SULPHUR. 217 



above, or suspend a piece of filter-paper, moistened with solution of 

 lead acetate, in the liberated gas, when the paper turns dark. Some 

 sulphides, for instance, those of mercury, gold, platinum, as also FeS 2 , 

 and a few others, are not decomposed by the acids mentioned, unless 

 zinc be added. 



Carbon disulphide, Carbonei disulphidum, CS 2 75.57. This 

 com pound is obtained by passing vapors of sulphur over heated 

 charcoal. It is a colorless, highly refractive, very volatile, and 

 inflammable neutral liquid, having a characteristic odor and a sharp, 

 aromatic taste. It boils at 46 C. (115 F.) ; it is almost insoluble 

 in water, soluble in alcohol, ether, chloroform, fixed and volatile oils ; 

 for the latter two it is an excellent solvent, but dissolves, also, many 

 other substances, such as sulphur, phosphorus, iodine, many alka- 

 loids, etc. 



Selenium, Se, and Tellurium, Te, are but rarely met with. Both elements 

 show much resemblance to sulphur; both are polymorphous; both combine 

 with hydrogen, forming H. 2 Se and H 2 Te, gaseous compounds having an odor 

 more disagreeable even than that of H 2 S. Like sulphur, they form dioxides. 

 Se0 2 and TeO 2 , which combine with water, forming the acids H 2 SeO 3 and 

 H 2 Te0 3 , analogous to H 2 SO 3 . The acids H 2 Se0 4 and H 9 TeO 4 , corresponding 

 to H 2 SO 4 , also are known. 



Ionic mechanism of the solution by acids of salts that are insol- 

 uble in water. The operation of dissolving by the aid of an acid, salts that 

 are insoluble in water is resorted to frequently in general chemical work, and 

 particularly in chemical analysis. It is a matter of observation, too, that a 

 salt will dissolve in some acids, but not in others; also that of salts of the same 

 acid with different metals, some will dissolve in a given acid, while others will 

 not. Thus zinc sulphide is soluble in dilute hydrochloric acid, but not in 

 acetic, and the same is true of calcium oxalate and calcium phosphate. Iron 

 sulphide is soluble in most any dilute acid, but copper sulphide is not appre- 

 ciably dissolved by the same acids. The student often wonders what the ex- 

 planation is of such facts as these. The ionic theory gives a physical basis for 

 accounting for them. 



Solution is the converse of precipitation. In the discussion of the latter 

 subject (see page 193) it is stated that whenever there are more ions of a sub- 

 stance than a saturated solution of that substance can maintain, the excess of 

 ions unite to molecules, which separate from solution as a precipitate. The 

 more insoluble the substance is the smaller is the concentration of molecules 

 and ions that can be maintained in its saturated solution, and the more com- 

 plete is the precipitation. Every "insoluble" substance is soluble to some 

 extent in water, even if only minutely. But many of the so-called insoluble 

 salts are slightly soluble in water, which is an important factor in accounting 

 for the solution of salts by acids. Now, water in contact with such a salt be- 



