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C E N BUS used for lining tunnels made by the modern shield system. Roman Cement is another cement of the Portland class which came into use some hundred years ago, shortly before the manufacture of artificial Portland cement was attempted. It is still in use, though only for special purposes where a quick-setting material is required. It is made from septaria nodules which are dredged up on the Kent and Essex coasts and consist of about 60 per cent of calcium carbonate mixed with clay, the mass being sufficiently indurated to remain coherent under water. The nodules are not prepared in any way, but simply burned at a moderate red heat. The resulting cement varies somewhat in composition, but approximates to the following figures:— Per cent. Insoluble silicious matter . . . 5'86 Silica (Si02) 19-62 Alumina (A^O;;) .... 10'30 Ferric oxide (FejO-j) . . . .7-44 Manganese dioxide (jvln02) . . .1-57 Lime (CaO) . . . . . 44'54 Magnesia (MgO) .... 2'92 Sulphuric anhydride (S03) . . .2-61 Carbonic anhydride (CO.,) . . 3'43 Water (H20). . . . . 0-25 Alkalies and loss .... 1’46 100-00 The most characteristic constituent is the oxide of iron, which gives the cement a reddish colour, and the presence of manganese also differentiates Roman from Portland cement, which rarely contains appreciable quantities of that element. The high percentage of alumina causes the cement to be quick-setting, and it becomes hard in about five minutes. It resists the action of water, salt or fresh, very well, and is therefore useful in situations where the work is likely to be submerged immediately after it has been put in place. The term Natural Cements is applied to cements made by burning mixtures of clay and carbonate of lime naturally occurring in approximately suitable proportions. They may be regarded as badly-mixed Portland cements, and need no special description. American “natural” cements are of a somewhat different class. They are usually made from a silicious limestone containing magnesia, and are comparatively lightly burned. The following analysis is typical of a cement of this kind :— Per cent. Silica (Si02) . . . .24-30 Alumina (A1203) . . . . 7"22 Ferric oxide (Fe203) . . . . 5 "06 Lime (CaO) . . . . . 33"70 Magnesia (MgO) . . . . 20"94 Water, carbonic anhydride, and loss . 8 "78 100 00 These irregular cements of the Portland class are good building materials for ordinary purposes, but are not so suitable as good artificial Portland cement for heavy and important undertakings. The chief use of hydraulic cements, whether of the puzzuolanic or Portland class, is to act as an adhesive material in work which is to be 0t n exPosed f° water. No doubt in times of remote cements, 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

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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. 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 on 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. Authorities.—D. B. Butler. Portland Cement. London, 1899. —F. H. Lewis. Article on cement in The Mineral Industry. New York, 1898.—Serranger and Blount. Article on cement in The Mineral Industry. New York, 1897 ; Paper on testing cements, Journal of the Society of Chemical Industnj. London, 1894.—G. R. Redgrave. Calcareous Cements. London, 1898. (b. Bl.) Census—United Kingdom.—The historical account of the census in the United Kingdom has been already given in detail up to and including that of 1871. Since then the changes in procedure have not been numerous, though some are of considerable importance. In England and Wales the growth of population necessitated the increase of the enumerating agency from 32,543 in 1871 to 34,711 in 1881, to 35,507 in 1891, and to 38,200 in 1901, while the operations have been complicated by the greater number of local subdivisions to be recognized in the returns. Between 1871 and the next census, for instance, the creation of sanitary districts entailed S. II. — 8o