362 Relations between the Lines of various Spectra. 



photographic enlargements show that the magnesium triplets 

 are the same in character as those of zinc and cadmium. First, 

 the lines are continuous ; secondly, they have a large nimbus; 

 thirdly, they are extended ; and, fourthly, the first and second 

 lines in each triplet are stronger than the third, which is the 

 most refrangible. There is, however, a difference in the first 

 and second strong lines of the principal triplet in cadmium, 

 and this difference is common both to my spectrum and that 

 of Mr. Ames ; namely, the lines are double. 



He suggests the probability of the corresponding zinc lines 

 being resolvable into doublets, and I should feel inclined to 

 suggest that in such an event the corresponding magnesium 

 lines are with equal probability doublets. But on the whole 

 I do not think the zinc lines are double, as it rather appears to 

 me that we must expect certain variations in the lines of 

 homologous spectra, and one of these is a splitting of strong 

 lines into two in the case of elements with greatly increased 

 atomic mass, just as the intervals between lines in the doublets 

 and triplets in different spectra become greater with a definitely 

 increased quantity of matter in the atoms. Of course, under 

 such a condition, lines with approximately the same wave- 

 length in one spectrum, and appearing as one line, would be 

 represented by corresponding lines in the spectrum of an 

 element of the same group, but with a higher atomic mass, 

 as two separate lines. The individual members of any homo- 

 logous series of organic substances possess certain peculiar 

 properties which are variations from those belonging to the 

 general type ; this is particularly the case with those terms of 

 the series of high molecular weight, and elements of the same 

 group exhibit properties which are still wider variations from 

 the general properties ; therefore, in such a matter as the con- 

 stitution of spectra it would indeed be strange if the latitude 

 in such variations were not still greater. For, as the properties 

 which the elements of the same group possess in common are 

 fewer in number than those held in common by homologous 

 series of organic substances, the constitution of the spectra 

 may be expected to be less strictly defined. In other words, 

 the spectrum of an element may be conceived as being the 

 expression of the vibrations which are caused by all the mole- 

 cular movements upon which the properties of the molecules 

 depend. 



With regard to the nine most refrangible lines in mag- 

 nesium, zinc, and cadmium, T can affirm that in each spectrum 

 they form a very striking group, particularly so in magnesium 

 and zinc. 



Of the groups comprising the first four of the nine lines in 



