The best-known cements are Portland and Roman cements, and are distinguished from mortar by hardening quickly, while mortar hardens slowly. Portland and Roman cements both set or harden under water ; hence they are generally spoken of as hydraulic cements, although they are often . used in superior masonry which is not intended to be covered by water. Portland cement is made of chalk or ground limestone mixed with clay or shale. Roman cement is made from a natural mixture of lime and clay. Of the natural cement stones found in the United States, the best are the Rosendale cements of New York. A cement made of carbonate of magnesia is superior in strength and hardness to all others. For ordinary cementing plaster of Paris is very useful, and there are many cements to mend broken glass, ivory, wood, etc.