xii.] IMPURITIES. 155 



cases in the spectra of calcium and strontium may be given. 1 

 The longest line in the visible portion of the calcium spectrum 

 (Thalen), wave-length 4226 '3, is found in the strontium 

 spectrum as a line of medium length. 4607 '5, one of the 

 longest lines of strontium, appears in the calcium spectrum as a 

 short line. 



Another very long line of strontium occurs at 42 15 '3, in close 

 proximity to the longest calcium line, and, according to Thalen, 

 occurs also in the spectrum of that metal. 



We have here, then, two metals with two lines common to 

 their spectra ; and it is found that the line which is long and 

 bright in one spectrum, is faint in the other ; and with regard 

 to a third line, one observer finds it in both spectra, the other in 

 one only, and after many attempts succeeds in observing it in 

 the second, but only in a specimen known to be contaminated with 

 the first. 



The simplest explanation of the case, bearing in mind the 

 facts already dwelt upon, is that the calcium used to produce 

 the spectrum was contaminated to a certain extent with stron- 

 tium, the strontium in turn containing calcium a state of 

 things which a moment's consideration will show to be not only 

 possible but most probable, the close chemical relation of the 

 two metals and the extreme difficulty of making even an 

 approximate separation when mixed, being well known. 



The long lines of calcium at wave-lengths 4226, 3968, and 

 3933, occur also in the spectrum of iron, cobalt, nickel, barium, 

 and other metals, as observed in the arc using carbon poles, and 

 assume very considerable proportions, equalling or surpassing in 

 length many undoubted lines of those elements which are less 

 easily volatilised by the action of the current; on the other 

 hand, the iron lines at wave-lengths 4071, 4063, and 4045, 

 occur in calcium, strontium, and barium, and in other metals 

 under like conditions. 



1 Proc. Roy. floe. No. 147, 1873, p. 511. 



