248 GENERAL SCIENCE 



form iron oxide; zinc and sulphur when heated combine 

 to form a white powder, zinc sulphide. Substances which 

 have been formed by the union of two or more substances 

 are called compounds. By proper means these com- 

 pounds may be separated again into the substances which 

 were used in their production. The question of course 

 arises as to whether these substances oxygen, hydrogen, 

 iron, zinc, sulphur may be further divided into other 

 substances. All we can say is that they have never been 

 divided in any of the numerous attempts that have been 

 made, and we therefore assume that they cannot be 

 divided. Substances which we have never been able to 

 decompose into other substances are called elements. 

 If powdered zinc and sulphur are placed in a dish and 

 stirred, they do not unite but remain as small particles 

 of zinc and sulphur. This is called a mixture. When a 

 flame is brought near this mixture, there is a flash of 

 light and a cloud of white smoke. The white powder 

 that is left in the dish is no longer a mixture of zinc and 

 sulphur but a new substance, zinc sulphide, having new 

 properties. Sulphur will dissolve in carbon disulphide. 

 Zinc sulphide will not. Sulphur is yellow and zinc is 

 gray. The new substance is white. 



There are numerous mixtures about us. Air is a mix- 

 ture of the gases nitrogen, oxygen, carbon dioxide, and 

 water vapor. Most rocks are mixtures of different sub- 

 stances. A mixture of oxygen and hydrogen does not 

 produce water until the temperature is raised to about 

 620 Centigrade. 



The Common Elements. - - There are about eighty dif- 

 ferent elementary substances or elements, but many of 

 these are found in very small quantities. In our daily 

 life we are not commonly concerned with more than 



