THE CHEMICAL ELEMENTS. 609 



Now this reduction has shown me that the hypothesis that identical 

 lines in different spectra are due to impurities is not sufficient. I show 

 in detail in the paper the hopeless confusion in which I have been 

 landed. 



I find short-line coincidences between many metals the impurities of 

 which have been eliminated, or in which the freedom from mutual im- 

 purity has been demonstrated by the absence of the longest lines. 



The explanation of this result on the hypothesis that the elements 

 are elementary does not lie on the surface, but it does on the assump- 

 tion that they are compounds and behave like them. 



This is the first point. We now pass from the results brought 

 about at the same temperature with different substances to those ob- 

 served at different temperatures with the same substance. 



I find that when the temperature is greathj varied^ the elements 

 behave spectroscopically exactly as compound bodies do, as we have 

 already seen. New lines are developed with increasing temperatures, 

 and others fade in precisely the same way as the metallic lines made 

 their appearance in the salts at the expense of the latter, which faded 

 too. 



In short, the observations and reasoning which I formerly employed 

 to show how acknowledged compounds behaved in the spectroscope 

 are now seen to indicate the compound nature of the chemical elements 

 themselves. 



In a paper communicated to the Royal Society in 1874, referring, 

 among other matters, to the reversal of some lines in the solar spec- 

 trum, I remarked : 



It is obvious that greater attention will have to be given to the precise char- 

 acter as well as to the position of each of the Fraunhofer lines, in the thickness 

 of which I have already observed several anomalies. I may refer more particu- 

 larly at present to the two H lines 3933 and 3968 belonging to calcium, which 

 are much thicker in all photographs of the solar spectrum (I might have added 

 that they were by far the thickest lines in the solar spectrum) than the largest 

 calcium line of this region (4226-3), this latter being invariably thicker than the 

 H lines in all photographs of the calcium spectrum, and remaining, moreover, 

 visible in the spectrum of substances containing calcium in such small quantities 

 as not to show any traces of the H lines. 



How far this and similar variations between photographic records and the 

 solar spectrum are due to causes incident to the photographic record itself, or to 

 variations in the intensities of the various molecular vibrations under solar and 

 terrestrial conditions, are questions which up to the present time I have been 

 unable to discuss. 



The progress of the work has shown that the differences here indicated 

 are not exceptions, but are truly typical when the minute anatomy of 

 the solar spectrum is studied. 



Kirchhoff, indeed, as early as 1869 seems to have got a glimpse of 

 the same thing, for in his memorable paper, which may justly be re- 



