150 APPLIED SCIENCE 



flavor of the cream and butter. To counteract such harmful 

 changes, growths of special protective bacteria called cul- 

 tures are introduced into the butter for the purpose of pre- 

 serving its flavor. Some bacteria are very harmful as they 

 produce disease in both the human body and in other sub- 

 stances, but others arc extremely useful in industry, as they 

 produce desirable chemical changes and assist in converting 

 raw materials into finished products. Such a beneficial 

 change is produced by bacteria in the case of tanning. 



166. Composition of the Earth. Most of the raw mate- 

 rials used in trade and industry have their source in the 

 earth. A few of these substances, such as gold, are found 

 in a free state, but as noted before, the more common sub- 

 stances, such as iron, lead, tin, zinc, etc., are found combined 

 with oxygen, sulphur, and dirt. To understand why these 

 are found in this state, it is necessary to study the condition 

 of the earth. 



The interior of the earth is a hot, molten mass, from which 

 constantly issues, on various parts of the earth, a stream of 

 hot, molten stone or hot steam, gases, and so on. The gases 

 are steam, carbonic acid, burning carbon, hydrogen, and 

 hydrogen sulphide. The surface of the earth is in a com- 

 paratively cold condition. As we dig below the surface we 

 find masses of stone and rock within which valuable metallic 

 particles are embedded. These particles are called minerals. 

 These combinations of mineral and rock are due to the mix- 

 ing of hot masses. As a result, the metals that are acted 

 upon by oxygen, acids (carbonic acid), hydrogen sulphide, 

 etc., are found in the earth as oxides, sulphides, carbonates, 

 etc. As gold is not acted upon by any of the ordinary gases 

 it is found in a free state. 



