Part 3

The chief use of hydraulic cements, whether of the pozzuolanic or Portland class, is to act as an adhesive material in work which is to be exposed to water. No doubt in times of remote antiquity it was found that the jointing of masonry which was to be immersed required the use of a cement indifferent to the action of water. Ordinary mortar failed in such positions; mortar made from lime prepared from limestones or chalks containing a little clay was found to stand; mortar made from lime mixed with trass or similar active silicious material was also found to stand. On this observation rests the whole of the present enormous employment of hydraulic cements. It was a natural transition to utilize these cements not merely for jointing masonry but also for making concrete, and the only reason why hydraulic cements, as distinct from cements which are not hydraulic (e.g. ordinary mortar), are used for the latter purpose is their great mechanical strength. Their use in above-water work is checked by the low price of common brick. Even in such work, where it would be thought that masses of burnt clay would be the cheapest conceivable material, concrete is at least on level terms with its rival. It must be remembered that one of the great advantages of concrete is that five-sixths of its total mass may be provided from local sand and gravel, on which no carriage has to be paid. The cement, on which alone freight is to be reckoned, converts these from loose incoherent material into a solid stone. Thus it comes about that the largest use of cement is for manufacturing concrete for dock and harbour work, and for the making of foundations. It is also employed for the building of light bridges, floors, and pipes constructed of cement mortar disposed round a skeleton of iron rods. Such composite structures take advantage at once of the high tensile strength of iron and of the high compressive strength of cement mortar. (See also CONCRETE.)

Good hydraulic cements are highly permanent materials provided certain conditions be observed. It might be supposed that hydraulic cements from their nature would be indifferent to the action of water, but this is only true if the structures of which they form part are sufficiently compact. In this case the action of the water is checked by the film of carbonate of lime which eventually forms oh the surface of calcareous cement. This, together with the compactness of the mortar, hinders the ingress and egress of water, and prevents the dissolution and ultimate destruction of the cement. But where the concrete or mortar is not well made and is porous, the continual passage of water through it will gradually break up and dissolve away the calcareous constituents of the cement until its strength is utterly destroyed. This destructive action is increased if the water contains sulphates or magnesium salts, both of which act chemically on the calcareous constituents of the cement. As sea-water contains both sulphates and magnesium salts, it is especially necessary in concrete for harbour work to take every care to produce an impervious structure. There are various minor external causes for the failure and ultimate destruction of cement mortar and concrete, but their discussion is a matter for the specialist. Failure from inherent vice in the cement has been already touched on; it can always be traced to want of skill and care in manufacture.

_Calcium Sulphate Cements._--Under this term are comprehended all cements whose setting properties primarily depend on the hydration of calcium sulphate. They include plaster of Paris, Keene's cement and many variants of these two types. The raw material is gypsum (q.v.). This may be almost chemically pure, when it is generally used for Keene's cement; or it may contain smaller or greater quantities of impurities, in which case it is suitable for the preparation of cements of the plaster of Paris class. The mode of preparation is to calcine the gypsum at temperatures which depend on the class of cement to be produced. If plaster of Paris is to be made, calcination is carried out at about 204° C. (=400° F.); at this temperature, gypsum, CaS04.2H20, loses three-quarters of its combined water and becomes 2CaSO4.H20. If a cement of the Keene's cement class is to be prepared the temperature used is higher, e.g. 500° C. (=932° F.), and the whole of the combined water of the gypsum is expelled, the anhydrous sulphate CaSO4 being obtained.

Plaster of Paris; Keene's cement.

To produce plaster of Paris European practice consists in baking the mineral in ovens, and in America in heating it in kettles. Both processes are inferior in economy to calcination in rotatory kilns, a process which may be regarded as the method of the present and the immediate future. Keene's cement and its congeners are made in fixed kilns so constructed that only the gaseous products of combustion come into contact with the gypsum to be burnt, in order to avoid contamination with the ash of the fuel.

The setting of plaster of Paris depends on the fact that when 2CaSO4·H2O is treated with water it dissolves, forming a supersaturated solution of CaSO4·2H2O. The excess held temporarily in solution is then deposited in crystals of CaSO4·2H2O. In the light of this knowledge the mode of setting of plaster of Paris becomes clear. The plaster is mixed with a quantity of water sufficient to make it into a smooth paste; this quantity of water is quite insufficient to dissolve the whole of it, but it dissolves a small part, and gives a supersaturated solution of CaSO4.2H2O. In a few minutes the surplus hydrated calcium sulphate is deposited from the solution, and the water is capable again of dissolving 2CaSO4·H2O, which in turn is fully hydrated and deposited as CaSO4·2H2O. The process goes on until a relatively small quantity of water has by instalments dissolved and hydrated the 2CaSO4·H2O, and has deposited CaSO4·2H2O in felted crystals forming a solid mass well cemented together. The setting is rapid, occupying only a few minutes, and is accompanied by a considerable expansion of the mass. There is reason to suppose that the change described takes place in two stages, the gypsum first forming orthorhombic crystals and then crystallizing in the monosymmetric system. Gypsum thus crystallized is in its normal monosymmetric form, more stable under ordinary conditions than the orthorhombic form. Correlatively in its process of dehydration to form plaster of Paris, monosymmetric gypsum is converted into the orthorhombic form before it begins to be dehydrated.

The principles which govern the preparation and setting of the other class of calcium sulphate cements, that is, cements of the Keene class, are not fully understood, but there is a fair amount of knowledge on the subject, both empirical and scientific. The essential difference between the setting of Keene's cement and that of plaster of Paris is that the former takes place much more slowly, occupying hours instead of minutes, and the considerable heating and expansion which characterize the setting of plaster of Paris are much less marked.

It is the practice in Great Britain to burn pure gypsum at a low temperature so as to convert it into the hydrate 2CaSO4·H2O, to soak the lumps in a solution of alum or of aluminium sulphate, and to recalcine them at about 500° C. On grinding they give Keene's cement. Instead of alum various other salts, e.g. borax, may be used. The quantity of these materials is so small that analyses of Keene's cement show it to be almost pure anhydrous calcium sulphate, and make it difficult to explain what, if any, influence these minute amounts of alum and the like can exert on the setting of the cement. It seems probable that the effect of the salts is inconsiderable, and that the governing condition is the temperature at which the cement has been burnt. The setting of Keene's cement takes place by the same sort of process which has been described for the setting of plaster of Paris, the chief differences being that the substance dissolved is anhydrous calcium sulphate and that the operation takes a longer time.

All cements having calcium sulphate as their base are suitable only for indoor work because of the solubility of this substance. They form excellent decorative plasters on account of their clean white colour and the sharpness of castings made from them, this latter quantity being due to their expansion when setting.

See D.B. Butler, _Portland Cement_ (London, 1905); E.C. Eckel, _Cements, Limes and Plasters_ (New York, 1905); G.R. Redgrave and Charles Spackman, _Calcareous Cements_ (London, 1905); F.H. Lewis, "Manufacture of Hydraulic Cements in the United States," _The Mineral Industry_ (New York, 1898); W.H. Stanger and Bertram Blount, "Cement Manufacture in Great Britain," _The Mineral Industry_, New York, 1897 and 1905; _Id_. "The Testing of Hydraulic Cements," _Journ. Soc. Chem. Ind.,_ 1894, 13, p. 455; _Id., Proc. Inst. Civ. Eng._, 1901; B. Blount, "Recent Progress in the Cement Industry," _Journ. Soc. Chem. Ind.,_ 1906, 25, p. 1020; H.L. le Chatelier, _Recherrhes experimenlales sur la constitution des mortiers hydrauliques;_ Desch, _Concrete_, No. 2, pp. 101-102; Davis, _Journ. Soc. Chem. Ind.,_ 1905, 26, p. 727. (B. Bl.)

_Adhesive Cements._--Mixtures of animal, vegetable and mineral substances are employed in great variety in the arts for making joints, mending broken china and other objects, &c. A strong cement for alabaster and marble, which sets in a day, may be prepared by mixing 12 parts of Portland cement, 8 of fine sand and 1 of infusorial earth, and making them into a thick paste with silicate of soda; the object to be cemented need not be heated. For stone, marble, and earthenware a strong cement, insoluble in water, can be made as follows:--skimmed-milk cheese is boiled in water till of a gluey consistency, washed, kneaded well in cold water, and incorporated with quicklime; the composition is warmed for use. A similar cement is a mixture of dried fresh curd with 1/10th of its weight of quicklime and a little camphor; it is made into a paste with water when employed. A cement for Derbyshire spar and china, &c., is composed of 7 parts of rosin and 1 of wax, with a little plaster of Paris; a small quantity only should be applied to the surfaces to be united, for, as a general rule, the thinner the stratum of a cement, the more powerful its action. Quicklime mixed with white of egg, hardened Canada balsam, and thick copal or mastic varnish are also useful for cementing broken china, which should be warmed before their application. For small articles, shellac dissolved in spirits of wine is a very convenient cement. Cements such as marine glue are solutions of shellac, india-rubber or asphaltum in benzene or naphtha. For use with wood which is exposed to moisture, as in the case of wooden cisterns, a mixture may be made of 4 parts of linseed oil boiled with litharge, and 8 parts of melted glue; other strong cements for the same purpose are prepared by softening gelatine in cold water and dissolving it by heat in linseed oil, or by mixing glue with one-fourth of its weight of turpentine, or with a little bichromate of potash. _Mahogany cement_, for filling up cracks in wood, consists of 4 parts of beeswax, 1 of Indian red and yellow-ochre to give colour. _Cutler's cement_, used for fixing knife-blades in their hafts, is made of equal parts of brick-dust and melted rosin, or of 4 parts of rosin with 1 each of beeswax and brick-dust. For covering bottle-corks a mixture of pitch, brick-dust and rosin is employed. A cheap cement, sometimes employed to fix iron rails in stone-work, is melted brimstone, or brimstone and brick-dust. For pipe-joints, a mixture of iron turnings, sulphur and sal ammoniac, moistened with water, is employed. _Japanese cement_, for uniting surfaces of paper, is made by mixing rice-flour with water and boiling it. _Jewellers'_ or _Armenian cement_ consists of isinglass with mastic and gum ammoniac dissolved in spirit. Gold and silver chasers keep their work firm by means of a cement of pitch and rosin, a little tallow, and brick-dust to thicken. _Temporary cement_ for lathe-work, such as the polishing and grinding of jewelry and optical glasses, is compounded thus:--rosin, 4 oz.; whitening previously made red-hot, 4 oz.; wax, ¼ oz.

CEMETERY (Gr. [Greek: koimêtêrion], from [Greek: koiman], to sleep), literally a sleeping-place, the name applied by the early Christians to the places set apart for the burial of their dead. These were generally extra-mural and unconnected with churches, the practice of interment in churches or churchyards being unknown in the first centuries of the Christian era. The term cemetery has, therefore, been appropriately applied in modern times to the burial-grounds, generally extra-mural, which have been substituted for the overcrowded churchyards (q.v.) of populous parishes both urban and rural.

From 1840 to 1855, attention was repeatedly called to the condition of the London churchyards by correspondence in the press and by the reports of parliamentary committees, the first of which, that of Mr Chadwick, appeared in 1843. The vaults under the pavement of the churches, and the small spaces of open ground surrounding them, were crammed with coffins. In many of the buildings the air was so tainted with the products of corruption as to be a direct and palpable source of disease and death to those who frequented them. In the churchyards coffins were placed tier above tier in the graves until they were within a few feet (or sometimes even a few inches) of the surface, and the level of the ground was often raised to that of the lower windows of the church. To make room for fresh interments the sextons had recourse to the surreptitious removal of bones and partially-decayed remains, and in some cases the contents of the graves were systematically transferred to pits adjacent to the site, the grave-diggers appropriating the coffin-plates, handles and nails to be sold as waste metal. The neighbourhood of the churchyards was always unhealthy, the air being vitiated by the gaseous emanations from the graves, and the water, wherever it was obtained from wells, containing organic matter, the source of which could not be mistaken. In all the large towns the evil prevailed in a greater or less degree, but in London, on account of the immense population and the consequent mortality, it forced itself more readily upon public attention, and after more than one partial measure of relief had been passed the churchyards were, with a few exceptions, finally closed by the act of 1855, and the cemeteries which now occupy a large extent of ground to the north, south, east and west became henceforth the burial-places of the metropolis. Several of them had been already established by private enterprise before the passing of the Burial Act of 1855 (Kensal Green cemetery dates from 1832), but that enactment forms the epoch from which the general development of cemeteries in Great Britain and Ireland began. Burial within the limits of cities and towns is now almost everywhere abolished, and where it is still in use it is surrounded by such safeguards as make it practically innocuous. This tendency has been conspicuous both in the United Kingdom and the United States. The increasing practice of cremation (q.v.) has assisted in the movement for disposing of the dead in more sanitary conditions; and the proposals of Sir Seymour Haden and others for burying the dead in more open coffins, and abandoning the old system of family graves, have had considerable effect. The tendency has therefore been, while improving the sanitary aspects of the disposal of the dead, to make the cemeteries themselves as fit as possible for this purpose, and beautiful in arrangement and decoration.

The chief cemeteries of London are Kensal Green cemetery on the Harrow Road; Highgate cemetery on the slope of Highgate Hill; the cemetery at Abney Park (once the residence of Dr Watts); the Norwood and Nunhead cemeteries to the south of London; the West London cemetery at Brompton; the cemeteries at Ilford and Leytonstone in Essex; the Victoria cemetery and the Tower Hamlets cemetery in East London; and at a greater distance, accessible by railway, the great cemetery at Brookwood near Woking in Surrey, and the cemetery at New Southgate. The general plan of all these cemeteries is the same, a park with broad paths either laid out in curved lines as at Kensal Green and Highgate, or crossing each other at right angles as in the case of the West London cemetery. The ground on each side of these paths is marked off into grave spaces, and trees and shrubs are planted in the intervals between them. The buildings consist of a curator's residence and one or more chapels, and usually there is also a range of family graves with imposing tombs, massive structures containing in their corridors recesses for the reception of coffins, generally closed only by an iron grating. The provincial cemeteries in the main features of their arrangements resemble those of the metropolis. One of the most remarkable is St James's cemetery at Liverpool, which occupies a deserted quarry. The face of the eastern side of the quarry is traversed by ascending gradients off which open catacombs formed in the living rock,--a soft sandstone; the ground below is planted with trees, amongst which stand hundreds of gravestones. The main approach on the north side is through a tunnel, above which, on a projecting rock, stands the cemetery chapel, built in the form of a small Doric temple with tetrastyle porticos.

Many of the cities of America possess very fine cemeteries. One of the largest, and also the oldest, is that of Mount Auburn near Boston. Others of importance are the Laurel Hill cemetery (1836) at Philadelphia; the Greenwood cemetery (1838) at Brooklyn (New York); the Lake View cemetery at Cleveland, Ohio; while the cemeteries at New Orleans (q.v.) are famous for their beauty.

The chief cemetery of Paris is that of Père la Chaise, the prototype of the garden cemeteries of western Europe. It takes its name from the celebrated confessor of Louis XIV., to whom as rector of the Jesuits of Paris it once belonged. It was laid out as a cemetery in 1804. It has an area of about 200 acres, and contains about 20,000 monuments, including those of all the great men of France of the 19th century--marshals, generals, ministers, poets, painters, men of science and letters, actors and musicians. Twice the cemetery and the adjacent heights have been the scene of a desperate struggle; in 1814 they were stormed by a Russian column during the attack on Paris by the allies, and in 1871 the Communists made their last stand among the tombs of Père la Chaise; 900 of them fell in the defence of the cemetery or were shot there after its capture, and 200 of them were buried in quicklime in one huge grave and 700 in another. There are other cemeteries at Mont Parnasse and Montmartre, besides the minor burying-grounds at Auteuil, Batignolles, Passy, La Villette, &c. In consequence of all these cemeteries being more or less crowded, a great cemetery was laid out in 1874 on the plateau of Méry sur Oise, 16 m. to the north of Paris, with which it is connected by a railway line. It includes within its circuit fully 2 sq. m. of ground. The French cemetery system differs in many respects from the English. Every city and town is required by law to provide a burial-ground beyond its barriers, properly laid out and planted, and situated if possible on a rising ground. Each interment must take place in a separate grave. This, however, does not apply to Paris, where the dead are buried, forty or fifty at a time, in the _fosses communes_, the poor being interred gratuitously, and a charge of 20 francs being made in all other cases. The _fosse_ is filled and left undisturbed for five years, then all crosses and other memorials are removed, the level of the ground is raised 4 or 5 ft. by fresh earth, and interments begin again. For a fee of 50 francs a _concession temporaire_ for ten years can be obtained, but where it is desired to erect a permanent monument the ground must be bought by the executors of the deceased. In Paris the undertakers' trade is the monopoly of a company, the _Société des pompes funèbres_, which in return for its privileges is required to give a free burial to the poor.

The _Leichenhäuser_, or dead-houses, of Frankfort and Munich form a remarkable feature of the cemeteries of these cities. The object of their founders was twofold--(1) to obviate even the remotest danger of premature interment, and (2) to offer a respectable place for the reception of the dead, in order to remove the corpse from the confined dwellings of the survivors. At Frankfort the dead-house occupies one of the wings of the propylaeum, which forms the main entrance to the cemetery. It consists of the warder's room, where an attendant is always on duty, on each side of which there are five rooms, well ventilated, kept at an even temperature, and each provided with a bier on which a corpse can be laid. On one of the fingers is placed a ring connected by a light cord with a bell which hangs outside in the warder's room. The use of the dead-house is voluntary. The bodies deposited there are inspected at regular intervals by a medical officer, and the warder is always on the watch for the ringing of the warning bell. One revival, that of a child, has been known to take place at Frankfort. The Leichenhaus of Munich is situated in the southern cemetery outside the Sendling Gate. At one end of the cemetery there is a semicircular building with an open colonnade in front and a projection behind, which contains three large rooms for the reception of the dead. At both Frankfort and Munich great care is taken that the attendants receive the dead confided to them with respect, and no interment is permitted until the first signs of decomposition appear; the relatives then assemble in one of the halls adjoining the Leichenhaus, and the funeral takes place. In any case there is, with ordinary care, little fear of premature interment, but in another way such places of deposit for the dead are of great use in large towns, as they prevent the evil effects which result from the prolonged retention of the dead among the living. Mortuaries for this purpose have also been established in many places in England.

In Italy the _Campo Santo_ (Holy Field) is best illustrated by the famous one at Pisa, from which the name has been given to other Italian burying-grounds. Of the cemeteries still in use in southern Europe the catacombs (q.v.) of Sicily are the most curious. There is one of these under the old Capuchin monastery of Ziza near Palermo, where in four large airy subterranean corridors 2000 corpses are ranged in niches in the wall, many of them shrunk up into the most grotesque attitudes, or hanging with pendent limbs and head from their places. As a preparation for the niche, the body is desiccated in a kind of oven, and then dressed as in life and raised into its place in the wall. At the end of the principal corridor at Ziza there is an altar strangely ornamented with a kind of mosaic of human skulls and bones.