On the Chemistry of the Hottest Stan 



173 



With increased temperature, in the case of the stars so far con- 

 sidered, the lines of hydrogen steadily increase in intensity up to the 

 highest temperature, as typified by Bellatrix. Meanwhile, as we have 

 seen, the second stage metallic lines gradually thin out, while the 

 enhanced lines become stronger up to the temperature of a. Cygni, 

 and ultimately all the lines of iron have disappeared, and magnesium 

 and calcium are only represented by traces of their enhanced lines. 



When the lines of the cleveite gases first become visible, as in 

 % Cygni, the lines of hydrogen have already become the strongest in 

 the spectrum, but at still higher temperatures, the intensification of 

 the lines of hydrogen is less rapid than that of the lines of the 

 cleveite gases. In none of the stars now considered do the hydrogen 

 lines reach their maximum development, as stated before. This 

 point will be considered later on. 



VII, The Temperature Eaxges of the Metallic axd Cleveite 



Gas Lines. 



Since there are regular sequences in the intensities of both the 

 metallic and cleveite gas lines in passing through stars of gradually 

 increasing temperature, it is important to compare the behaviour of 

 the lines of the different substances between the limits of the 

 temperature range I have chosen for discussion. 



The metallic lines characteristic of the fourth stage of tempera- 

 ture behave differently in the case of different metals, although all 

 the comparisons have led to the same general conclusion with 

 regard to the temperature of the absorbing vapours in any particular 

 star. 



The Lower Limit in the case of the enhanced Metallic Lines. 



In the case of calcium, the enhanced lines, namely H and K, appear 

 faintly at the temperature of the oxy-hydrogen flame (Stage I), and 

 are strongly developed at the arc temperature (Stage 2). The 

 appearance of H and K as strong lines in oc Orionis is therefore to be 

 expected. 



In the case of iron, the enhanced lines do not occur at all at the 

 first stage of temperature, and only very feebly at the second. They 

 would therefore not be expected at the low temperature of cc Orionis, 

 and, as a matter of fact, unlike H and K they first appear as well 

 developed lines in a. Cygni. 



Magnesium behaves almost exactly like iron in regard to its 

 enhanced line (4481), since, absent from cc Orionis, it appears as a 

 prominent line in # Cygni. 



