GREENHOUSES 



Stone. Either crushed storre or gravel 

 may be used in making concrete, the only 

 difference being that the crushed stone usual- 

 ly has a cleaner surface and the cement will 

 cling to it more tightly. When gravel is 

 used it should be free from clay, and the in- 

 dividual stones should be clean and bright 

 and not covered with a layer o clay or soil. 



The size of the stones may range from a 

 fourth to two and a half inches in diameter, 

 the size depending on the use to which the 

 concrete is put. The best results are ob- 

 tained when the sizes vary regularly from 

 small to large, in order that they may settle 

 well together when the concrete is poured. 



Run of the Bank gravel is sometimes used. 

 This is economical only when it contains 

 sand and gravel in the correct proportions, 

 as explained in a succeeding paragraph. 



Crushed Stone may also contain very fine, 

 medium and coarse stone in the correct pro- 

 portions, so that no sand need be added, but 

 such a condition is rare, unless tlie stone is 

 ground and furnished for this special pur- 

 pose. 



