Polytechnic Association. 903 



structions in the sea, etc., chips or small stones, gravel, broken bricks, 

 burned clay, cinders, etc., are generally used in combination with 

 sand. Such a mixture is then termed beton or concrete. The sub- 

 stitution of common lime for a portion of the cement, however, 

 results always in a sacrifice of strength in proportion to the extent of 

 tiie adulteration. When mixed with a small quantity of lime milk, 

 it loses proportionally but little of its solidity, while it can be worked 

 much better and more safely, as the setting is greatly retarded by such 

 an admixture. In the following table are given the results of Pro- 

 fessor Eankine's experiments as to the crushing strength of bricks 

 and natural stone : — 



Crushing weight per 

 square inch in 

 Materials. pounds. 



Brick, weak red 550— 800 



Brick, strong red 1,100 



Brick, first quality hard 2,000— 4,368 



Brick, fire 1,700 



Chalk 330 



Granite, Patapsco 5,340 



Granite, Quincy 15,300 



Marble, Montgomery county, Penn 8,950 



Limestone, granular 4,000 — 4,500 



Limestone, marble 5,500 



Sandstone, strong 5,500 



Sandstone, ordinary 3,300 — 4,400 



Sandstone, Connecticut 3,319 



Caen stone 1,088 



Durability ; Ilealthfulness. 



European experience, extending over a period of more than forty 

 years, has established the fact that constructions of Portland cement 

 or beton will resist the influences and changes of climate equally as 

 well as the very best building stones. Portland cement stone,- if 

 properly made, is almost impervious to water, while this cannot be 

 said of brick and sandstone. Since warmth and moisture are pecu- 

 liarly favorable to vegetable growth, these latter building materials 

 are more liable to disintegration than others with less absorptive 

 power. The resistance to frost is absolute, even in structures the 

 roofs of which are terraced with this material. With regard to the 

 absorption of moisture by brick, Mr. Edwin Chadwick, who was 

 appointed to report on improved dwellings at the Paris Exhibition, 

 says : 



" There is another great source of evil attaching to walls of the 

 common brick and common soft stone construction — the absorbency 



