Cements, certain substances which by their interposition cause the surfaces of solid bodies to adhere together or to unite, the action being either mechanical or chemical, or both. The history of the fabrication of cements, like that of many other arts, reaches so far back into the early ages that it is impossible to ascertain with much exactness where it was first skilfully practised. The ancient Egyptians 4,000 years ago possessed the knowledge not only of making building mortar, but of mixing earthy materials which would set and harden under water. In the construction of the pyramid of Cheops, a cement made of Nile mud and gypsum is believed to have been used. Many of their sculptures in bass relief were executed in cement, and examples are still preserved of Egyptian ceilings in painted stucco of a date much earlier than Solomon's temple. The pictures in relief which have been discovered in the excavations at Nineveh were mostly executed in alabaster; but the Babylonians, not having such materials, covered their bricks with plaster, on which they made their designs. It was upon the "plaster of the wall" of Belshazzar's palace that the mystic hand traced the fatal letters.

Under Nebuchadnezzar Babylon became the first city in the world, and mortars and ce-ments of various kinds, of a bituminous and earthy character, were used in enormous quantities in the construction of edifices and public works. The Greeks gave the subject much intelligent attention, as is evidenced by the chemical composition and state of preservation of mortars and cements which have been found in their ancient temples; and it is a matter of history that in the earlier development of the architectural and engineering arts by the Romans, the Greeks were often consulted by them. The Romans, however, attained to the greatest distinction for the magnitude and durability of their works. They prepared an excellent cement for hydraulic purposes, which they used in making concrete with broken stones for the construction of various piers and harbors on the Mediterranean. They early became acquainted with the properties of poz-zuolana, which mixed with burned lime gave them a hydraulic cement that can scarcely be said to have been since excelled. The mole or breakwater of Pozzuoli is one of the monuments of the durability of their hydraulic structures. It was composed of 24 arches, sustained upon piers, built of brick faced with stone, and held together with cement made of pozzuolana and lime.

Thirteen of the piers are still above the water, although they were built more than 1,800 years ago. The arched construction was for the purpose of preventing a collection of sand behind the mole. Vitruvius, in his work Be Architectural says: "There is found in the neighborhood of Baiae, and the municipal lands lying at the foot of Vesuvius, a kind of powder which produces admirable effects; when mixed with lime and small stones it has not only the advantage of giving great solidity to common buildings, but possesses the further property of forming masses of masonry which harden under water." - Cements may be divided into those which are chemical and those which are mechanical, or into the stony and the resinous and glutinous. The stony cements may be again subdivided into those which harden on exposure to the air, such as common building mortar; those which harden when immersed in water, as the hydraulic cements; and those which harden principally by combining with water, as gypsum and gypseous cements. Common building mortar is made of lime and sand.

The preparation of lime by calcination from limestone, chalk, marble, and other forms of carbonate of lime, is treated of in the article Lime. Many kinds of limestones contain carbonate of magnesia as well as carbonate of lime, and are called dolomitic limestones. When the proportion of carbonate of magnesia is 46 per cent., the stone is called dolomite, and has been pronounced unfit for making building mortar. It does not become so hot in slaking nor set so soon as pure lime; but those who use it assert that in time it becomes harder. Some of the best lime in this country is made from stone obtained along the Hudson river, much of which is almost pure dolomite. Before mixing with sand, the lime is slaked, a process that requires to be carefully performed in order to secure good results. Three volumes of quicklime moistened with one of water slakes with much violence and the evolution of heat, which often reaches 300° F. If enough water is used to cover the lime, it will be made to boil. Slaked lime is a hydrate of the protoxide of calcium, and is two and a half or three times the volume of the quicklime used. All the water used in slaking should be added at once.

Less than is required to convert all the lime to a hydrate will cause a granular powder to be formed, which is of inferior value. After the slaking is completed the mass should be allowed to lie one or two days before it is made into mortar. In the south of Europe it is often put in boxes and kept for months, a practice which is thought to increase its power of conferring hydraulic energy to pozzuolanas. When used, the slaked lime should be made into a creamy paste by the addition of about an equal quantity of water. The quantity of sand to be added depends upon the use which is to be made of the mortar, and also upon the quality of the sand. In order to prevent shrinking, there should not be much more lime paste than is sufficient to fill the void spaces between the particles of sand. Coarse sharp sand will therefore take more lime than that which is fine, and will in time generally become harder. For bricklaying and the rough or foundation coat of wall plastering, the proportions vary from two to three parts of sand to one of lime paste. In the rough coat in wall plastering about one sixth part by measure of cows' hair is mixed, to aid in binding. Mortar may be mixed by hand, in which case a common hoe is the implement generally used.

A mill, however, mixes the materials more thoroughly, and when a large quantity is required will repay the outlay. A common pug mill, such as is used for grinding clay for brick, answers the purpose very well, or one having a pair of heavy rollers moving round an axis in a circular pan. A form used in Europe is a conical mill, which empties into a trough or tube in which a spiral band wound round a shaft revolves, while it mixes the mortar and carries it to the end, when it is delivered into tubs. The setting and hardening of mortar are two distinct processes. Setting is caused partly from the adhesion that takes place between the particles of sand and lime, and which is no doubt increased by incipient chemical action, partly from the formation of more or less crystalline hydrate, and probably to some extent by evaporation. The hardening of common building mortar consists in the slow 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 another.

There has been a long and an unsettled 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 conversion 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 Lisle, in regard to the action of alkaline silicates upon limestones and chalk, it is probable that the lime after becoming 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 presence of caustic lime. Some portions effervesced in cold dilute hydrochloric acid, yielding in a short time a stiff jelly, thus revealing the presence 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 independent 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 hardening 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 in,to two principal classes, natural and artificial cements.

The former are entirely made from certain rocky strata or earthy substances, without any admixture of foreign material; while those which are artificial are made by combining earthy substances with caustic lime, and sometimes small portions of an alkali. . Certain geological formations contain beds which are composed of such proportions of lime, alumina, silica, the alkalies, and a few other bodies of less consequence, that after they are calcined insoluble compounds are formed on the addition of water.

Other formations exist, of a less calcareous constitution, 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 stratified rocky beds of aqueous deposits, lying principally in strata of the Silurian system connected 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 expanding, 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:

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, 000 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 containing too little or too much lime. Those of the Trenton group which furnish hydraulic cement extend from Vermont, in a narrow belt, southward through the eastern tier of counties in New York, passing into Orange county at Newburgh; thence through Sussex and Warren counties, New Jersey, into the state of Pennsylvania, appearing in Berks, Chester, Lancaster, and York counties, and passing through Maryland into Virginia, where they are found in Rockingham, Augusta, Rock. bridge, Roanoke, and other counties.

The tentaculite, or water limestone of the lower Helderberg formation, however, furnishes most of the hydraulic cement that is used in the United States. Large deposits are found in Oneida, Onondaga, Madison, Niagara, Erie, Ulster, and Sullivan counties, New York. The largest development of the manufacture has taken place in Ulster county. The deposits there occupy a narrow belt in the valley of the Rondout creek, along the line of the Delaware and Hudson canal, and pass into New Jersey and Pennsylvania, but have not been found east of the Hudson river. The following table, made from a larger one in the work of Gen. Q. A. Gillmore " On Limes, Hydraulic Cements, and Mortars," gives analyses made by Prof. E. C. Boynton of four samples of hydraulic limestone, and the localities from which they were taken:

The limestone from which cement is made, as has been intimated, varies considerably in quality even in the same locality, the difference depending much upon the relative position of the layer from which it is taken. The nearer it lies to those strata that are decidedly calcareous, the more it approaches in character what is called a rich lime, slaking and expanding more or less. Again, when the more highly argillaceous strata are approached, and the deposit contains less lime, it loses energy and capability of hardening. Some of the stones require greater time for calcination than others, from 20 to 40 hours being the range. If the temperature is raised too high, the silicate is partially fused and the property of setting under water destroyed. It is therefore evident that such stones should not be burned together, although they may be mixed afterward. Careful inspection is not always sufficient to detect the difference, nor is chemical analysis always to be relied on, although it is quite probable that those limes which contain much alkaline silicates should not be so highly calcined as others. Trial kilns capable of burning from 100 to '500 lbs. should be frequently used.

In preparing hydraulic cement for the market, similar kilns to those used for calcining common limestone are employed, and the processes are also similar. The perpetual flame kiln, or the draw kiln, such as is used on the continent of Europe, particularly the one invented by Friedrich Hoffmann of Berlin, is preferable. Kilns which are charged with alternate layers of fuel and limestone do not furnish so equable or controllable a degree of heat, and it is more important to secure this condition in calcining hydraulic limestone than in making quicklime. The process of calcination is described in the article Lime. After burning, the stone is passed through a crushing mill and reduced to small pieces of the size of beech nuts, and is then finely pulverized by ordinary millstones. The crushers in general use are capable of preparing for the mill about 250 bbls. per day each; and one millstone will grind about 10 bbls. per hour. One cubic yard of stone from the quarry yields about 9 bbls. of ground cement, of 300 lbs. each. In Ulster county it is packed in barrels as it comes from the spout. The color of most of the hydraulic cement made in this country is a lighter or darker drab, the depth of hue depending principally upon the amount of oxide of iron which may be present.

It will be observed from the table that the hydraulic limestones of Ulster and Erie counties, X. Y., as well as theoneatShepherdstown, Va., contain a large proportion of carbonate of magnesia. This might be regarded as a defect from what has been said of the effect of the presence of magnesia in common lime; but the case is altered in respect to hydraulic cements, and they are considered by Vicat and Marshal Vaillant, as well as by MM. Rivot and Cha. toney, as being superior for hydraulic purposes on this account. When hydraulic cement is to be used, it is made into a paste with water, but no definite rules as to the proportion can be prescribed; the best plan being to add just that quantity which will form a paste of such a consistence as can be manipulated with facility by the trowel. When the cement is used to form grout for filling in walls, it requires to be made thinner. The setting and hardening of hydraulic cement are processes not so distinctly separated as they are in the case of common building mortar.

The setting and much of the hardening take place simultaneously, or rather the setting and hardening are mostly one and the same process; and with some cements it takes place almost immediately, further hardening proceeding slowly by cohesion, and by further chemical changes that have been commenced. The time varies with the kind and quality of the cement, some kinds taking not more than ten minutes, while others require more than as many hours, to become hard enough to fracture. MM. Rivot and Chatonay state that in calcination of an argillaceous limestone, when it has beenjust sufficient to expel the carbonic acid from the carbonate of lime, there will be formed a silicate and an aluminate of lime, SiOs, 3CaO, and A12O3, 3CaO, which on the addition of water become hydrates, Si03, 3CaO, 6HO, and A13O3, 3CaO, 6HO. But if the calcination has been effected with intense heat, so as to produce partial vitrification, the reaction will become more complex, silicate of alumina and silicate of lime being formed first; after which, on the addition of water, decomposition takes place with the formation of hydrosilicate and hydroaluminate of lime, each containing three equivalents of water instead of six.

From the fact that increased hydraulicity is conferred on some earths which are used in making hydraulic cement by mixing them with lime that has been slaked a long time and exposed to the air, and therefore partially reconverted to a carbonate, it would appear that double decomposition is often favorable to the silicification of the lime; or in other words, that on quitting its union with carbonic acid it more readily combines, in consequence of being in a nascent state, with the silica, which on leaving its union with an alkali is also in a nascent condition. What influence oxide of iron may have on the hardening of cements is still a matter of discussion. Experience sometimes seems to favor the idea that it is beneficial in ultimate hardening; but very hard cements have been made which contained scarcely a trace of iron, and the subject must be considered as involved in doubt. - The hydraulic cements principally employed in Europe are the Roman, Portland, Medina, Mul-grave, and those made on the continent from pozzuolana, trass, santorin, and Teil lime. Modern Roman cement, so called because of its similarity to the old Roman cement, is principally made in England and France. The English article is made from nodules, or septaria, found in the Kimmeridge and London clays.

The first modern Roman cement was made by Mr. Parker of London, who patented his process in 1796; it consisted in calcining the septaria stones nearly to the point of vitrification, and then crushing and grinding them to powder without admixture of any other material, the product being therefore a natural cement. Roman cement is also made at Boulogne from nodules of septaria lying in the same geological formation, and does not essentially differ in quality from that made in England. Roman cement is now made in England from a clay shale found above the chalk formation in the isle of Sheppey and the isle of Wight. In Germany Roman cement is also made from limestones of a similar character. All the Roman cements contain a notable quantity of oxide of iron, as will be seen in the following table of analyses by Michaelis:

No. 1 is Roman cement from the Rudersdorf limestone; 2, from limestone shale from the isle of Sheppey; 3, from limestone forming the under bed of the lead ores at Tarnowitz; and No. 4 is from Hausbergen limestone. Portland cement, an artificial compound, so named from its resemblance to Portland stone, was first manufactured by Joseph Aspdin of Leeds, England. His method was, according to his letters patent, to pulverize limestone and burn it in a kiln; then to add an equal weight of clay and thoroughly knead the mixture with water to a plastic mass, which was dried, broken in pieces, and again burnt to expel all the carbonic acid. Since that time, however, great improvements have been made in its manufacture, both in England and on the continent. To obtain the best results, it has been found that the lime should be in a state of carbonate when it is mixed with the clay, and therefore, on account of greater facility and cheapness, chalk is employed instead of limestone. Two methods are practised, one in England and the other in Germany, which are somewhat different.

In England the wet method is followed, which consists in mixing the materials together with water and grinding them to a pulp, which, after evaporation to the proper degree of stiffness, is made into balls or bricks, which are dried and then calcined to near the point of vitrification, after which they are ground in a mill. The proportions of the ingredients are from 65 to 75 per cent, of chalk to 25 or 35 per cent, of clay. In Germany the dry method is preferred, which consists in first drying the materials and reducing them to powder separately, after which they are mixed with water in the proper proportions, and made into bricks, which are then dried, calcined, and ground, as in the English method. The Germans also add a certain quantity of alkali, either soda or potash, depending upon the quantity which may already be contained in the clay, but enough to bring the amount to 3 or 4 per cent, of the whole, for the purpose of forming a soluble silicate to act upon the lime in setting; and sometimes a small amount of finely pulverized sand or quartz rock is added. In making mortar of Portland cement Mr. Lipowitz, a German manufacturer, recommends the use of about 100 parts of cement to 44 parts of water, but the proportions are subject to variation.

Analyses of four varieties of Portland cement made in England and Germany are given by Michaelis:

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 proportions 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 calcined in a common kiln, mixed with coke, a lower degree of heat being employed than in burning Portland cement. Gen. Pasley is regarded as the founder of artificial cement manufacture in England. In 1826 he made a cement 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 rivers. 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 Jess 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 water is explained by the theory of MM. Rivot and Chatoney that where cements are calcined at a high heat silicate of alumina and silicate of lime are formed, which on the addition 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 decomposition 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 Martinique, and in several places in France. It is mixed, without calcination, with caustic lime.

Sometimes, however, the lime is allowed to stand for several months after slaking before being used, and this practice is said in some instances to increase the hydraulic energy of the product, probably in consequence of the partial recarbonization of the lime facilitating the process of silicification which takes place between the lime and the alkaline silicates 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 the Romans also employed it many centuries ago. It possesses qualities nearly identical with Italian pozzuolana, and is treated with lime in the same maimer. It is of a grayish color, and lies in beds composed of fine particles and small lumps. The following table showing the chemical composition of pozzuolana and trass is taken from Wagner's "Chemical Technology:"

A hydraulic lime obtained at Teil, in the department of Ardeche, France, has been found to possess properties which eminently fit it for use in marine constructions, as it sets and hardens well in salt water. As analyzed by Prof. Rivot, it has the following composition:

- Various methods are employed for ascertaining the hardness and tensile strength as well as the power to resist pressure. The apparatus of M. Vicat for testing the hardness and determining the time of setting employs a pointed pin, which, by means of a suitably adjusted frame, is driven against the surface of a prepared 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 perfectly plane sides, under a weight which may be increased at pleasure. The crushing and tractile force necessary to overcome the resistance of the cement may be found by mathematical calculation, by supporting a brick edgewise 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 sand, and exposed for several weeks to the action of the air. This has also been used with good results in the harbors of Marseilles and Toulon, and at various other places, in both fresh and salt water. The piers enclosing the harbor at the ocean terminus of the North sea canal of Holland are laid in Portland cement, and the same material enters into the structure of the Cherbourg breakwater. The foundations of Fort Tompkins and Fort Rich. mond in New York harbor are composed of concrete made of hydraulic cement, sand, and fragments of granite, in the proportion of one part of cement to three of sand and five of granite. Exterior stucco work is now to a great extent made of hydraulic mortar. - For further information in regard to the details of the technical manipulation employed in manufacturing, using, and testing hydraulic cements and mortal's, see "Practical Treatise on Calcareous Mortars and Cements," by L. J. Vicat (London, 1837); "Observations on Calcareous Cements," etc, by Major Gen. Sir C. W. Pasley, K. C. B. (London, 1847); Henry Reid's treatises on Portland cement and on concrete (London, 18G8 and 18G9); the practical essay of A. Lipowitz on Portland cement (translated by W. F. Reid, London, 1868); the work of Dr. W. Michaelis, Die hydraulwchen Mortel (Leipsic, 1869); Gen. Q. A. Gillmore's treatise on "Lime, Hydraulic Cements, and Mortars" (New York, 1872); and "Report on the Hydraulic Lime of Teil," by Leonard F. Beckwith, C. E. (New York, 1873). - The following table of the composition of ancient mortars, with the names of the analysts, is taken from Mr. Reid's treatise on concrete, and will be found interesting to peruse in connection with the various theories of the hardening of mortars:

- The mastics and other oily cements of London have nearly gone out of use on account of their want of durability. They were compounds of lime, sand, litharge, and linseed oil. Gypsum and gypseous cements are much used for internal plastering and decoration. The rapidity with which plaster of Paris sets when mixed with water makes it an article of great value. The gypseous cements mostly used in England are Keene's and the Parian. The former is obtained by recalcining plaster of Paris which has been slaked with a saturated solution of alum and water, and again slaking it with a saturated solution of borax. Bituminous cements are employed as substitutes for flagging in paving streets, and to protect the extrados of arches from the effects of water; and for this they have much utility, because in all new masonry there are movements which fissure the coatings made in hydraulic cements, so that it is almost impossible to render them impermeable to water, and the elasticity of bituminous cements is well calculated to overcome the defects of the more unyielding stony compounds and render them particularly serviceable in making repairs. They are made from natural asphaltum, which is mixed with chalk or carbonate of lime, or with hydraulic cement.

A cement which is made in Silesia for covering roofs is composed of Portland cement, coal or wood tar, and a small portion of sulphur. Among the resinous cements are Varley's, which is made by drying by means of a red heat 10 parts of pulverized whiting, and when cold stirring it in a melted mixture of 16 parts of black resin and one of beeswax. Singer's electrical and philosophical apparatus cement consists of 5 lbs. of resin, 1 lb. each of beeswax and red ochre, and two tablespoonfuls of plaster of Paris, melted together. The ochre and plaster should be previously calcined, and added after the other ingredients are melted. Lapidaries' cement for holding gems while they are being cut is made of resin and red ochre or Spanish brown, melted together and tempered with a little beeswax and tallow. Opticians' cement, for a similar purpose, the fixing of glasses for grinding, is made of sifted wood ashes and melted pitch. White lead cement, made by grinding linseed oil varnish and white lead together, is used for repairing fractured bodies of all kinds, but requires a considerable time to harden. Plumbers' cement is made of two parts of brick dust and one of black resin melted together. Iron rust cement is made by mixing 50 or 100 parts of iron tilings with one part of sal ammoniac.

When used it is moistened with enough water to form a paste. The subsequent rusting and the consequent packing of the joints to which it may be applied gives it a particular value. Sometimes clay may be added to the mixture with advantage. A solution of shellac in alcohol, added to one of isinglass in proof spirit, makes a cement which has considerable power to resist the action of moisture. Common glue, melted with half its weight of resin and a small quantity of red ochre to impart body, is used in cementing hones to their frames. A strong cement for stoneware is made by boiling the cheese of skimmed milk in a large quantity of water, and incorporating the solution with quicklime in a mortar. It may be used cold, but it is better to warm it. It joins earthenware, marble, or any kind of stone so that the seam can scarcely be discovered. A cement composed of from four to six parts of potters1 clay and one of iron tilings, with enough linseed oil to form a paste, makes a cement which is often used in stopping cracks in steam boilers.

In the distillation of caoutchouc, the residue left in the retort, when again mixed with the distilled oil, or caou-tchoucine, forms an elastic cement which is much used by shipwrights; and another tenacious cement is made by dissolving one part of caoutchouc in four parts of coal tar, adding two parts of shellac, and heating the mixture in an iron vessel.