On the Chemistry of the Hottest Star, 



169 



Some of the arc lines, including 4352*2 and 4167'8 remain, so that in 

 the absence of a record of the ultra-violet triplet, it may be concluded 

 that the absorbing magnesium in 7 Cygni is probably at a tempera- 

 ture not differing greatly from that of the spark, that is the third 

 stage of temperature. The wave-length of the new spark line about 

 4395 is not yet known Tilth sufficient accuracy, on account of its 

 great breadth, to justify its use in this inquiry. The actual appear- 

 ance of the magnesium lines in 7 Cygni thus confirms the conclusion 

 with regard to the temperature of this star which has already been 

 derived from the discussion of the lines of calcium and iron. 



In the case of x Cygni the most prominent magnesium line is the 

 spark line at 4481. The I lines, the ultra-violet triplet commencing 

 with 3838*4, and the line 4352*2 are also present, while 457T3 and 

 4167"8 are absent, or very feeble. The great intensity of 4481, which 

 is only a short line in the spark, indicates that the temperature is, 

 in all probability, a little higher than that of the experimental spark, 

 that is, intermediate between the third and fourth stages. 



Passing to 9 Leonis, 4481 is a little less intense than in a. Cygni, 

 while the line at "4352 is considerably reduced in intensity as com- 

 pared with x Cygni. A temperature a little higher than that of 

 a Cygni is therefore indicated. 



In Rigel, where the temperature is higher than in y Leonis, 4481 

 is one of the few strong lines recorded in the spectrum, and it 

 appears without the other Mg lines. 



The same is true of cT Tauri and Bellatrix, except that 4481 is now 

 reduced in intensity. These varying aj)pearances are indicated in 

 Map I. 



The study of magnesium thus perfectly accords with what we have 

 learned as to relative stellar temperatures from a discussion of the 

 lines of iron and calcium. 



General Results ivith regard to Calcium and Magnesium. 



Map I indicates that in the case of the stars so far discussed, the 

 same order of temperature is arrived at by a consideration of the 

 lines of calcium and magnesium as that deduced in the first instance 

 from the relative lengths of continuous spectrum, and afterwards by 

 an inquiry into the presence of the enhanced iron lines. Four indica- 

 tions of stellar temperatures are therefore now available, namely, the 

 extent of the continuous radiation, the lines of iron, the lines of 

 calcium, and the lines of magnesium. 



The enhanced lines of calcium and magnesium, unlike those of 

 iron, do not disappear from the spectrum in the case of the hottest 

 stars yet studied, but they become very feeble, so that an approach 

 to disappearance is indicated. 



