that, to a certain point, the shrinking power of the clay increased with its progressive meagreness (the water being constant), and the porosity decreased. This point is termed the "point of greatest density" of the mass.

From the point of greatest density, further impoverishing diminishes the shrinking for an equal amount of water in the pores, but increases the porosity.

Sometimes refuse materials may be converted into a passable quality of fire-bricks; the material alone has no plasticity, but possesses quick drying qualities.

The refuse from the China-Clay Works of Devonshire, in England, is satisfactorily used for this purpose. After the kaolin has been washed out, the quartz and mica are mixed with a small portion of fat clay, thoroughly tempered in the pug-mill, and moulded into bricks. They are found to resist the effects of heat very well, and are commonly employed in the construction of some classes of metallurgical works.

In England, the lining for Bessemer convertors is often produced by mixing pulverized Sheffield sandstone with two and one half per cent. of alumina and oxide of iron.

The introduction of the Bessemer process for the manufacture of cast-steel, and of the Siemen's gas-furnace into countries which are but poorly supplied with fire-resisting materials, has developed the difficulty of securing bricks of a sufficiently refractory character to withstand the extremely high degree of heat exhibited in the melting chamber, as well as the sudden, and often violent, alterations in various other portions of the furnaces.

In many portions of this country and in England, there

is not a great deal of trouble in meeting these demands; but in supplying less favored countries and colonies, Mr. Joseph Khern, the well-known Austrian metallurgist, gives a method for obviating this difficulty.

The plan which he has introduced for the manufacture of these silicious bricks is an excellent one, and the material produced he describes as being much superior to any other refractory product obtainable in Austria.

The chief ingredient employed is quartz, which must be of the highest degree of purity, especial care and watchfulness being exercised to reject all such portions as show any admixture of iron or copper pyrites, carbonate of lime, or even mica or feldspar. The preparation is similar to that observed in the manufacture of Dinas, and the silicious firebricks made at Stolberg, near Aix.

The quartz, having been selected in the manner described, is heated in quantities of from twelve to fifteen tons in a Rumford oven, or a continuous kiln as for lime may be used. At the end of twelve hours, having reached a full red heat, it is thrown into water; the best fragments are then selected and afterwards cleaned by a simple jigging process, and then subsequently crushed under a tilt-hammer until sufficiently fine to pass through a sieve having sixty holes to the square .inch, which leaves the particles coarse and sharp.

Two varieties of fat clay are used to bind the coarselypulverized quartz; the clays differ slightly in plasticity, and are very carefully prepared by thorough weathering, pulverization under light stamp heads, and fine grinding under

edge rollers; a final sifting is performed through a very fine sieve of six hundred apertures to the square inch.

The tilt-hammer used for pulverizing the quartz weighs two hundred and fifty pounds, and is capable of crushing three and one-half tons of the burned quartz in twelve hours.

In selecting the quartz the purest quality is reserved for the first quality of bricks, which have to resist the greatest temperature and sudden changes; while the second and third classes of bricks are made for less exposed positions, and are composed chiefly of the remains of bricks which have been once used, but again pulverized and sifted afresh.

The following are the compounds employed for the different classes of bricks: First class, 16 parts of quartz to 1 of plastic clay, or 14 parts of quartz to 1 of leaner clay: Second class, 16 parts of ground bricks of the first class to 1 of clay: Third class, 8 parts of ground bricks of the second and third classes to 1 of clay.

The third class bricks are made more with an idea for their employment in portions of the furnace requiring greater mechanical strength than fire-resisting qualities. The materials are first mixed together in a dry condition on a large, clean, and tight platform of wood, and are then thrown into a tight wooden pugging-box, six feet square and nine inches deep. In this box the dry mixture should be about six inches deep, and be thoroughly incorporated by kneading with water and treading under men's feet, and occasionally turning over the mass with shovels, care being taken not to draw splinters from the wooden box into the

clay. A sufficient quantity of water must be added to allow the mixture to be worked into a ball between the fingers without crumbling.

The second and third class bricks are formed in open moulds, the pug being beaten down by a metal rammer of about four and one-half pounds weight; the first class, however, are subject to a pressure of about three and one-half tons to the square inch during a period of three-quarters of an hour before they are removed from the moulds.

The drying is done in chambers through which a current of air passes, at the ordinary temperature in summer, but artificially warmed in winter. The bricks are fit for burning in seven days. The kilns are rectangular chambers, each having two step grate fireplaces in one of the shorter sides, and a flue communicating with a high chimney at the opposite end. The capacity is small, being only about twenty-five hundred bricks. As soon as the kiln is filled the charging opening is partly closed, and a gentle fire is kept in the grates, the damper in the flue being closed.

At the end of thirty-six hours the charging hole is entirely closed, and the draft is stimulated by opening the damper in the flue inch by inch at intervals, until at the end of seventy-two hours the whole of the bricks have attained a strong white heat. The fires are then removed, the damper closed, the grates filled with sand, and cracks that may have been discovered in the kiln are carefully luted or smeared over with soft mud.

The charging opening should also receive careful attention and be faithfully daubed. After standing in this way

for thirty-six hours, the charging place is gradually opened, and in from sixty to seventy-five hours the burned bricks may be removed.

In the United States, superior qualities of fire-clays are found in various localities. The fire-bricks most esteemed are those made from the "Amboy clay" of New Jersey, and the Mt. Savage fire-bricks made at Mt. Savage, Md. The first are produced from a cretaceous clay, which is first burned in a kiln, its plasticity being lost in the process, and resulting in what is known as "cement;" the second or Mt. Savage bricks are produced from two qualities or varieties of carboniferous fire-clay, one of which has in its natural state the properties of the "cement" just mentioned. It is non-plastic, and is treated in the same manner as the "Amboy clay."

At Mineral Point, Tuscarawas County, Ohio, a clay nearly similar to the Mt. Savage clay is found; its appearance and properties are about the same; it is non-plastic, and is treated in the same manner.

For all these clays the "cement" is coarsely ground, mixed with from one-sixth to one-tenth plastic clay, gradually dried and tempered and then hard-burned.

The fire-bricks made from the clay of the coal-measures of Pennsylvania, Ohio, Illinois, and Missouri are also held in high esteem. In some portions of the State of Michigan a very fair quality of bricks is produced from non-plastic clays.

Cheaper bricks, and of somewhat inferior quality, but yet adapted to many purposes for which fire-bricks are used, are