8 APPLIED BIOLOGY 



more than one atom in the compound. Examples are : water, 

 written H 2 O, meaning that two atoms of hydrogen (H) are 

 combined with one .of oxygen (O) ; and sulphuric acid, 

 written H 2 S04, meaning that two atoms of hydrogen, one 

 of sulphur, and four of oxygen are combined to form the 

 acid. 



In the above example of burning magnesium there are two 

 elements; but more than two elements are often involved 

 in a chemical change. One compound of several elements 

 may cause a chemical change in another compound, as dis- 

 solving baking soda (HNaCOs) in sulphuric acid (H 2 S0 4 ) 

 illustrates. This dissolving of baking soda in acid illustrates 

 the most common kind of chemical change, for most sub- 

 stances on the earth are compounds. It has been noted 

 that the burning of magnesium involves only two elements, 

 but the ordinary burning of coal and other common fuels is 

 a union of compounds with oxygen, resulting in several 

 new compounds in the smoke and ashes. 



Disintegration of Compounds. Not only may elements 

 unite to form compounds, but these may be separated into 

 simpler ones or even into their constituent elements. For 

 example, water may be formed by burning hydrogen gas 

 so that oxygen of the air unites with the hydrogen as shown 

 by the formula H 2 O, but water may also be decomposed by an 

 electric current passed through it in a suitable apparatus 

 and the two constituent elements (hydrogen and oxygen) 

 in gaseous form be collected separately. Such disintegra- 

 tion of compounds into the constituent elements is not so 

 common in nature as is change of compounds into simpler 

 ones. 



11. Composition and Changes of Living Matter. The 

 principles stated in this chapter have been illustrated by 

 lifeless matter, but we shall see in later lessons that they 

 are also applicable to living matter. Living things are 

 composed of a number of common elements united in very 



