FILLING OF CAVITIES 109 



water and any form of aggregate being concrete. 

 The word cement is, however, frequently used to 

 designate concrete, especially when the aggregates 

 employed are small. 



Concrete, then, is made by mixing cement with 

 aggregates and with water. In order that the 

 concrete may be good the materials of which it is 

 made must be carefully selected. Portland ce- 

 ment is very uniform and it is safe to use the prod- 

 uct of any of the large mills, or any brand sold 

 by a reliable dealer. If kept for any length of 

 time it must be protected from moisture, and must 

 be dry and powdery when used. It comes in 

 paper or cloth sacks containing about ninety-five 

 pounds. Four sacks make a barrel and cost about 

 a dollar and a half. 



The aggregates present greater difficulties. In 

 the first place, the type of aggregate best suited to 

 the purpose must be determined. It has been the 

 usual custom to employ simply a mixture of cement 

 and sand for filling trees, on account of the fact 

 that coarser aggregates make it difficult to work 

 the exposed surface of the filling down smooth. 

 A concrete made of small aggregates, however, is 

 not so strong nor so dense as one in which larger 

 aggregates are used. W. A. Radford, in 

 " Cement and How to Use It," makes a clear 

 statement of this principle: 



" There is the closest relation between the 



