CEMENTS 185 Nos. 1 and 4 are English Portland cements, No. 2 made at Stettin, and No. 3 at Wildau. It will be remarked that in these instances the English cements contain the largest propor- tions of alkalies. In France a similar method is employed. The hydraulic lime of Saint- Leger is made of four parts of chalk to one of clay. The ingredients are ground together in a mill with water, and when sufficiently firm the mixture is made into bricks, which are cal- cined in a common kiln, mixed with coke, a lower degree of heat being employed than in burning Portland cement. Gen. Pasley is re- garded as the founder of artificial cement man- ufacture in England. In 1826 he made a ce- ment by burning the river mud from the Med- way with limestone or chalk. This mud, on account of its argillaceous composition, as well as from the presence of sodium salts, is well adapted to the purpose; but other materials are now used, such as marls mixed with clay, and generally the deposits at the deltas of riv- ers. A natural Portland cement is made at Boulogne! from a layer of Kimmeridge clay which lies about 160 ft. below the strata in which the septaria nodules are found. It is burned and ground without the addition of lime or any other material. In burning this earth the best results are obtained by bringing it to a white heat, so that vitrification begins to take place. It has a greater specific gravity than English Portland cement, and requires less water for making into paste than either that article or the Boulogne Roman cement ; and it also sets more slowly, requiring from 10 to 20 hours. The reason of its taking less wa- ter is explained by the theory of MM. Rivot and Chatoney that where cements are cal- cined at a high heat silicate of alumina and silicate of lime are formed, which on the addi- tion of water undergo decomposition with the formation of aluminate and silicate of lime, containing each three instead of six equivalents of water, which is the case when a heat only sufficient to drive off" the carbonic acid of the carbonate of lime is employed ; and the de- composition which must take place before the final hydration also explains the slow setting. Pozzuolana is found not only at the foot of Mt. Vesuvius, but in various places in Europe : in Sicily, in Mauritius, in Guadeloupe and Mar- tinique, and in several places in France. It is mixed, without calcination, with caustic lime. Sometimes, however, the lime is al- lowed to stand for several months after sla- king before being used, and this practice is said in some instances to increase the hydraulic en- ergy of the product, probably in consequence of the partial recarbonization of the lime facil- itating the process of silicification which takes place between the lime and the alkaline sili- cates contained in the volcanic pozzuolana. A substance of volcanic origin called trass is found in the valley of the Rhine, which has long been much used, especially by the Dutch engineers, for hydraulic works in Holland, and CONSTITUENTS. Tn IM. Sol. In HC1. InioLInHCl. Silica 44-5 11*50 87-44 Lime 8*8 8 - 16 2-25 Magnesia 4'7 2-15 0-27 Potash ( r * 1 0-29 0*08 Soda 1 5 ' 5 f 2-44 1-12 Alumina ( 150' 17-70 1-25 Oxide of iron 12-0 11-17 0-75 Water 9-2 7-65 Total 99-7 56-06 48-16 the Romans also employed it many centuries ago. It possesses qualities nearly identical with Italian pozzuolana, and is treated with lime in the same manner. It is of a grayish color, and lies in beds composed of fine parti- cles and small lumps. The following table showing the chemical composition of pozzuo- lana and trass is taken from Wagner's " Chem- ical Technology : " A hydraulic lime obtained at Teil, in the de- partment of Ard^che, France, has been found to possess properties which eminently fit it for use in marine constructions, as it sets and hard- ens well in salt water. As analyzed by Prof. Rivot, it has the following composition : THE UNBUBNED STONE. Lime 46-8 Oxide of iron 07 Silica, quartz sand, and clay 15-0 Carbonic acid and water. 87-6 Total, 99-6 THE BURNED LIME. Lime 78-29 Oxide of iron traces Silica 18-20 Alumina 1-80 Quartz sand 1-71 Total. . . ... 100-00 Various methods are employed for ascertain- ing the hardness and tensile strength as well as the power to resist pressure. The apparatus of M. Vicat for testing the hardness and deter- mining the time of setting employs a pointed pin, which, by means of a suitably adjusted frame, is driven against the surface of a pre- pared brick of cement with any given force, and which is determined by different weights which are used as the motive power. The tensile strength is found by fastening each end of a brick in a suitable clamp or socket, and suspending it with a weight attached at the lower end. The force required to produce fracture in this case will represent the power of cohesion attained by the particles of the brick. The power of resisting pressure is found by placing a prepared brick, having per- fectly plane sides, under a weight which may be increased at pleasure. The crushing and tractile force necessary to overcome the resist- ance of the cement may be found by mathe- matical calculation, by supporting a brick edge- wise and applying a weight in the middle, which will exert a crushing force at the upper, and a tractile force at the lower edge. The jetties which form the harbor at Port Said, the Mediterranean terminus of the Suez canal, were constructed of blocks of concrete made of hydraulic lime from Teil mixed with beach
