292 THE CHEMISTRY OF THE SUN. [CHAP. 



in the spectrum of B would be truly produced by the action of 

 high temperature, while a,f y and / would be but remnants of 

 the spectrum of a lower temperature. 



So much then by way of explanation ; it is clear that to make 

 this reasoning valid we must show that the spark, or better still 

 the arc, provides us with a summation of the spectra of 

 various molecular groupings into which the solid metal which we 

 use as poles is successively broken up by the action of heat. 



We are not limited to solid metals ; we may use their salts. 

 In this case it has already been shown 1 that in very many cases 

 the spectrum is one much less rich in lines. 



Second Consideration. 



Since at different temperatures the brilliancy of the spectral 

 lines of the same substances as ordinarily observed changes 

 enormously, it is important to see if these changes can be pro- 

 duced at the same temperature by employing those experimental 

 conditions which will be most likely to bring about different 

 molecular conditions if such exist. 



This experiment is founded on the behaviour of compound 

 bodies when they are distilled at different temperatures. If we 

 take, for instance, a mixture of hydrocarbons, some of them 

 very complex in their nature, and others more simple ; when a 

 Low temperature is employed it is found that the simpler hydro- 

 carbons come over in the shape of vapour in greater abundance, 

 when a higher temperature is employed more complex forms 

 come over in greater abundance. If therefore we were fortunate 

 enough to be able to observe the spectra of these different 

 vapours, assume that the series of hydrocarbons, for instance, 

 shown in the accompanying diagram (Fig. 98), had each of 

 them a distinct spectrum, we should be able to follow spectro- 

 scopically the effect of each change of temperature, and we 

 could in that way associate the known fact of the greater 

 1 Phil. Trans. 1873, p. 258. 



