304 Our Surroundings 



wise and are again heated until they soften and flatten into sheets 

 of glass which are slowly cooled. This second heating and slow 

 cooling process is called annealing. It strengthens glass against 

 cracking and breaking. 



Bottles and small glass objects are made by machines which 

 mold, press or squeeze the molten glass into shape. Plate glass 

 is made by pouring the molten material onto a flat, metal table 

 where it is rolled by machinery to its proper thickness. When 

 cooled, it is rubbed and treated until flat and smooth. The shap- 

 ing and finishing of glass is made possible because molten glass 

 solidifies slowly. 



Glass-making is among the oldest of industries and figures 

 of glassmakers with their blow pipes may be seen on many 

 an ancient monument. It has only been since the be- 

 ginning of the century that glass has been made by machin- 

 ery. Within the past thirty years great machines have been 

 invented which take the place of glass-making by hand and 

 which turn out various glass products in enormous quantities 

 daily. 



Importance of Knowledge of Soil. The dependence of all 

 living things on the soil for the production of the food essential 

 to their growth and development makes necessary some knowl- 

 edge of the nature and composition of soil. The life of all crea- 

 tures depends directly or Indirectly upon food substances provided 

 by plants. Plants, in turn, get substances from the soil to aid in 

 the manufacture of their food. If any of the essential substances 

 were for any reason removed from the soil, all life on the earth 

 would perish. This fact makes plain the importance of some 

 knowledge of the soil. 



Necessity for Cultivation of Soil. Unless soil is cultivated, 

 that is, plowed and harrowed, it soon becomes covered with a 

 growth of weeds. Their removal, however, is not the only reason 

 for cultivating soil. Cultivation crumbles the soil into small par- 

 ticles, thus bringing it into the best possible condition for holding 

 moisture until it is absorbed by the root hairs of plants. Besides, 

 it aids the movement of the air in the soil. Air and moisture 

 are both necessary for vigorous plant growth. 



