88 



Transactions of the 



Professors KirchhofF, Angstrom, and Thale'n, and Mr Huggins 

 have made very accurate maps of the metallic lines. In the latter's 

 map several of the bright lines appear to coincide, namely — 



Division of 

 Scale. 



Tellurium and nitrogen, . 1366 

 Osmium and arsenic, . 1737 

 Chromium and nitrogen, 2336 



I have no doubt that with more accurate instruments these lines 

 will appear to be perfectly distinct, yet it appears truly wonderful 

 that, in examining many hundreds of bright lines of twenty-four 

 elements, Huggins found only six cases of coincidence. 



Now we come to the examination of the spectra of gases and 

 other non-metallic bodies. We again have recourse to electricity. 

 Angstrom was the first to point out that the ordinary electric 

 spark, in passing between the poles in air, yields a double spec- 

 trum — one of the metallic poles, and the other of the air. If we 

 diminish the density of tlie gaseous medium through which the 

 spark passes, we can get a much longer one ; we thus obtain the 

 beautiful phenomenon of the discharge in vacuo. By filling tubes 

 with diiferent gases, and then nearly exhausting them by the air- 

 pump, we obtain lights of different colours. The well-known 

 Geissler's tubes are made on this princijDle. When the electricity 

 passes through, the gas becomes heated to incandescence, and we 

 can thus easily obtain its spectrum. 



Hydrogen gives a peculiar red colour, and it is characterised 

 by three bright lines — one of the most intense red, another of a 

 bright gTeenish blue colour, and the third a dark blue or indigo. 

 These same three lines coincide exactly with three dark lines in 

 the solar spectrum, and we shall presently see that it has been 

 established without a doubt that hydrogen exists in the solar 

 atmosphere. The spectrum of nitrogen is much more complicated. 

 It consists of a great number of lines in the blue and violet, green 

 and red. It has been found that it undergoes a change when the 

 intensity of the electric discharge varies, and when highly rarified 

 nitrogen is used, not when it is at the ordinary atmospheric pres- 

 sure. It has been explained by saying that nitrogen, as oxygen, 

 exists in allotropic modifications. 



When intensely heated, and under certain circumstances, gaseous 

 bodies can be made to yield continuous spectra, and certain 

 elements on an increase of temperature give new lines. We may 

 compare with this phenomenon the overtones or harmonics of a 

 vibrating string, as observed in acoustics. 



The examination of the spectrum of carbon is of great interest 



