Building Materials, Their Use and Origin 419 



volved in the manufacture of most common necessities of 

 modern life, such as steel, glass, copper, lead, aluminum, illu- 

 minating gas, hydrocarbons, cotton, wool and silk textiles, paper, 

 soap, and glycerin, in preparing and cooking food products as 

 bread, meat, and vegetables, or preparing sugar, starch, etc., 

 and also in many operations concerned with building and con- 

 struction. It is important in the industry of the household as 

 well as in the industries of the world. Chemistry has con- 

 tributed greatly to human comfort and welfare in the past ; but 

 this is only an earnest of what it will contribute in the future. 

 One can hardly overestimate the importance of the study of 

 chemistry in any field in which one is likely to labor. 



199. Cement. The use of cement as a building material 

 is rapidly increasing. In the United States more than 

 80,000,000 barrels of cement are manufactured every year, 

 valued approximately at $70,000,000. Cement is widely utilized 

 in all kinds of construction, such as buildings, sidewalks, bridges, 

 aqueducts, dams, and so on. When other materials are used 

 for buildings, cement is used generally in the construction of 

 the foundation. 



Cement sets quickly and hardens in a few days after being 

 wet, and it must therefore be kept dry in shipment and handling. 

 The setting does not depend upon the presence of carbon dioxid 

 as is the case with slaked lime ; and it takes place also under 

 water. In this process cement absorbs a definite amount of 

 water and forms minute crystals, thus making an artificial 

 rock. The rock becomes harder with the passing of years. 

 Cement is usually mixed with sand, or with sand and gravel 

 or crushed rock, to form concrete. . Concrete may be made into 

 any shape by being poured into molds made of wood, which 

 are removed after the concrete has set. To secure greater 

 strength, concrete work is often reinforced with steel rods 

 arranged in the molds before the concrete is put in. 



200. Brick. Some sedimentary deposits have in the course 

 of geological history been converted into shale, and some shales 



