Hydraulic limestones, or those which yield a lime capable of setting under water, are not so abundant as ordinary limestones. The blue Lias, stretching from Whitby, on the north-eEtst, to Lyme Regis, in the, south-west of England, is the chief source. Available beds, whie,h have been igno rantly regarded as " bastard " limestones, and therefore neglected, occur among the Carboniferous limestones of Flintshire, Northumberland, Lanarkshire, East Fife, and the Lothians, These may always be distinguished, according to Dr. Page, by their tougher, earthier, and less crystalline texture, by not effervescing so violently under acids, and by weathering more slowly into a deeper brown surface. Some of the argillo-caleareous ironstones, known as " curl," and " cone in cone," and containing about 10 per ceut. of iron, are used, as at Coalbrookdale, in the manufacture of hydraulic cements. The Septaria, or argillo-calcareous nodules, of the London clay and lower Lias, are well known for their strong and energetic hydraulic qualities. Recently, the beds of exceed ingly puro gypsum, disclosed by the sub-Wealden borings, have been drawn upon by cement makers.
There are some scores of hydraulic cements in the market ; but their composition varies rather in method and proportion of admixture, than in the ingredients themselves. The essential com ponents are lime, clay, and oxide of iron ; the lime may vary from 50 to 80 per cent. ; the clay, from 25 to 40 ; and the oxide of iron, from 3 to 14. In some cases, the limestone employed is naturally hydraulic ; but more often, that quality is attained by an artificial admixture of the required materials, The following are some of the best-known hydraulic cements :— Parher's.—This cement is made from the nodules of induilted and slightly ferruginous marl, known as Septaria, belonging to the London clay, and found in the Isle of Sheppy, at Harwich, and on other parts of the south-easiern coast. These, as well as the argillo-caleareous nodules from the lower Lias, are naturally hydraulic limestones ; when well selected and prepared, they furnish a quick-setting, strong, and durable cement. They are burnt with pit coal in conical kilns, in the same manner as other limestones, care being taken to avoid excessive heat, as, if the lumps undergo the slightest fusion, even on the surface, they will be unfit for cement-making. After proper roasting, the calx is ground to very fine powder, and immediately packed in barrels, to exclude sir and moisture. For use, it is tempered with water, and applied at once ; it soon hardens, aod will not bear being softened down again with water.
Peds.—Quicklime, 1 part ; baked clay, 2 parts ; powdered, mixed, and calcined ; then, gypsum, fresh baked and in fme powder, 1 part, is Etdded to powdered baked clay, 2 parts; mixed well, added to the former compound, and the whole thoroughly incorporated. It is very hard and durable. fortland.—This cement, so largely manufactured on the Thames, the Tyne, and other rivers, consists of about 80 parts ef chalk or rich lime, and 20 parts of fluviatile mud, or clay ; the twe ingredients are incorporated wet, then dried, calcined, and reduced to powder.
Roman.—Genuiue Roman cement manufactured from pozzuolana, a ferruginous volcanic ash from Vesuvius and other Italian volcanoes, and lime ; or, from a combination of lime and a Tertiary volcanic earth, or kind of pumice, called trass, which occupies wide areas in the Eifel district of the Rhine. The only preparation required is grinding to an impalpable powder. The Roman cement made in this country is obtained from the Septaria of the London clay 8,nd the lower Liss, from the cement stone of the upper Lias, 8,nd from tbe shale beds of the Kimmeridge clay ; it is also manufactured from several artificial admixtures of lime and ferruginous clay, calcined together. It must be kept in closed vessels, and is mixed with water for use.
For ful ther information on the subject of Calcareous Cements, the reader is referred to Spons" Dictionary of Engineering,' and Supplement ; to Reid's Cement '; and to Page's 'Economic G eology.'
Gelatinous animal tissues contain an adhesive substance, which anatomists call " histose." When the tissues are boiled in water, the histose is changed into a substance called " gelatine," which is dissolved by the water. It may afterwards be separated from the water by simple evaporation, when it forms a dry, hard substance, which has different names according to the source from which it has been manufactured. That obtained from cartilage is called " chondrine "; that from bones, hoofs, and bides, " glue " ; that from the air-bladders and intestines of fishes, " isinglass "; and that from the less tenacious and adhesive constituents of parchment scraps, and some other animal membranes, " size." The process of manufacture, in all the,se cases, consists in boiling in water ; the hot water causes the animal substances to change into gelatine, which it dissolves. (See Bones.) Of the products mentioned above, two only are employed as cements, viz. Glue, and Size.
Glue.—This useful article is made from fresh bones, freed from fat by previous boiling ; from the refuse scraps produced in trimming skins for tanners ; from the hoofs and horns of cattle ; and from leather cuttings. The best glue is obtained from the " soundings" of sheep-skins and cattle-hides, known a.s " fleshings," stnd also, from tbeir industrial application, as " glue-pieces." These are first placed in pits containing milk of lime, where they are allowed to soak for several days, or even three weeks ; the milk of lime is changed every six or seven days, and the pieces are occasionally turned over. When sufficiently soaked, the pieces are taken out to drain and dry, for which purpose they are placed en hurdles, or in layers on a sloping pavement, and turned over three times daily. When dry and hard, they a,re ready to be sold to the glue-manufacturer, and are a better material for his purpose than the fresh skin-pieces. The first operation of the glue-maker is to soak the pieces in weak lime-water, and then to wash them in baskets under a stream of water. They are then drained, and exposed to the air, so as to enable the adhering lime to absorb carbonic acid from the atmosphere, and thus to lose its caustic properties, which would destroy part of the glue during the subsequent boiling operations. Gine which is to be used as gelatine, for culinary purposes, is derived from perfectly clean and fresh bones. For the manufacture of this material, beef-bones are preferrcd to all others, as they yield a perfectly transparent article, sold under the name of gelatine or isinglass. Calf-bones give a milky glue ; hog-bones create a blackish foam in the solution ; while the product from sheep-bones always retains the peculiar odour of the fat of these animals. The glue made from hoofs and bones is always brown and of common quality. Whatever the substance used, whether glue-pieces, bones, or horns, the process is essentially the same. The raw materiel is put into a flat copper boiler, provided with a perforated false bottom, at a little distance above the bottom, so as to prevent the solid material from touching the shell, where it would stick fast and be burned. The boiler is two-thirds filled with water, and heat is applied. In a few hours, after stirring repeatedly, the pulpy liquid is drawn off in successive portions, as soon as it is perceived that a sample taken out gelatinizes on cooling. Experience has taught that too long boiling injures the glue. The test for this cooled gelatinized material is that it must be fit to be cut into slices with a wire. Before drawing off the solution, the fire is let down, so as to stop the boiling, and allow the liquid to clarify by settling. The liquor is then drawn off into a deep boiler, where it settles for the second time, remaining hot for from five to six hours The longer it stays in this vat, the better it will be clarified, and the higher .will be its market price. Often a certain proportion of alum is added to it at this stage, to assist the separation of the impurities.