The streets of "Old London" at the recent Inventions Exhibition at South Kensington were paved with a material in imitation of old, worn bowlder stones and red, herring-boned brickwork, all in one piece from one side of the street to the other. The composition is made by Wilkes' Metallic Flooring Company, out of a mixture consisting chiefly of iron slag and Portland cement, a compound possessing properties which won the only gold medal given for paving at that Exhibition. At the present time the colonnade in Pall Mall, near Her Majesty's Theater, is being laid with this paving, which is also being extensively used in London and the provinces for roads, tramways, and flooring; the composition is likewise sometimes cast into artistic forms for the ornamentation of buildings, or into slabs for roofing, facing, and other purposes. The subway from the Exhibition to the District Railway is laid with the same material.

The works of the Wilkes Metallic Flooring Company are in the goods yard of the Midland Railway Company at West Kensington. The Portland cement, before it is accepted at the works, is tested by means of an Aidie's machine. The general strain the set cement is required to bear is 750 lb. to the square inch. All samples which will not bear a strain of 500 lb. are rejected. The various iron slags are carefully selected, and rejected when too soft, and at the works a small percentage of black slag, rich in iron, is mixed in with them. The lumps of slag are first crushed in a Mason & Co.'s stone breaker, and then sifted through 1/8 in., 1/4 in., and 1/16 in. wire meshes into these three sizes for mixing. Next the granulated substance is thoroughly well washed with water to remove soluble matter and impalpable dust, and afterward placed where it is protected from the access of dust and dirt. The washing waters carry off some sulphides, as well as mechanical impurities. The Portland cement is not used just as it, comes from the works, but is exposed to the air in a drying room for about fourteen days, and turned over two or three times during that period.

The slag is also turned over three times dry and three times wet, and mixed with the Portland cement by means of water containing 5 per cent. of "Reekie" cement to make the whole mass set quickly. The mixture is then turned over twice and put into moulds; each mould is first half filled, and the mixture then hammered down with iron beaters. The rest of the composition is then poured in, beaten down, and the whole mould violently jolted by machinery to shake down the mixture and to get rid of air holes. While it is still wet the casting is taken out of the mould, its edges are cleaned, and after the lapse of one day it is placed in a bath, of silicate of soda. Should the casting be allowed to get dry before it is placed in this bath, no good results would be obtained; it is left in the bath for seven days. When delicate stone carvings have to be copied, the moulds are of a compound of gelatine, from the flexible nature of which material designs much undercut can be reproduced. For the foregoing particulars we are indebted to Mr. William Millar, the working manager at West Kensington. Sometimes the composition is cast in large, heavy slabs, moulded on the top to resemble the surface of roads of granite blocks. A feature of the invention is the rapidity with which the composition sets.

For instance, the manager states that a roadway was finished at the Inventions Exhibition at seven o'clock one night, and at six o'clock next morning four or five tons of paper in vans passed over it into the building, without doing any harm to the new road. In laying down roads, much of the preparation of the material is done on the spot, and the composition after being put down unsilicated in a large layer has the required design stamped upon its wet surface by means of wooden or gutta-percha moulds. As regards the durability of the composition, Mr. T. Grover, one of the directors, says that the company guarantees its paving work for ten years, and that the paving, the whole of the ornamental tracings, and some of the other work at Upton Church, Forest Gate, Essex, were executed by means of Wilkes' metallic cement three years ago, and will now bear examination as to its resistance to the action of weather. Some of this paving has been down in Oxford Street, London, for more than six years. Mr. A.R. Robinson, C.E., London agent of the company, states that the North Metropolitan Tramway Company has about 25,000 yards of it in use at the present time, and that the paving is largely used by the War Office for cavalry stables.

The latter is a good test, for paving for stables must be non-slippery and have good power of resisting chemical action.

In the Wm. Millar and Christian Fair Nichols patent for "Improvements in the means of accelerating the setting and hardening of cements," they take advantage of the hydraulicity of certain of the salts of magnesia, by which the cements set hard and quickly while wet. For accelerating the setting of cements they use carbonate of soda, alum, and carbonate of ammonia; for indurating or increasing the hardening properties of cements they use chloride of calcium, oxide of magnesia, and chloride of magnesia or bittern water; for obtaining an intense hardness they use oxychloride of magnesia. The inventors do not bind themselves to any fixed proportions, but give the following as the best within their knowledge. For colored concretes for casts or other purposes they use Carbonate of soda, 8.41; carbonate of ammonia, 1.12; chloride of magnesia, 0.28; borax, 0.56; water, 89.63; total, 100.00. For gray concrete for any purpose they use: Alum, 8.46; caustic soda, 0.28; whitening or chalk, 0.56; borax, 0.56; water, 90.14; total, 100.00. For floors or slabs in situ they add to cement, well mixed and incorporated with any required proportion of agglomerate for a base, liquid composition of the following proportions: Oxide of magnesia, 0.29; chloride of magnesia, 0.29; carbonate of soda or alum, 4.74; water, 94.68; total, 100.00. Articles manufactured by the invention are afterward wetted with chloride of calcium and placed in a bath containing a solution of silicate of soda or chloride of calcium.

The strength of the chloride of calcium is equal to about 20 deg. specific gravity.

C.A. Wilkes and William Millar's improved "metallic compound for flooring, paving, and other purposes," has for its object to provide a paving compound which is not slippery or liable to soften in hot weather, which sets rapidly, and is durable. To three parts of blast furnace slag are added one part of hydraulic cement and enough water to give the proper consistency. To each gallon of water used is added one part of bittern water - the dregs from the manufacture of sea salt - or one part of brine, or about 5 per cent. of carbonate of soda, and 2½ per cent. of carbonate of ammonia. In the compound they sometimes use potash in the proportion of about 5 per cent. of the carbonate of ammonia and carbonate of soda, and when potash is used with bittern water or brine, the proportion of the latter is correspondingly reduced. The compound is of a blue gray color; but when a more striking color is desired, red or yellow oxide of iron may be added. When more speedy induration is necessary, they add about 1 oz. of copperas to every gallon of compound used.

The claim is the admixture of bittern water, carbonate of soda, and carbonate of ammonia with the washed slag and cement.

Another improvement, by C.A. Wilkes, relates, in laying in situ any metallic or other materials for street roadways, to completing the convenience thereof by roughening or grooving the surfaces. The concrete is laid in a plastic condition upon a bed of hard core, broken stone, or preferably rough concrete. For footpaths the material may be laid in convenient sections, say 4 ft. to 8 ft. square and 2 in. to 4 in. thick; and in order to allow for the expansion of the material during the setting of the sections or subsequent variations in temperature, he packs the joints between the sections with a layer of felting cloth or other compressible material, thus forming expansion joints. Sometimes he slightly roughens the surface of the material, to give better foothold to pedestrians. Sometimes the grooving is made in imitation of ordinary granite paving sets. In tramway pavement there are grooves to give a grip to the horses' feet, and a slight camber between the rails. He states that a great advantage in laying a pavement by the method is that, when any repairs are necessary, a piece of the exact size can be manufactured at the works, and stamped to the same pattern as the adjoining pavement, then placed at once in position on the removal of the worn portion, thus saving the time necessary for the setting of the concrete on the spot. - The Engineer.