VI.] CHEMISTRY OF THE STARS. 63 



us to determine them. The blank spaces indicate that so far no star 

 has been photographed in the spectrum of which the enhanced lines 

 exactly match those on the opposite side. 



The names of the various chemical substances included in the dis- 

 cussion are given at the top. I have retained the prefix " proto " to 

 that condition of each metallic vapour which gives us the enhanced 

 lines alone, and I have added it to that form of hydrogen seen only in 

 the hottest stars. 



The behaviour of the most typical line of each chemical substance 

 is indicated by a double line looped at the top at its highest range. 

 The length and varying thickness of the lines in stars on both sides of 

 the temperature curve are derived from the observed appearance and 

 intensity of the lines, noted in the different stars. 



The wave-lengths of the lines discussed are shown at the bottom of 

 the map. 



Details of Changes observed. 



The facts embodied in the map present to us the spectral changes 

 noted in stars of Groups III, IV, and V of my classification,* and are 

 a result of a more general inquiry than those referred to in my pre- 

 vious papers f the origins of a very considerable number of stellar 

 lines having since then been traced to enhanced lines of metals and to 

 known gases. 



It will be seen that this more general inquiry entirely justifies the 

 prior statement J that the metallic lines are thickest in stars increasing 

 their temperature, and the hydrogen lines thickest in stars decreasing 

 their temperature, in other words, on the opposite arms of the tempera- 

 ture curve. I have already stated a possible explanation^ 



It will be observed that, so far, I have not been able to find stellar 

 spectra on the downward side corresponding to those of 7 Argus and 

 Orionis ; but it is more than probable that near the apex of the curve 

 only a small change will be observed ; their default, therefore, is of less 

 consequence than it might have been. 



The same remark applies to a Cygni and Sirius : but here it is cer- 

 tain that the differences in the relative intensities of the gaseous and 

 enhanced lines will be considerable, judging from what happens above 

 and below the heat stages represented by them. 



The stars used in the discussion give us very definite results, show- 

 ing that the various chemical forms are introduced at six very distinct 

 heat levels. 



* Proc. Eoy. Soc., vol. xliii, p. 117 (1887). 



f Proc. Roy. Soc., vol. xliv, p. 1 (18S) ; ibid., vol. xlv, p. 380 (1889) ; PHI. 

 Trans., A., 184, (1893), p. 725. 



J Proc. Eoy. Soc., vol. Ixi, p. 182. 

 Proc. Boy. Soc., vol. Ixi, p. 183. 



