182 CEMENTS- alow conversion of the hydrate of lime into carbonate and silicate, and the deposition of crystalline hydrate, and also in the cohesion that naturally results from the long-continued apposition of particles of matter with one an- other. There has been a long and an unset- tled controversy among chemists, architects, and engineers in regard to the changes which take place in mortar after it has been laid in walls exposed to the air, some maintaining that the hardening is chiefly caused by the conver- sion of the hydrate of lime into carbonate; others believing that it is principally due to the slow formation of a silicate ; and others again that it is due to both these changes, and also to the crystallization of hydrate and consequent packing of void spaces aided by the force of cohesive attraction. From the investigations of Prof. F. Kuhlmann of Lille, in regard to the action of alkaline silicates upon limestones and chalk, it is probable that the lime after becom- ing carbonated is susceptible of combining with a certain portion of silica, if this is present in a soluble condition in combination with an alkali, and forming a silico-carbonate of lime. Petz- holdt examined several old mortars. Some that were 300 years old yielded lime water when digested in fresh water, thus showing the pres- ence of caustic lime. Some portions effervesced in cold dilute hydrochloric acid, yielding in a short time a stiff jelly, thus revealing the pres- ence of a soluble silicate of lime. He moreover found that in similar mortars, those which were 300 years old contained three times as much soluble silicate of lime as those which were only 100 years old. These experiments not only show that silicification takes place in building mortar made of lime and sand, but that it takes place in a pretty constant rate of progress, which would appear to be somewhat indepen- dent of the degree of carbonization attained by the lime. Hydraulic cements are used in the construction of fortifications, breakwaters, aqueducts and reservoirs, canals, foundations of bridges, and other works of military and civil engineering, as well as in the construction of cellars and cisterns. The useful property which they possess of rapidly setting when immersed in water, and of continuously hard- ening under the same influence, chiefly results from the strong affinity of caustic lime for silica and alumina, and from the affinity the resulting compounds have for water, and their insolubility. They are divided into two princi- pal classes, natural and artificial cements. The former are entirely made from certain rocky strata or earthy substances, without any admix- ture of foreign material ; while those which are artificial are made by combining earthy sub- stances with caustic lime, and sometimes small portions of an alkali. Certain geological for- mations contain beds which are composed of such proportions of lime, alumina, silica, the alkalies, and a few other bodies of less conse- quence, that after they are calcined insoluble compounds are formed on the addition of water. Other formations exist, of a less calcareous con- stitution, which need the addition of caustic lime to enable them to perform the same or similar reactions ; and there are others which contain too much lime to be capable of being used for hydraulic cement, but which may be employed in making what is termed hydraulic lime. The stone from which hydraulic cement is made in the United States is found in strati- fied rocky beds of aqueous deposits, lying prin- cipally in strata of the Silurian system connect- ed with the Appalachian chain of mountains. It is an argillaceous limestone, which yields on calcination the proper proportions of lime, alumina, and silica to unite with water and form a hard substance without slaking or ex- panding, and also to indurate continuously in consequence of other chemical reactions. The Silurian system, in the classification of the New York geologists, lies in strata in the following order : SILURIAN BTBTEM. ! Lower Helderberg. Lower Helderberg. Salina. Saliferous. Niagara. Niagara. Clinton. Medina. Oneida. Hudson. Hudson river. Utica. Trenton. Trenton. Black river. Birdseye. Cbazy. Potsdam. Calclferous sandstone. Potsdam sandstone. This formation covers a large space in Canada, N. of New York, about the size of that state, and also a large area N. W. of Lake Ontario. In the United States it covers about half of the state of New York and the western part of Vermont, whence it extends in a belt S. W. through eastern New York, and through the states of New Jersey, Pennsylvania, Maryland, Virginia, and East Tennessee. It also appears in Middle Tennessee and Kentucky, and covers a vast area W. and N. W. of Lake Michigan, 600 m. in longitude and from 200 to 500 in latitude. Most of the hydraulic cement in New York is obtained from beds in the Trenton and lower Helderberg formations. In Ohio beds are found in the upper coal measures, and in Illinois in the lower carboniferous formation and in the Niagara group. In Canada there are beds in the saliferous group. Only a few layers of these groups are used, most of them either contain- ing too little or too much lime. Those of the Trenton group which furnish hydraulic cement extend from Vermont, in a narrow belt, south- ward through the eastern tier of counties in Ifew York, passing into Orange county at Newburgh ; thence through Sussex and War- ren counties, New Jersey, into the state of
