Polytechnic Association: 901 



numerous cracks and fissures prove that great strength and durability 

 were not, under the circumstances, to be expected. 



With regard to plastering, a high temperature is equally disadvan- 

 tageous, in causing a too rapid evaporation. The best season is the 

 spring. Thus, a building which has been completed in the autumn 

 can be provided with a coating of cement the following spring. For 

 common plastering, the summer and the fall are, as is well known, 

 preferred. 



The Properties of Cement Stone. 



Cement stone possesses an agreeable bright gray stone-color ; it is 

 capable of being polished ; attains in its best quality a hardness and 

 power of resistance equal to that of the most valued limestones, even 

 to those of the oldest formations. To these properties it owes the name 

 Portland cement. Joseph Aspdin intended to intimate, by the use 

 of this name, that this product equaled the Portland stone in appear- 

 ance and quality. If this important member of the Upper Oolite, 

 and of that local fresh-water formation, the Wealden, are left out of 

 consideration, the final member of the English cretaceous formation 

 includes the most excellent building stones. 



The most splendid edifices of the metropolis, the most beautiful and 

 the grandest buildings of England, are made of this stone, and the 

 extensive quarries of Purbeck are widely known on account of their 

 beautiful and homogeneous material. Like this stone, the solidified 

 Portland cement is of a fine-grained texture, and, offering no cleavage 

 whatever, it yields evenly to the chisel. But this property can be 

 easily dispensed with in the case of cement, on account of the fact 

 that while soft it can be adapted to any form. Herein lies the great 

 value of this artificial product. It is true that Portland cement shares 

 that deportment with other materials like plaster of Paris, but when 

 we consider its degree of hardness, strength and binding qualities, we 

 find that the cement surpasses all similar materials, and what is of the 

 greatest importance, that it will fill a given space without contracting 

 or expanding. If we conclude that Portland cement stone contains 

 on an average sixteen per cent of water in chemical combination, and 

 that the burned cement has an average specific gravity of 3.2, the 

 specific gravity of solidified Portland cement, calculated as follows, 

 (3-2xS4) + 16 _ 9m 



100 ' - 2 - T04 > 



while direct determinations give the figures 2.676. 



