Atoms, Ions and Molecules. 407 



Silver arsenate. — Chocolate color; dissolves in the same 

 solvents as the last and also gives colorless solutions. 



Silver phosphate. — Triargentic phosphate dissolves with 

 facility in nitric acid, in ammonia water and in mercuric 

 nitrate. All three solutions are colorless. 



Cuprous sulphite* — Eed ; dissolves in ammonia water and 

 in dilute hydrochloric acid, giving colorless solutions. ■ 



Stannic sulphide precipitated. — Ochre yellow, dissolves in 

 warm potash or soda solution ; the resulting solution is nearly 

 colorless. 



It also dissolves in potassium sulpharsenite to a colorless 

 solution. 



Red antimony pentasulphide dissolves in alkaline mono- 

 sulphides ; solutions colorless. 



Orpiment (As 2 S 3 ) dissolves easily in warm potash or soda 

 solutions ; colorless. 



II. Combinations of Ions. 



Ions may combine. A. Two or more similar colorless ions 

 may unite to form a colored elementary molecule. 



The ions of iodine and bromine are colorless as shown in 

 solutions of HBr and KBr, HI and KI. The molecules are 

 strongly colored. The ions of lead are colorless, as shown in 

 solutions of lead nitrate or acetate. The molecule (or the 

 polymer) of lead is blue. The ion of sulphur is colorless — 

 water is not colored when saturated with sulphydric acid. The 

 oxygen acids of sulphur are colorless. But the molecule is 

 yellow. Selenium has a colorless ion, but a strongly colored 

 molecule. Colorless cuprous ions unite to form red copper. 



B. Two or more similar ions, colored, may unite to form a 

 colorless (or white) molecule or polymer. The ion of platinum 

 is yellow in Pt iv , red in Pt u ; in each case the ions unite to 

 form white platinum. The nickel ion is green; these unite to 

 form white metallic nickel. 



C. Two or more similar ions, colored, may unite to form a 

 molecule of a wholly different color. Blue copper ions may 

 unite to form red copper. 



D. Two or more dissimilar colorless ions may unite to form 

 a colored molecule. Sulphur and nitrogen unite to form 

 orange-colored nitrogen sulphide. Sulphur and silver to form 

 black silver sulphide. 



It is an interesting fact, and one I believe that has not been 

 previously noticed, that no ion and, therefore, no atom, is black, 

 but is always transparent to some portion or portions of the 

 visible rays. In this respect atoms and ions differ absolutely 



* Gmelin Handbook, Ger. ed., ir, p. 622. 



