MATTER AND ITS PROPERTIES 47 



vibrating that is, moving back and forth in their places 

 with great rapidity. 



40. The Divisions of Matter. Any separate portion 

 of matter is called a body. A molecule, a book, a pebble, 

 a boulder, a star, a fish, a tree are bodies. 



A particular kind of matter is called a substance. Iron, 

 sugar, salt, water, wood are substances of which bodies are 

 made. 



If the molecules of a substance are all of one kind of mat- 

 ter, that substance is called an element. Iron is an element, 

 and nothing but iron has ever been made from iron alone. 

 Gold and silver are elements. 



41. Some Important Elements. Of the eighty known 

 elements, only about one half are of much importance at 

 present. The elements most abundant in the sun and in 

 the land upon which we live, in the air and water surrounding 

 it, and in the bodies of plants and animals are : 



Certain solid metals: aluminum, calcium, copper, gold, 

 iron, lead, nickel, platinum, potassium, silver, sodium. 



One liquid metal : mercury. 



Certain solids which are not metals: arsenic, carbon, 

 iodine, phosphorus, silicon, sulphur. 



Certain gases: chlorine, hydrogen, nitrogen, oxygen. 



These elements very seldom exist by themselves in the land, 

 or in the water, or in our bodies. They are united with each 

 other, forming another class of substances, called compounds. 



42. Compounds. If we heat a little of the element mer- 

 cury (a heavy silvery liquid) and drop into it the right quan- 

 tity of the element iodine (a bluish-black scaly solid), neither 

 color remains. A new substance appears, not liquid like 

 mercury, not in flat scales like iodine, but in the form of red 

 powder. This is mercury iodide, a compound of mercury 

 and iodine. 



A compound is a substance made up of two or more ele- 

 ments combined in definite proportions by weight, and 



