found in another direction. As Lockyer 

 says: 



" For twenty years I longed for an incandescent bottle 

 in which to store what the centre of the spark produces. 

 The stars have provided it." 



The sun and stars really constitute so many " incandes- 

 cent bottles " in which to study the variations of spectra 

 under different conditions and afford, as we shall see, proof 

 of the most reasonable sort of the dissociation of the ele- 

 ments. 



THE CASE OF IRON IN THE SUN. 



In one part of the sun, called the " reversing layer," the 

 spectrum of iron is represented by nearly a thousand lines. 

 In another part of the sun, called the " chromosphere," 

 which is apparently at a much higher temperature, the 

 spectrum of iron is reduced to two lines only. It is dif- 

 ficult to see what other explanation we can assign to this 

 remarkable fact than that at the higher temperature of the 

 chromosphere the atom of iron is decomposed or dissociated 

 into some simpler constituent which appears at that point. 

 This explanation is rendered additionally valid by the 



