Influence of Atomic Weight. 471 



and Hautefeuille, appears to be in accordance with the above 

 rules, giving a more refrangible spectrum than Zr. 



Aluminium Group (Al, Gra, In, Tl). — Aluminium is charac- 

 terized by the large number of bright lines lying close together, 

 of which the most important in the red are those of wave-lengths 

 6243 and 6245, and in the blue 4661 and 4662. The alumi- 

 nium-bands seen in the ultra-violet are extremely characteristic. 

 Gallium. This spectrum consists of two violet lines, of which 

 the brighter Ga «, of wave-length 4170, lies a little more 

 towards the blue than In j3, and the second, Gra /3 (wave-length 

 4031), a little more towards the violet than K/3. Indium. 

 The spectrum of this metal is made up of an intense indigo- 

 blue line In a (wave-length 4509) and a less intense violet 

 line In /3, 4101. Thallium is characterized by the intense 

 green line Tl a (wave-length 5349). From the above it is 

 seen that, as the atomic weight increases, the rays become 

 less and less refrangible : thus Al, the metal with lowest 

 atomic weight, is remarkable for bright lines in the ultra-violet, 

 whereas Tl, with the greatest atomic weight, is recognized by 

 its well-known green line. 



Calcium Group (Ca, Sr, Ba). — Here also the rays towards 

 the violet end of the spectrum become less and less refrangible 

 as the atomic weight increases. 



Magnesium Group (Be, Mg, Zn, Cd, Hg). — In the case of 

 these elements the rays towards the violet end tend, on the 

 other hand, to become more and more refrangible as the 

 atomic weight increases. Zinc, however, is an exception to 

 this rule. 



Potassium Group (Li, Na, K, Rb, Cs). — The spectra of this 

 group become less refrangible from K to Cs, i. e. as the atomic 

 weight increases. In the case of Na and Li, which are excep- 

 tions, may they not give lines in the ultra-violet beyond K /3 

 which have not yet been observed ? 



Copper Group (Cu, Ag, Au). — Here the rays towards the 

 violet end become less and less refrangible as the atomic weight 

 increases. 



The general conclusions, therefore, which are to be drawn 

 from the above remarks on the spectra of the elements are as 

 follows : — 



(1) That for non-metals and quasimetals (including the CI, 

 S, P, and C groups) the spectra become more and more re- 

 frangible as the atomic weight increases. 



(2) That for true metals (Al, Ca, K, and Cu groups) the 

 reverse holds good, the spectra becoming less and less refran- 

 gible as the atomic weight increases. The Mg group, and 



