412 M. C. Lea — Color Relations of 



any periodic system. The position required is already occupied 

 by calcium and a place cannot be made without dislocating all 

 the following groups, a strong argument in favor of the lower 

 figure. 



Helium with an atomic weight of probably 4 offers equally 

 great difficulties. As the old periodic system excludes hydro- 

 gen and commences with lithium with an atomic weight of 7 ? . 

 any element having an atomic weight less than 7 is, like hydro- 

 gen, entirely outside of a system which does not recognize the 

 existence of such elements. 



In the system which I have proposed, helium, argon with the 

 atomic weight of 19*9, and their congeners would constitute a 

 group of their own, taking position between the hydrogen 

 group and the lithium group. 



In the first part of this paper I endeavored to show that the 

 relations of atoms to the visual rays of the spectrum might be 

 made the basis of a classification of the elements composed of 

 these atoms and that such a classification harmonized extremely 

 well with the known properties of the elements. Not that 

 the particular color exhibited by an atom has for this purpose 

 any importance ; the question is simply whether color is present 

 or absent. In other words, we have to inquire whether a given 

 atom does or does not exercise selective absorption amongst the 

 visual rays. It was there shown that the atoms of all the ele- 

 ments having atomic weights below 48 did not exhibit selec- 

 tive absorption.^ After these came two elements which I call 

 transitional because at some valencies their atoms exhibit selec- 

 tive absorption and at others do not. Next, elements whose 

 atoms at all valencies exhibit selective absorption. Next came 

 a transitional element and then more elements having colorless 

 atoms, and so on through the entire series of elements arranged 

 in the numerical order of their atomic weights. 



It is even probable that the distinction so established between 

 these three classes of elements may eventually prove to have a 

 far-reaching significance and that those elements whose atoms 

 always exercise selective absorption may prove to be differently 

 constituted in some important way from those whose atoms 

 always allow free passage to all the visual rays. 



About the same time that the first part of this paper 

 appeared, Julius Thomsen proposed another and a different 

 system of classification,* to which later he added a supplementf 

 in reference to my paper with the object of showing that the 

 elements which I had indicated as possessing colored ions were 



* Zeits. fur Anor., Ch. ix, 192. f Ibid., x, 155. 



