On the Chemistry of the Hottest Stars. 



151 



different lengths, and I indicated by means of photographs how very 

 definite these phenomena were. The adoption of this method of 

 work enabled me to establish that when a metallic vapour is sub- 

 jected at any one temperature to admixture with another gas or 

 vapour, or to reduced pressure, its spectrum becomes simplified by 

 the abstraction of the shortest lines and by the thinning of many 

 lines. 



In the same paper, I pointed out that "these observations have 

 an important bearing upon the solar spectrum, for the reason that, 

 as is well known, all the lines known to exist in the spectrum of an 

 element supposed to be present in the sun's atmosphere are not in all 

 cases reversed" (p. 261). Maps were given which showed "that 

 invariably the reversed lines are simply those ivhich are longest in the 

 spectrum. ... It supplies us at once with the true test to apply to 

 the reversal of solar lines and a guide of the highest value in 

 spectrum observations of the chromosphere and photosphere " 

 (p. 264). 



In another communication,* in November, 1873, I added that 

 " the test formerly relied on to decide the presence or absence of a 

 metal in the sun (namely, the presence or absence of the brightest 

 and strongest lines of the metal in question in the average solar 

 spectrum) was not a final one ; and that the true test was the 

 presence or absence of the longest line, being that which remains 

 longest in the spectrum when the pressure of the vapour is 

 reduced." 



In this paper I gave a photograph of the spectrum of iron pro- 

 duced when an image of a horizontal arc was projected on to the 

 vertical slit of a Steinheil spectroscope. A spectrum of long and 

 short lines is obtained in this way, precisely as in the case of the 

 spark, except that the phenomena are much better seen. The long 

 lines represent the vapours which extend furthest from the centre 

 of the arc, the short lines those which exist only at the centre. 



On the strength of the criterion thus established, I was enabled 

 to announce the presence of many metallic elements in the sun's 

 atmosphere. 



Some Short Lines indicate the Effects of High Temperatures. 



It was generally assumed in the first instance that the short line 

 were true products of the greater heat of the core of the arc. 



Subsequent work with the jar spark, in 1876 f and 1878,+ showed 

 that at spark temperatures some of the shorter arc lines behaved 



*• ' Phil. Trans.,' vol. 164, Part 2, p. 490. 

 f ' Eoy. Soc. Proc.,' vol. 24, p. 352. 

 t Ibid., vol. 28, p. 157. 



