28 EXPERIMENTAL GENERAL SCIENCE 



they cannot be made liquid except under great pressure and at 

 an extremely low temperature. Table salt, the .familiar 

 white solid, consists of sodium, a silvery solid, and chlorine, 

 a greenish and poisonous gas. Still more remarkable is the 

 union of carbon and sulphur to form carbon disulphide (82). 

 Carbon is the well-known black solid represented by lamp- 

 black, or charcoal, and sulphur is a tasteless and odorless yellow 

 solid. When combined in the proportion of two of sulphur and 

 one of carbon, the result is a colorless substance instead of 

 either black or yellow, liquid instead of solid, and having a 

 strong and disagreeable odor. 



Moreover, charcoal or sulphur alone may be eaten without 

 harm, and are often taken as medicine, but when combined as 

 carbon disulphide (82), they are poisonous, and the suffo- 

 cating gas which they produce is often used to destroy insects 

 and other vermin. In numerous cases, the union or disinte- 

 gration of two substances is influenced by a third which does 

 not form a part of the substance produced. Thus, when man- 

 ganese dioxide is mixed with potassium chlorate and heated, 

 the latter gives up its oxygen much more readily than it would 

 by itself. Substances that influence chemical action in this 

 way are called catalyzers. 



33. Proportions of Elements in Compounds. In forming 

 the molecules of a substance, the chemical elements unite with 

 one another in definite and unvarying proportions. It is this 

 feature that makes the molecules of a given substance all alike, 

 no matter how many chemical elements compose it. When 

 carbon and oxygen unite to form carbon dioxide (62), two 

 atoms of oxygen always unite with one of carbon. Though 

 great quantities of oxygen may be present, it does not affect 

 the result; one atom of carbon will take on only two of oxygen. 

 When oxygen is scarce however, one atom of carbon may unite 

 with one atom of oxygen, forming carbon monoxide (CO). We 

 see, therefore, that while the atoms of different chemical ele- 



