ing paper and place it in the sun, or near a heater to 

 dry. White paper is best, for that can be rolled back 

 and forth, and leave the fine loam on the edge. Crunch 

 the sand in your hand near the ear, to determine 

 whether it is sharp. Keep a magnifying glass on the 

 work to examine sand. 



In the laboratory for testing the material used in the 

 Philadelphia subway construction the percentage of 

 "loam" (which might better be termed "silt"), in the 

 finer aggregates, was determined by agitating the sam- 

 ple in water, pouring off the water containing the 

 matter not settled within three-quarters of a minute, 

 and repeating the operation until the water became 

 clear in that time. The effluent containing the sus- 

 pended matter is poured into a glass cylinder into 

 which the matter which settled in three-quarters of a 

 minute had been previously placed, and after settle- 

 ment the fine material is measured and recorded as a 

 percentage of the total volume of the material in the 

 glass cylinder, after the water has become clear. 



AGGREGATES. 



The different rocks used to produce concrete aggre- 

 gate are: 



Trap. Very good, hard and with sharp faces. This 

 includes crushed granite and other igneous rock. 



Limestone. Good for some kinds of work, not as 

 hard as granite. Don't use where exposure to fire is 

 possible, because heat will free the carbon dixoide 

 (Co 2 ) from the limestone (CaCo :: ) and leave quicklime 

 CaO, which is slaked and disintegrated by \v;> 



Sandstone. Don't use sandstone unless it is tough 

 and close grained. 



Gravel. A well graded bank gravel is excellent. The 

 proportions of larger and smaller inerts is already well 

 determined. A \vi-hecl gravel is very good. If gravel 

 runs too high in sand it should be screened. 



Slag. Furnace slag is specially prepared by running 



