24 



THE WORLD WE LIVE IX 



Forms of matter. Both living and non-living things are made 

 up of chemical elements. There are over ninety elements. The 



common ones that are found in 

 a tree are oxygen, carbon, nitro- 

 gen, and hydrogen ; while a 

 number of others, less com- 

 mon, such as sulphur, potas- 

 fsiuru, iron, and phosphorus, 

 ^^""^j 1 are also found in the composi- 



\j tion of most plants and ani- 

 cacyo&n 76% I ma l s - Many of these same 



elements are found in soil, in 

 air, and in water. Some ele- 

 ments are gases, such as oxygen 

 and nitrogen. Some are solids, 

 such as carbon and sulphur, 

 and two which are not found 

 in the composition of the tree 

 are liquids, mercury and bromine. Elements are simple substances. 

 For example, iron, so far as we know, has nothing but iron in it : 

 and oxygen nothing but oxygen in it. It is easy to separate some 

 elements from their compounds and not so easy to get others. 

 Carbon, for example, in its pure state is obtained when we collect 

 on a sheet of white paper the black substance from the smoke 

 of a candle. Soot is almost pure carbon. The yellow sulphur that 

 we buy at the drug store is an element. It is not so easy to 

 obtain oxygen in a pure state. This element is often combined 

 in nature with other elements to form substances called com- 

 pounds. A simple compound containing oxygen is water. 



The percentage of different chemical elements 

 that are found in the human body. How do you 



account for such a large proportion of the gas 

 oxygen? 



Demonstration 2. The separation of water into its elements. If 

 by means of the apparatus shown in the diagram an electric current 

 is passed through water to which a little sulphuric acid has been added, 

 we find that the water separates into two gases. In one tube the gas 

 present occupies just half as much space as in the other tube. The 

 gas present in the smaller quantity proves upon test to be oxygen 

 as it causes a glowing splinter to burst into flame. The other gas. color- 

 less, tasteless, and odorless like the oxygen, differs from it by igniting 

 with a slight explosion when a burning match or splinter is introduced 



