266 EEPORT OF NATIONAL MUSEUM, 1899. 



Their property of induration out of contact with the air is assumed 

 to be due to the formation of calcium and aluminum silicates. Inas- 

 much as these silicates are practically insoluble in water, it follows that 

 quite aside from their greater strength and tenacity they are also more 

 durable; indeed, there seems no practical limit to the endurance of 

 a good hydraulic cement, its hardness increasing almost constantly 

 in connection with its antiquity. Certain stones contain the desired 

 admixture of lime and clayey matter in just the right proportion for 

 making hydraulic cement. In the majority of cases, however, it has 

 been found that a higher grade, stronger and more enduring material, 

 can be made by mixing in definite proportions, determined by experi- 

 ment, the necessary constituents obtained, it may be, from widely sep- 

 arated localities. The exact relationship existing between composition 

 and adaptability to lime making does not seem as yet to be fully worked 

 out. As is well known, the pure white crystalline varieties yield a 

 quicklime inferior to the softer blue-gray, less metamorphosed varie- 

 ties. Nevertheless there are certain distinctive qualities, due to the 

 presence and character of impurities, which led Gen. Q. A. Gillmore 

 to adopt the following classification: 



(1) The common or fat limes, containing, as a rule, less than 10 per cent of impurities. 



(2) The poor or meager limes, containing free silica (sand) and other impurities in 



amounts varying between 10 per cent and 25 per cent. 



(3) The hydraulic limes, which contain from 30 to 35 per cent of various impurities. 



(4) The hydraulic cements, which may contain as much as 60 per cent of impurities 



of various kinds. 



As above noted most cements are manufactured from a variety of 

 materials, and their consideration belongs therefore more properly to 

 technology. Nevertheless it has been thought worth the while here 

 to give in brief the matter below relative to a few of the more impor- 

 tant and well-known varieties now manufactured. 



Portland cement. — This takes its name from a resemblance of the 

 hardened material to the well-known oolitic limestone of the island of 

 Portland in the English Channel. As originally made on the banks 

 oi the Thames and Medway it consists of admixtures of chalk and clay 

 dredged from the river bottoms, in the proportions of three volumes 

 of the former to one of the latter, though these proportions may vary 

 according to the purity of the chalk. These materials are mixed with 

 water, compressed into cakes, dried and calcined, after which it is 

 ground to a fine powder and is read} for use. The following analyses 

 from Heath's Manual of Lime and Cement will serve to show the vary- 

 ing composition of the chalk and clay from the English deposits. 



