callis sand, white of eggs, and the strongest wort, that it defied all hammers and hatchets whatsoever." The mortar used in bishop Gundulph's works at Malling and Rochester is described by B. Ferrey as consisting of a sort of tua found only in the cliffs at Dover, which appears to have been exclusively used in his works.

1859d. Slug is applied to the vitrified earths left in furnaces, either for glass or iron. Scoria are the lighter, more porous, and less vitrified earths arising from the puddling and refining of iron. The cinders used are the earthy residues derived from the combustion of coal. When ground into powder, the two former, which contain a large proportion of the mineral ox des, make very good mortars i' mixed with middling or perfectly hydraulic imes. Cinders appear to render the rich limes moderately hydraulic when properly mixed. They require a large quantity of water to render perfect the crystallization of the hydrate of lime. All these mortars may be usefully employed for works out of water.

1859e. The stones whereof the Dutch terras is made are found in the neighbourhood of Liege, and also, we believe, at Andernach on the Rhine, from the size of a pea to that of a middle-sized turnip. From their being brought down the rivers to Holland the cement has been called Dutch; the only operation they undergo in that country is the reduction of them to a coarse powder by means of mills. They are beaten by iron-headed stampers on an iron bed till they will pass through a sieve whose wires are about one eighth of an inch apart. This cement is sent from Holland in casks. Trass, terras, or tarras, is a blueblack trap. It is obtained from pits of extinct volcanoes, and has nearly all the distin guishing elements of puzzuolana, resembling it in composition, and in the requirements of its manipulation, having to be pulverised and added to rich lime to develope its hydraulic properties.

1859f. The Puzzuolana, or terra Puteolana of the Italians, which, as well as the last-named cement, has been almost if not quite superseded by the introduction of the Roman cement, is brought from Civita Vecchia. Its name is however derived from Puzzuoli, where it is principally found, though produced in other parts of Italy, in the neighbourhood of extinct volcanoes. It suddenly hardens when mixed with one third of its weight of lime and water, forming a cement more durable under water than any other. Bergman found 100 parts of it to contain 55 to 60 parts of siliceous earth, 20 of argillaceous, 5 or 6 of calcareous, and from 15 to 20 of iron; this last constituent is considered to be the cause of its property of hardening under water. The iron decomposes the water of the mortar, and thus in a very short time a new compound is formed. According to Vitruvius, when used for buildings in the water, 2 parts of puzzuolana were mixed with 1 of mortar. Artificial puzzuolana may be made by slightly calcining clay, and driving off the water of combination at a temperature of 1,200°.

1859g. Subsequently to the use of this material from Puzzuoli, a similar material has been found near Edinburgh; and in the Vivarais, a site of extinct volcanic action in the centre of France. Its aspect and colour, however, vary very much even in the same locality. Berthier gives the following analysis of two of these materials:

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1859h. In the use of blue lias lime for mortar, workmen ignorant of its qualities invariably spoil it. In important works the lime should be supplied in an unground state, to prevent the core being mingled with the good lime. In slaking, the lumps should be broken into pieces of about the size of a nutmeg; then immersed upon a sieve in water, and kept therein until air bubbles freely rise to the surface: the lime so wetted is to be left in a heap, and covered with damp sand, for twenty-four hours. At the expiration of that time it should be screened and mixed with sand and the least possible quantity of water. When slaked, it does not sensibly increase in bulk, unlike the ordinary chalk or stone lime of the neighbourhood of London. The best descriptions of blue lias lime will not bear more than 1 parts of sand to 1 of lime. Wood, of Bath, in his work on Cottages, 1788, has stated that blue lias lime mixed with coal ashes in the manner prescribed by M. Loriot, will make the hardest cement I ever saw, as I have found by various experiments; it will hold water, resist frost, harden in a few hours in water, and will bear a very

な

good polish. Coach or carriage ways are laid or pitched with blue lias, which wears very well, though it will not bear the frost."

1859i. A very useful hydraulic mortar for executing sea-walling, consists of 1 part of chalk lime, or of Halkin lime, with one part of puzzuolana from Civita Vecchia, and Iļ parts of sand; but the value of this mixture depends upon the influence exercised by the puzzuolana on the setting of the lime. A mixture of the natural calcareous cements, or of Portland cement with sand, is another good mortar. The presence of sulphate of lime in any composition intended to resist the action of sea water would be fatal, as it crystallises at a different rate of rapidity, and it is more easily soluble than the carbonate of lime. French authorities lay particular stress on the following qualities for the formation of good hydraulic mortar: I. It is essential that the materials should be perfectly pulverised before mixing, so that the combination may be as perfect as possible. II. Sufficient free lime must be present to allow the carbonic acid in the water to combine with it, and form a protective coating of carbonate. III. Long soaking of the mterials is advisable, in order that the chemical combinations necessary for the ultimate stability of the mortar may take place before it is actually used.

1859k. Mr. Smeaton discovered, by a course of experiments, that the scales (grey oxide of iron) that fly off under the forge hammer froin r. d hot iron, pulverised, sifted, and mixed with lime, form an admirable cement, equal to puzzuolana. He found, in pursuing his experiments, that roasted iron ore produced an effective water cement, by using a greater proportion of it than either terras or puzzuolana. Equal quantities of iron scales and argillaceous lime, with half the quantity of each of these of sand, produced a cement in every respect equal to terras mortar. If pure carbonate of lime be used, equal parts of each of the ingredients ought to be incorporated. We do not think it necessary here to give any account either of Loriot's cement, or that proposed by Semple: neither are to be depended on: indeed the first, as a water cement, is of inferior utility, and very little better than common mortar dried before the admission of water upon it.

1860. Grout, or liquid mortar, is nothing more than e mmon mortar mixed with a sufficient quantity of water to make it fluid enough to penetrate the interstices and irregularities of the interior of brick walls, which common mortar will not reach. The mortar whereof it is made will bear 4 of sand to 1 of lime, but it should be thoroughly beaten. It may be kept a little longer, whereby its quick setting will be facilitated.

1861. CONCRETE is a compound of ballast, or stone chippings, and lime mixed together. It is so called from the speedy concretion that takes place between these particles. If, however, gullets or small stone chippings are used, sand in a large proportion to the lime must be used. The use of concrete was well known at an early period; it is mentioned by De Lorme in his work published in 1568; and it is by no means, therefore, a discovery of modern days. Wherever the soil is soft, and unequal for the reception of the foundations of a building, the introduction of concrete under them is an almost infallible remedy against settlement. The Thames ballast, commonly used for concrete, is a mixture of sand and small stones. With this, and lime in the proportion of never less than 4 to 1, and never properly exceeding 9 to 1, of stone lime, or such as is known to set hard in water, a mixture is made. The lime is generally used in powder, and the whole being shovelled together, it is wheeled in barrows to a stage over the spot where it is to be used, and let fall into the trench dug out for the reception of the foundation. The greater the height the concrete is made to fall, the sounder and stronger it becomes. It must always be recollected that no more lime is necessary than with the thinnest coat to surround the particles of the ballast, and that therefore the size of the pebbles or stones should influence the quantity of the lime. As the ground is more or less to be trusted the thickness of the concrete must be regulated; when used on the best ground, a foot in thickness will be sufficient; while on the worst, as many as four feet or more may be required. The upper surface being levelled, it is usual to lay on it a tier of Yorkshire stone landings, for the reception of the brick-work or mason's work: in some cases, after carrying the wall a certain height, a second tier of landings has been introduced. When the soil is watery, no water should be put to the concrete, but the ballast, and lime merely mixed and tumbled in. The usual practice of making concrete as above stated, is objected to by many practitioners, who recommend that the French method of making béton should be followed in lieu of it. 1862. In forming concrete, the stones or pebbles used should never exceed the size of a hen's egg, of which 2 parts may be combined with 1 part of the smaller substances used; this makes it about equal to Thames ballast. It has been calculated, that as the lime absorbs the water, and with the sand fills up the interstices of the larger material, if the proportion of the lime be about one eighth of the ballast, then 33 cubic feet of ground lime, and 30 cubic feet of ballast, with a sufficient quantity of water to effect the admixture (and this is generally rather less than a gallon to a cubic foot of ballast, or than equal measures of water and lime), will be required to make 27 cubic feet of concrete; that is, there is a loss of bulk equal to all the lime, and of about 10 per cent. of the ballast. But some experiments made in 1857-8, in which the present editor assisted, showed that the same measure

which gave a cubic yard of ballast, held precisely the same ballast with the addition of one-sixth in bulk of ground stone lime made with it into concrete, besides about fourteen Jails of water; and likewise tended to disprove the assertion that concrete swells in etting. This cubic yard of concrete weighed 27 cwt. In estimating, allowance must be made for the loss of material.

1862a. Expansion taking place in concrete made of unground lime, during its slaking, has been taken advantage of by G. L. Taylor in the underpinning of some walls at Chatham, as detailed in the Transactions of the Institute of British Architects, 1835. This expansion has been found to average about of an inch for each foot in height, and the size thus gained the concrete never loses. Care must be taken when using it for floors and for the spandrel of arches, to allow sufficient space, and to lay it in such a way that this increase may take place without thrusting out the walls, as has occasionally happened, In old malthouses in the West of England, with concrete floors 5 to 6 inches thick, stone walls 2 feet 6 inches to 3 feet thick have bulged out 3 or 4 inches on each side by the expansion of the concrete, as also noticed in the Transactions of the above named society, 1854, p. 74. When ground lime is used the assertion that concrete swells is very questionable, as stated in the previous paragraph. The Metropolitan Board of Works, under the Met. Man. and Building Acts Amend. Act, 1878, sec. 16, requires the cement "to be Portland cement, or other cement of equal quality, mixed with clean sharp sand or grit in the proportions of one of cement to four of sand or grit." Concrete for walis to be "of Portland cement and of clean Thames or pit ballast, or gravel, or broken brick or stone, or furnace clinkers, with clean sand in the following proportions: viz., 1 of Portland cement, 2 of clean sand, and 3 of the coarse material, which is to be broken up sufficiently small to pass through a 2-inch ring. The proportions of the materials to be strictly observed, and to be ascertained by careful admeasurement; and the mixing, either by machine or hand, to be most carefully done with clean water, and if mixed by hand, the material to be turned over dry before the water is added."

18626. For water works required to set rapidly, an excellent concrete may be made by a mixture, the proportions of which were found by Treussart as follows:-30 parts of hydraulic lime, very energetic, measured in bulk, and before being slaked; 30 parts of terras of Andernach; 30 parts of sand; 20 parts of gravel; and 40 parts of broken stone, a hard limestone. These proportions diminish one-fifth in volume after manipulation; the mortar is made first. When the Italian puzzuolana is used, the proportions should be 33 parts of lime as before; 45 parts of puzzuolana; 22 parts of sand; and 60 parts of broken stone and gravel. The first of these concretes should be employed immediately it is made; the second requires to be exposed about twelve hours before it it is put in place. When burnt clay or pounded bricks are used, 30 parts will suffice, but this mortar must not be used in sea water. If only rich, instead of hydraulic, limes be used, the quantity of the natural or artificial puzzuolanas must be increased, and that of the stones and gravel be decreased. (Burnell, Limes, &c.) See par. 1864c.

1862c. After many experiments, M. Kuhlmann recommends a cement composed of 30 parts of rich lime, 50 of sand, 15 of uncalcined clay, and 5 of powdered silicate of potash, as having all the requisite hydraulic properties, especially for cisterns intended for spring water. In marine constructions care should be taken to add an excess of silicate to those portions of cement which are exposed to the immediate contact of the sea.

1862d. The object to be aimed at in making hydraulic concrete, is to give such a sufficiency of mortar as will produce the aggregation of the whole mass of rough rubble materials. In Portland cement concrete, for instance, the proportions for the mortar may be 1 of cement to 3 of sand, and this mortar may then be mixed with 6 parts of ballast or shingle. In blue lias lime concrete, the proportions may be 1 of unground lime to 2 or 2 of sand, and this mortar may be mixed with 3 or 4 parts of ballast; and it must be understood in all cases that the mortar must be made first, and that it then should be thoroughly incorporated with the ballast or shingle. This concrete as used at the recent extension of the London Docks by Mr. Rendel, consisted of 1 part of blue lias lime with 6 parts of gravel and sand. The proportions for the blocks of the mole at Marseilles were 3 parts of Theil lime to 5 parts of sand mixed up into mortar, and then added to 2 parts of broken stone. At the Metropolitan Main Drainage works, the proportion of 1 of Portland cement to 5 of ballast for sewers, and 1 of cement to 8 of ballast and sand for backing walls and other works except sewers. The usual proportions are 1 to 6. A report was delivered to the Aberdeen Harbour Board on the damage caused by the chemical action of the sea-water on the (Portland ?) concrete entrance works of the graving dock. The surface had softened from the foundation up to the bottom of the ashlar lining, three feet above low water. The concrete behind four courses of the ashlar, between high and low water, was also softened, loosening the bond. The softened concrete under the water had been removed, and the face of the wall rebuilt up to lowwater level with Roman cement concrete in bags plastered with Roman cement. The pressure on the foundations amounts at low water to 5 lbs. on the square inch of surface, and at high water to 11 lbs.; this caused a current of sea-water through the porous

structure of concrete of theoretical velocity from 1500 to 2250 feet per minute, which continually washed the decomposed cement into the dock, and brought new particles of concrete and sea-water into contact. (British Architect, July 29, 1887; and Architect, March 9, 1888, p. 16 of Supplement.)

1862e. Béton, or couerete, as made in France, is invariably composed as follows:I. The mixture of lime and sand, either by hand or by a prg-mill, as for ordinary mortar. Great importance is attached to the choice of the lime and to the mode of slaking it; and if a sufficiently good one cannot be obtained, artificial puzzuolanas are introduced. The mode of slaking is prescribed in the specification according to the nature of the lime, instead of being left to the choice of the workmen. II The mortar so prepared is then well mixed by rakes with broken stones or ballast in such proportions as shall insure its filling up the intervals between them; the volume having been ascertained by immersing the stones in a known quantity of water. These spaces are equal to about 0.38 to 0:46 of the cubical contents of the vessel; but in practice, about one fourth more mortar is added than necessary to ensure solidification of the mass, especially when the beton is intended to resist water pressure. III. The material is then wheeled to its situation, and rammed down carefully until the mortar begins to work up to the surface.

1862f. In an English patent, 1859, No. 2757, M. Coignet, of Paris, argues that the tenacity of mortar is not produced, as hitherto supposed, by the formation of silicate of lime and alumina, but by the crystallisation of lime. His concrete, called Béton Aggloméré, consists of about 180 parts of sand, 44 of lime produced by slaking, 33 of Portland cement, and 20 of water, combined by a process of two main operations: I. A complete consolidation of the materials with little water; and II., the steady but not violent compression of the consolidation in moulds. The cement is mixed with the sand and lime, and sprinkled whilst mixing with a little water. This mixture is thrown into a machine, formed like an endless screw enclosed in a cylinder, at the rate of two shovelfuls followed by about a quart of water, until the cylinder is full. The screw, turned by two men, delivers the mixture through a series of holes in the bottom of the cylinder; but on a large scale, a machine is used of 10 to 15 horse-power. This mixture, after its delivery from the machine, is put by degrees into moulds, and each layer is rammed in by workmen. He found by experience that the purer the lime the quicker was the crystallisation; and that, although pure hydrate of lime will take carbonic acid, silicate of lime and alumina will not take it, because silicic acid took the place which carbonic acid did with the pure lime; and frankly admitted that his first experiments in 1855, in marine works, had not entirely succeeded, but claimed perfect success for those at Marseilles since 1859, and for those executing (1864) in Paris and elsewhere.

1862g. The resistance of béton and concrete should never be regarded as being superior to those given for limes, if the superstructure be commenced upon them immediately. In both cases the resistances are found to increase with comparative rapidity during the first six or seven months.

CEMENT.

1863. Among those cements used in England, Parker's, also called Roman and Sheppey cement, was discovered in 1796 by Mr. James Parker, of North fleet. It was manufactured principally from stone found in the Isle of Sheppey, and at Harwich, being septaria from the London clay, and properly classed among the limestones indigenous to this country. It consists of ovate or flattish masses of argillaceous limestone, arranged in nearly horizontal layers, chiefly imbedded in the clay of the cliffs. It was found at first on the beach, but as it became scarcer it was sought for by dredging out at sea. The substance, being coated with a calcareous spar or sulphate of barytes, forms the basis of the cement. About 1810-15 it was found possible to use this material in the depth of winter, but with inferior manufacture this is impossible. In 1840 it was stated that "the genuine Sheppey cement is now almost only a name," arising from the nodules first found having nearly disappeared in consequence of the great consumption of the cement. If this cement be of extremely good quality, 2 parts of sand to 1 of the cement may be used. The cement itself is a fine impalpable powder; yet when wetted it becomes coarse, and, unless mixed with great care, it will not take a good surface. When mixed with the sand and water, it sets very rapidly; it is necessary, therefore, to avoid mixing much at a time, or a portion will be lost. The colour of this cement, when finished, is an unpleasant dark brown, bence it has received the name of black cement." The surface requires frequent colouring for appearance. It is impervious to water almost the moment it is used; hence it becomes highly serviceable on the backs of arches under streets, for the lining of cisterns, and for carrying up in it, or coating with it, damp walls on basement stories. It will not resist fir so well; and it should therefore never be employed for setting grates, ovens, coppers, or furnaces. This, with many other hydraulic cements, has been eclipsed by Portland cement.

1834. Atkinson's cement is a good material, preferable in colour to the last named, but, as we think, inferior in quality. It takes a much longer time to set than Parker's cement,

than which it absorbs more moisture. It answers well enough in dry situations. Vicat formed a factitious Roman cement; but its efficacy was doubtful, though it had, for want of a better substitute, been much employed at Paris.

1864a. Portland cement, the latest (about 1843) of all these cements, is made from limestone and clay. The mud of the river Medway, corresponding to the argillocalcareous stone of Roman cement, is mixed with chalk and ashes from former makings, and calcined at a heat amounting almost to that of vitrification. A larger quantity of Band may be mixed with it than with Roman cement, to which it is superior in colour and hardness of setting. The heaviest, considered the best in quality, weighs 110 lbs. to 112 lbs. per striked bushel.

18646. The distinguishing peculiarities which should render Portland cement a permament substitute for Roman cement have been explained by a London manufacturer of both materials (Builder, 1863, p. 761). It may be condensed into the statement:-That the stone from which Roman cement is made, though composed of lime and the silicate of alumina, yet the proportion of the latter preponderates to such an extent as to prevent a perfect amalgamation of the ingredients in burning. The result is a cement loose in its texture, because containing inert foreign matter, which is retentive of moisture, and consequently attackable by frost and vegetable growth. In Portland cement the case is otherwise. The dose of lime to clay is in the ascertained correct proportion of two to one, and with this condition there is the power thoroughly to combine the ingredients by burning, and thus to give a density and compactness to the product which, in enabling it to resist water, frost, and other decomposing agencies, are the elements of its durability and of its superiority to the natural cements. Carelessness, or want of proper knowledge in its manufacture; an improper mixture of the ingredients; an imperfect calcination; its bad manipulation; and unfair handling when used as a cement, are all likely to result in disastrous effects on being used. When employed as a mortar or as a concrete, it has seldom been known to fail. 1864c. It is usual for the manufacturer to grind the cement after burning it. It is then placed in well-closed casks, which should not exceed 6 cwt. each, when the cement may be preserved for some time; but by contact with the atmosphere it is said to absorb humidity and carbonic acid, and thus becomes deteriorated. It should be ground very fine. For the sieve in sifting it, the French engineers required 185 meshes to the square of 4 inches on a side. One-third of the volume of the cement for the quantity of water is the best proportion, and the more that the cement is beaten up, the harder it becomes. The best cement will harden in about five or six minutes, and under water in about an hour; when mixed with sand it takes a little longer. When mixed with sea-water, and used in sea-water with a large quantity of sand, it may take even twenty-four hours before setting. (See pars. 1862b, c, and d.)

1864d. The resistance to rupture of pure cement after 20 days' exposure to the air is about 54 lbs. per inch square; if sand be added in the proportion of to 1 of cement, it falls to 37 lbs.; and if it be in equal proportions, it falls to 27 lbs. The permanent load in any large works should never be more than one-sixth of that required to produce rupture: and if small materials be employed, only one-fifteenth should be calculated upon.

1864e. In testing Portland cement, the Admiralty, at the Chatham Dockyard extension works, specified that samples would be taken from about one sack in ten, and gauged in moulds, which, when set, would be placed in water and tested at the end of seven clear days. Each must bear without breaking a weight of 650 lbs. upon the test-block of 11⁄2 inches square in section. In 1878 the Metropolitan Board of Works required the cement to be of the best quality, ground so fine that it will pass through a sieve of fifty meshes to the lineal inch. It must have a specific gravity of not less than 31, and weigh as delivered 114 lbs. or more to the imperial striked bushel. When brought upon the works it is to be put into dry sheds or buildings, which the contractor is to provide for the purpose, having wooden floors and all necessary subdivisions. The cement is to be emptied out upon this floor, every fifty bushels being kept separate, and is not to be used until it has been tested by samples taken out of every tenth sack. The samples to be gauged neat in moulds, put into water 24 hours after the briquettes have been made, and remain till tested, to bear without breaking a weight of 400 lbs. per square inch 7 days, and 600 lbs. 28 days after they have been made. The first to be considered as preliminary, and the second as decisive. Mr. John Grant's, C.E., specification is of a more extended character, and includes the quality of sand. The briquettes with three of sand to bear a weight of 150 lbs. per square inch after 28 days.

1864f. With cement at 112 lbs. per bushel, a cubic foot weighs 87.13 lbs., a cubic yard 2,352 6 lbs., and a ton occupies a space of 25.7 cubic feet.

1864g. With this cement, the ordinary proportions for walls may be 1 to 12 of gravel for common, and 1 to 6 of slag and saud for facing, concrete. A cubic yard of concrete takes about 1 yard, or 31 cubic feet, of loose gravel, exclusive of the cement, as made in a gauge or measuring-box. One-twelfth of 31 cubic feet, or a little more than 2 feet cube, goes to each gauge, and is easily calculated and prepared; or 218 lbs. by weight, if the tement weighs 112 los. per bushel. For making good solid concrete, there should be