CEMENTS. 119 



The artificial quays and islands built by the Romans for 

 the purpose of erecting villas in the Bay of Baiaj, a spot of 

 fashionable resort for the wealthy, were constructed of a 

 peculiar earth, called by the Romans pulvis puteolanus, and 

 now known under the name of Puzzolana. It is a porous, 

 friable mineral, of volcanic origin and various color. On 

 being reduced to a powder, and incorporated thoroughly with 

 lime, either with or without sand, it forms a mass of firm, 

 solid substance, that, not only in the air, but immersed in 

 water, concretes to stony hardness. 



Cements are also formed of other substances. Some of 

 the ores of manganese form water cements ; and baked clay 

 reduced to powder, and the common greenstone calcined and 

 pulverized, make with lime tolerable hydraulic cements. 



Some limestones, when calcined and mixed with simple 

 sand and water, form water cements, and usually in con- 

 sequence of these stones containing a certain portion of 

 argillaceous earth united with lime. 



Various parts of the United States afford very good 

 hydraulic cements. 



Different theories have been broached to account for sub- 

 stances hardening under water ; the most probable appears 

 to be, that the attraction for water in certain argillaceous 

 earths causes them to quickly absorb the superabundance of 

 moisture from the lime, and thus hasten its solidification ; 

 which explanation receives collateral aid in the fact, that 

 burnt clays, which, properly managed, afford hydraulic ce- 

 ments, cease to do so if the burning has been continued till 

 vitrification has ensued. (Bigelow's Useful Arts.) 



COMMON CEMENT FOR BROKEN VESSELS. 



Put to half a pint of boiling milk the same quantity of 

 vinegar. Take out the curd, and when the whey is only 

 milk-warm beat into it the whites of five fresh eggs. Beat 



