Rocks and Soil 303 



and worked by machinery until plastic and is then pressed into 

 molds. After these bricks have been slightly dried naturally they 

 are placed in ovens and dried artificially. A better class of brick 

 is formed from clay which is forced by machinery through open- 

 ings in a big container, coming out as a brick bar which is cut into 

 individual bricks of proper size. These bricks are then dried and 

 baked. Bricks of still finer texture are made by using less mois- 

 ture in the original mixture and by pressing the material to the 

 shape desired by very powerful machinery. These bricks also are 

 baked and form the high class pressed brick we see today. 



Drainage pipes and building tiles, or hollow blocks, are made 

 of brick clay which has been molded to shape and baked. Orna- 

 mental tiles are glazed and finished like the better grades of 

 pottery. 



Glass. Ordinary glass is a mixture of sands containing silica, 

 soda ash, and limestone in proportions varying with the kind of 

 glass desired. For window glass and cheap plate glass the pro- 

 portion is 6, 2 and 1. Since the quality of glass depends in large 

 measure upon proper proportioning and mixing of these ingredi- 

 ents, a machine has been devised to give absolute accuracy of 

 proportion as well as uniform and perfect mixing. 



These carefully combined raw materials are placed in large 

 tanks, or furnaces, lined with fire brick. They are heated 

 to a very high temperature by flames of gas until, by melt- 

 ing, they form glass. When at the right consistency, this glass 

 is taken from the furnace and blown, pressed or rolled into 

 shape. 



In making glass tubes, like those used in science laboratories, 

 a small amount of melted glass is removed from the furnace on 

 the end of an iron tube, or blow pipe. By blowing through the 

 other end of this tube, the glass blower forms a giant glass ball 

 like a soap bubble. He then clamps one end of the ball on a 

 machine and pulls on the iron tube, drawing the glass into the 

 form of a tube. 



To make window glass, the proper amount of the molten mix- 

 ture is conveyed to a glass blowing machine which blows the glass 

 and then draws it into large cylinders. These are cut length- 



