THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XLYII. — On the Color Relations of Atoms, Ions and 

 Molecules ; by M. Carey Lea. Part II. 



I. Interaction of Ions. 



If a colored substance be formed by the union of a color- 

 less kation with a colorless anion, the color belongs to the 

 molecule only. The colorless ions have so modified each 

 others' vibration periods that selective absorption is exercised. 

 As soon, therefore, as the molecule is divided into ions the 

 color must disappear. Consequently if we find a solvent 

 which, like water, is capable of separating the ions, the result- 

 ing solution when dilute must be colorless, no matter how 

 intense the color of the compound. 



The truth of this law of interaction has been tested experi- 

 mentally in a considerable number of cases. The 1'esults, 

 which were found to be without exception confirmatory, are 

 given below. 



Alkaline metals. — These form comparatively few colored 

 products, soluble in water, by uniting with colorless atoms. 

 Well marked instances are, however, found in the monosul- 

 phides. 



Potassium sulphide, K 2 S, is in the solid form cinnabar-red. 

 It dissolves in water to a colorless solution.* The sodium salt 

 acts similarly. As to the alkaline selenides and tellurides, their 

 tendency to form poly-salts is so great that it is not possible to 

 judge of the color shown by mono-salts in solution. It is of 

 interest, however, that hydrogen selenide and probably also the 



* Graham Otto, 5th Ger. ed., vol. iii, p. 240. The reactions here described have 

 been either found or verified by the writer except in a few instances, and in these, 

 as in the present case, the authority on which they are stated is given. 



Am. Jour. Scl— Fourth Series, Vol. I, No 6.-— June, 1896. 

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