274 THE CHEMISTRY OF THE SUN. [CHAP. 



When it first began to be conceded that the fluted spectrum 

 of a substance was not due to an impurity, but to an allo- 

 tropic state, it was thought that it was produced by molecules 

 intermediate in complexity between thoie which in the case of 

 any one body produced the continuous and line spectrum respect- 

 ively. Some experiments which I undertook soon led me 

 to the conclusion that this molecular grouping was not the only 

 one between the extremes named. It was an inquiry which 

 carried us into lower temperatures than had been employed 

 before ; it was less wonderful therefore that it should show that 

 more " orders of spectra," to use Pllicker's term, were necessary. 



These experiments were undertaken in consequence of the 

 following considerations : 



I. Most solids when heated if they can be heated and yet 

 retain their solid state give us continuous radiation spectra. 



II. Many of the metalloids in vapour give us fluted absorp- 

 tion spectra at ordinary temperatures, and others do the same 

 at higher temperatures. 



III. Most elements driven into vapour by the temperature of 

 the voltaic arc give us line radiation spectra, with line absorption. 



IV. All elements driven into vapour by the induced current 

 give us line radiation spectra. 



Now we have here, prima facie, good reason for supposing 

 that the molecular structure of the vapours which give us such 

 different effects is not the same. To take the lowest ground. 

 If, in the absence of all knowledge on the subject, it could be 

 shown that all vapours at all stages of temperature gave us 

 spectra absolutely similar in chara^dter, then it would be more 

 likely that all vapours were truly Homogeneous and similar 

 among themselves, as regards molecular structure, than if the 

 spectra varied in character, not only from element to element, 

 but from one temperature to another in the vapour of the same 



