where it is to be laid or moulded. The water for the mixing is either thrown in as needed, by hand into the top of the mill, or else supplied by a circular trough perforated with holes, which is placed around the inside of the mill at top. When cement is one of the ingredients, it is first made into a suitable paste with water, and then added to the others, from a vessel over the top of the mill, from which it is poured in a uniform manner, and in the requisite amount.

164. For all ordinary work, one passage through the pugmill is sufficient, but where greater thoroughness in the mixture is a requisite, the concrete may be passed through the mill a second time.

165. The concrete when laid or moulded is put in in successive layers, from one to three inches in thickness, and packed moderately by hand with pestles weighing from fifteen to thirty pounds.

166. To increase the rapidity of the setting, when necessary, the materials may be heated, in process of mixing, by a spiral tube or worm, through which heated air, steam, or hot water is caused to circulate.

167. Among other artificial conglomerates, that known as Ransome's artificial stone, from the name of the inventor, is now coming into use in England. This material consists of clean river sand the grains of which are cemented with the silicate of lime. To effect this union a silicate of soda is formed, by digesting common flints in a solution of caustic soda, in iron air-tight cylindrical vessels, by means of steam, under a pressure of seventy pounds, which circulates through a coil of iron pipes. The sand, after being thoroughly dried, is mixed with a sufficient volume of finely ground carbonate of lime to fill the voids between the grains. To each bushel of this mixture a gallon of the silicate is added, and the whole thoroughly mixed in a loam mill. The mixture is then moulded, and immediately after the solution of the chloride of calcium is thrown over it with ladles; the moulded blocks are then immersed in the solution, in open tanks, which is kept boiling, by steam passed through it in pipes, for several hours, according to the size of the blocks. This process expels any air that may have been retained in the blocks and facilitates the forming of the silicate of calcium. The block is then taken out and the chloride of sodium, that has been formed, thoroughly washed out with fresh water poured over the block.

This artificial stone is found to be very hard, and some specimens to have offered as great a resistance to rupture, by

compression and extension, as the best sandstones and marbles.

168. General Gillmore in his Report, Professional Papers, Corps of Engineers, No. 19, gives the following account of béton-Coignet or aggloméré.

Beton Agglomere. This name is given to a béton of very superior quality, or, more properly speaking, an artificial stone of great strength and hardness, which has resulted from the experiments and researches, extending through many years, of M. François Coignet, of Paris.

The essential conditions which must be carefully observed in making this béton are as follows:

First. Only materials of the first excellence of their kind, whether common or hydraulic lime, or hydraulic cement, can be used for the matrix.

Second. The quantity of water must not exceed what is barely sufficient to convert the matrix into a stiff, viscous paste.

Third. The matrix must be incorporated with the solid ingredients by a thorough and prolonged mixing or trituration, producing an artificial stone paste, decidedly incoherent in character until compacted by pressure, in which every grain of sand and gravel is completely coated with a thin film of the paste. There must be no excess of paste when the matrix is common lime alone. With hydraulic lime this precaution is less important, and with good cement it is unnecessary.

Fourth. The béton or artificial stone is formed by thoroughly ramming the stone paste, in thin, successive layers, with iron-shod rammers.

169. The materials employed in making his béton are sand, common lime, hydraulic lime, and Portland cement.

The sand should be as clean as that ordinarily required for mortar, for stone or brick masonry of good quality. Sand containing 5 or 6 per cent. of clay may be used without washing, for common work, by proportionally increasing the amount of matrix. Either fine or coarse sand will answer, or, preferably, a mixture of both, containing gravel as large as a small pea, and even a small proportion of pebbles as large as a hazel nut. There is an advantage in mixing several sizes together, in such proportion as shall reduce the volume of voids to a minimum. Coarse sand makes a harder and stronger béton than fine sand. The extremes to be avoided are a too minute subdivision and weakening of the matrix, by the use of fine sand only, on the one hand, and an undue enlargement of the volume of voids, by the exclusive use of coarse sand, on the other.

The silicions sands are considered the best, though all kinds are employed. When special results are desired in the way of strength, texture, or color, the sand should be selected accordingly.

170. The common lime should be air-slaked, or, better still, it may be slaked by aspersion with the minimum. quantity of water that will reduce it to an impalpable powIt should be passed through a fine wire screen to exclude all lumps, and used within a day or two after slaking, or else kept in boxes or barrels protected from the atmosphere.

It is scarcely practicable, under ordinary circumstances, to employ fat lime alone as the matrix of béton aggloméré, particularly in monolithic constructions, in consequence of its tardy induration. Even when used in combination with hydraulic lime or cement it acts as a diluent.

171. Attempts to make béton of even average quality, without good hydraulic ingredients, have failed in the United States; and it is extremely doubtful whether any characteristic excellence can be attained, after the lapse of weeks or even months, by a mixture of this character.

172. The most suitable hydraulic limes are those derived from the argillaceons limestones, in contradistinction to the magnesian or argillo-magnesian varieties. These limestones contain before burning from 15 to 25 per cent.-generally less than 20 per cent.-of clay. After burning, the lime is slaked to powder by aspersion with water, and sifted to exclude unslaked lumps.

Hydraulic lime cannot be considered an essential ingredient of béton aggloméré, except in comparison with common lime. It may be altogether replaced by good hydraulic cement, or it may be used alone, or mixed with common lime, to the entire exclusion of cement. A stiff paste of this lime should set in the air in from ten to fifteen hours, and sustain a wire point one-twenty-fourth of an inch in diameter, loaded with one pound, in eighteen to twenty-four hours. Its energy, and therefore its value, varies directly with the amount of clay which it contains, which generally will not exceed 20 per cent. before burning, although it may reach 25 per cent. Beyond this point the burnt stone can seldom be reduced by slaking and becomes a cement.

No hydraulic lime of this variety has ever been manufactured in the United States. It is not known that stone suitable for it exists here.

173. The heavy slow-setting Portland cements, natural or

artificial, are the only ones suitable for béton aggloméré. They are manufactured extensively throughout Europe.

This cement is produced by burning, with a heat of great intensity and duration, argillaceous limestones, containing from 20 to 22 per cent. of clay, or an artificial mixture of carbonate of lime and clay in similar proportions, and then reducing the product to fine powder between millstones. In this condition its weight should not fall short of 101 pounds and will seldom exceed 128 pounds to the bushel, poured in loosely and struck, without being shaken down or compacted. Between these limits additional weight may always be conferred in the burning, by augmenting the intensity and duration of the heat; and both the tensile strength, and the time required to set, increase directly with the weight. For example, a Portland cement weighing 100 pounds to the United States bushel, that will set in half an hour, and sustain when seven days old a tensile strain of 200 pounds on a sectional area of one square inch, would have its time for setting increased to four or five hours, and its tensile strength to about 400 pounds, if burnt to weigh 124 pounds to the bushel. An increase in weight of 24 pounds to the bushel nearly doubles the ultimate tensile strength of Portland

cement.

When the matrix of béton aggloméré is Portland cement alone, it is customary to prolong the process of trituration, in order to retard the set; or, if more convenient, the mixture may be passed through the mill twice or even three times, with an interval of an hour or more between each mixing. This course is specially desirable when the cement weighs less than 100 hundred pounds to the bushel, and is correspondingly quick-setting.

174. English engineers generally require that the cement shall be ground so fine that at least 90 per cent. of it shall pass a No. 30 wire sieve, of 36 wires to the lineal inch, and shall weigh not less than 106 pounds to the struck bushel, when loosely poured into the measure. When made into a stiff paste without sand, it should be capable of sustaining without rupture a tensile strain of 400 pounds on a sectional area 1 inch square, or 21 square inches (equal to 178 pounds to the sectional square inch), seven days after being moulded the sample being immersed six of these days in fresh water.

175. Experience has repeatedly demonstrated, and they have become well recognized facts, that in order to obtain uniformly good béton or artificial stone, with sand, and

either hydraulic lime or Portland cement, or both, it is necessary

First. To regulate, in a systematic manner, the amount of water employed in the manufacture thereof.

Second. To obtain, with a minimum quantity of water, the cementing material or matrix in a state of plastic or viscous paste.

Third. To cause each grain of sand or gravel to be entirely lubricated with a thin film or coating of this paste; and Fourth. To bring each and every grain into close and intimate contact with those which surround it.

It is also equally true, that the best results possible to be produced from any given materials will be attained when the above-named conditions are enforced.

176. It is impossible to produce a cementing material, of suitable quality for béton aggloméré, by the ordinary methods and machinery used for making mortars; for if we take the powder of hydraulic lime or Portland cement, and add the quantity of water necessary to convert it into a paste by the usual treatment, it will usually contain so much moisture, even after being incorporated with the sand, that it cannot be compacted by ramming, but will yield under the repeated blows of the rammer like jelly. If the quantity of water be reduced to that point which would render the mixture, with the usual treatment, susceptible of being thoroughly compacted by rammers, much of the cementing substance will remain more or less inert, and will perform but indifferently well the functions of a matrix.

177. To prepare the matrix, there is taken of the hydrau lic lime or cement powder, say one hundred parts, by measure, and of water from thirty to thirty-five or forty parts, which should be the smallest amount that will accomplish the object in view. These are introduced together into a suitable mill, acting upon the materials by both compression and friction, and are subjected to a thorough and prolonged trituration, until the result is a plastic, viscous, and sticky paste, of a peculiar character, in both its physical appearance and the manner in which it comports itself under the subsequent treatment with rammers. There would appear to be no mystery in this part of the process, yet the excellence of the béton aggloméré is greatly dependent on its proper

execution.

If too much water be used, the mixture cannot be suitably rammed; if too little, it will be deficient in strength.

178. The sand should be deprived of surplus moisture,