290 KEPOKT — 1880. 



The strongest-lines of aluminium in the green and blue are not seen 

 when a spai'k is taten from the chloride, but the longest lines falling 

 between the solar H and K are seen. 



Cadmium is also a striking case, for while the longest lines — 5085, 

 4799, 4677 — are seen in the specti'um of the chloride, the equally bright 

 but shorter lines, 5378, 5338, do not appear. 



Other examples are given by Mr. Lockyer. 



In a subsequent paper the same author ' has examined the spectra of 

 the com^Jounds of lead, strontium, barium, magnesium, and sodium with 

 chlorine, bromine, iodine, and fluorine, and has confirmed his previous 

 results. 



An alloy behaves in the same manner as a chemical compound : ' For 

 instance, it is possible to begin with an alloy which shall only give us the 

 longest line or lines in the spectrum of the smallest constituent, and by 

 increasing the quantity of this constituent the other lines can be intro- 

 duced in the order of their length. This reaction is so delicate that I 

 learnt from it a thing I had not before observed, that the least refrangible 

 line of C, the triple line of magnesium, is really a little longer than its 

 more refrangible compound ; for the spectrum of magnesium was reduced 

 to this one line in an alloy in which special precautions had been taken to 

 introduce the minimum of magnesium.' 



This behaviour of alloys was subsequently made the basis of a quanti- 

 tative spectrum analysis by Messrs. Lockyer and Roberts.^ Comparing the 

 spectra of metals as obsei-ved by this method with their reversal in the 

 solar atmosphere, Mr. Lockyer found that it was the longest line and not 

 necessarily the strongest line which was first reversed in the sun. 



Subsequent work has shown that the longest lines are also generally 

 those which are most persistent on a reduction of temperature. The short 

 lines, which disappeared on a reduction of temperature and were not 

 visible in the spectrum of the chlorides, also disappear when the metal is 

 volatilized in the arc instead of the spark. Thus the strong zinc lines 

 4924, 4911, which we have already mentioned, disappear in the ai'C. A 

 similar i"em ark applies to the two cadmium lines, 5377, 5336 which Profs. 

 Liveing and Dewar ^ found to be absent in the arc, and which, as we have 

 seen, are also absent in the spectrum of the chloride. The strong but 

 short magnesium line 4481, is also absent in the arc, as is shown in 

 Rand Capron's 'Photographed Spectra.'^ 



In this way many facts which have often puzzled observers, are 

 brought under one general law. Take, for instance, the three tin lines 

 to which we have already drawn attention. The least refrangible is the 

 longest, but it is faint, while the two others are strong. We conclude from 

 this that at low temperatures this faint line is stronger than the other 

 two, while if the temperature is raised the two most refrangible lines are 

 the strongest. This is fully confirmed by expei'iment. Lecoq de Bois- 

 baudran,^ who usually employed the spark without condenser from a 

 solution of the metallic salts, gives the least refrangible of the lines as the 

 strongest, and mentions that the line is weakened by the introduction of 

 the condenser. 



It is a corollary of what has been said that if we produce the reversals 



' Pldl. Trans, clxiii. p. 639 (1873). = Phil. Trans, clxiv. p. 495 (1874). 



3 Proe. Roy. Soc. xxix. p. 402 (1879). ■• Photofjvaplied Spectra, p. 35 (1877). 

 5 Spectres Luminevx, texte p. 143 ; C. R. Ixiii. p. 943 (1871). 



