CHANGES OF LIFELESS AND LIVING MATTER 7 



10. Compounds of Elements. Now, the elements have 

 the power of combining with each other so as to form various 

 compounds. To illustrate : the burning of magnesium is 

 a chemical combination between magnesium and oxygen of 

 the air two elements are here united to form a new sub- 

 stance, which is a compound of magnesium and oxygen 

 and is known as oxide of magnesium. Such a union of any 

 substance with oxygen is called oxidation. The burning 

 of coal is in part a combination between the elements oxygen 

 and carbon, but coal contains elements besides carbon. 

 All ordinary burning or combustion is an oxidation; that is, 

 the forming of a combination between oxygen of the air and 

 some other elements. In all cases of chemical change there 

 is a combining of elements into new or different substances. 

 Such substances composed of two or more elements are 

 called compounds. Most of the materials in the solid matter 

 of the earth are compounds; water, which is the most 

 abundant substance, is a compound of the elements hydrogen 

 and oxygen; and most of the materials in the bodies of 

 animals and plants are compounds. Air, however, is not a 

 chemical compound; the nitrogen and oxygen are not 

 united into a new substance, but are simply mixed together, 

 just as dry sand and sugar can be mixed without change of 

 composition in either. Other minor constituents of the 

 air will be mentioned later. 



Using the chemical symbols for the elements, chemists 

 write MgO for magnesium oxide formed by burning magne- 

 sium (Mg) in the oxygen of the air, and ZnO for zinc oxide 

 formed by burning zinc in the same way. The formula MgO 

 expresses the composition of a molecule of magnesium oxide 

 and means that it is made up of one atom of magnesium 

 united with one of oxygen. This is the simplest possible 

 compound. In most compounds of two or more elements 

 there is a greater proportion of, certain elements, and this 

 is expressed by numbers after the elements of which there is 



