CHEMICAL CHANGE AND THE METHODS OF STUDYING IT 15 



be a form of combustion spreads through the mass. The heating 

 employed at the start falls far short of accounting for the much 

 greater heat produced. When these phenomena have ceased, and 

 the test-tube has been allowed to cool, we find that it now con- 

 tains a somewhat porous-looking, black solid. This material is 

 brittle; it is not magnetic; it does not dissolve in carbon disul- 

 phide; and close examination, even under a microscope, does not 

 reveal the presence of different kinds of matter. This substance 

 is known to chemists as ferrous sulphide and, as we see, its prop- 

 erties are entirely different from those of its constituents. 



In this connection we must not omit to notice that, as in rust- 

 ing, a certain fixed proportion will be used in forming the com' 

 pound. 



Iron (55.84) + Sulphur (32.06) - Ferrous sulphide (87.90). 



If more iron is put into the original mixture, then some 

 unused iron will be found in the mass after the action. If too 

 much sulphur is employed, some may be driven off as vapor 

 by the heat and any that remains, beyond the correct propor- 

 tion, can be dissolved out of the ferrous sulphide with car- 

 bon disulphide. The sulphur which has combined with the iron, 

 however, is no longer present as sulphur it has no longer the 

 properties of sulphur, and therefore cannot be dissolved out. 



Another Illustration: Mercuric Oxide. It has long been 

 known that air contains an active and an inactive gas. The 

 Chinese called them yin and yang, respectively. Mayow (1643- 

 1679) showed that the active gas caused rusting, that it was ab- 

 sorbed by paint (really by the linseed oil) in " drying," that it 

 supported combustion of wood and sulphur, and that it is 

 necessary to life, being absorbed by the blood from the air en- 

 tering the lungs. It was not until 1774, however, that a pure 

 specimen of this gas was obtained, by Bayen, and was recog- 

 nized to be a special kind of gas different from ordinary air. 



