cement puzzuolani, but this has been shortened to 

 puzzolana, as the modern name for any cement 

 made of volcanic dust, or powdered burnt clay, mixed 

 with powdered hydrates of lime. It is much lighter in 

 weight than true Portland cement, and is of a light-lilac 

 color, rather than the familiar bluish-gray of the modern 

 cement. At the present time its use is limited to struc- 

 tures that are exposed to a moist atmosphere, or those 

 under water. 



The manufacture of Portland cement, which gets its 

 name from its resemblance to the famous Portland build- 

 ing-stone of England, began in the early years of the 

 nineteenth century. It is produced by calcining a mixture 

 of calcareous and argillaceous substances, and grinding 

 the resulting clinker to extreme fineness. When this 

 is thoroughly mixed with certain proportions of sand, 

 gravel, or broken rock, and thoroughly moistened, it 

 sets into an apparently homogeneous rock, of a quality 

 superior to most building-stone, and less expensive. 

 Its great flexibility in working, along with its other re- 

 markable qualities, make it the favorite medium of 

 modern construction. It can be cast in molds as bricks 

 or building-stone, which may then be laid in mortar; or 

 the molds can be so arranged that an entire wall, or 

 even a building, may be cast, the whole structure 

 being as homogeneous as if hewn from solid rock. 

 If steel rods are laid in the concrete during the course 

 of construction making "reinforced concrete," to 

 which we shall refer a little later the resulting build- 

 ing will be stronger and more enduring than if hewn 

 out of granite itself. 



