a kind of artificial stone made of baked clay.

1. The art of making bricks is so simple that it must have been practised in the earliest ages of the world; probably before mankind had discovered the method of fashioning stones to suit the purposes of building. The book of Genesis informs us, that burnt bricks were employed in the construction of Babel. Now, as this structure appears to have been raised about four hundred years after the period of the Flood, we may say, without much exaggeration, that the method of making bricks existed from the very origin of society. Bricks seem to have been in common use in Egypt while the Israelites were in subjection to that nation; for the task assigned them was the making of brick; and we are informed in Exodus that the Israelites built two Egyptian cities. No particulars are given in Scripture of the method of making bricks; but as straw was one of the ingredients, and as it very seldom rains in Egypt, it is probable that their bricks were not burnt, but merely baked by the heat of the sun. The same mode of making bricks seems still to be practised in the East; for the ruins of the tower near Bagdad are formed of unburnt bricks. We have seen specimens from that place; they are large, but thin, and have a brown colour. It is not at all likely that structures of unburnt brick should have been able to resist the weather since the time of Nebuchadnezzar; and hence it is probable that the tower in question was raised by the Arabs in comparatively modern times.

The art of brick-making was carried to considerable perfection by the Greeks. Pliny informs us that they made use of bricks of three different sizes, distinguished by the following names; *didoron*, or six inches long; *tetradoron*, or twelve inches long; and *pentadoron*, or fifteen inches long (lib. xxxv. c. 14). That the Romans excelled in the art of making bricks we have the amplest evidence, since brick structures raised at Rome 1700 years ago still remain as entire as when first built. Brick-making has been carried to great perfection by the Dutch, who have long been in the habit of forming their floors, and even in some cases of paving their streets with bricks. And it is remarkable how long their bricks will continue uninjured in such situations. Though brick-making has long been carried on in England, and especially in the neighbourhood of London, upon a very great scale, and though the process upon the whole is conducted in this country with very considerable skill, yet it must be acknowledged that English bricks are by no means so durable as Dutch bricks. We are disposed to ascribe this inferiority not so much to the nature of the materials employed in the manufacture of English bricks, as to the mode most frequently employed in London in building houses. Few of the London houses, comparatively speaking, are freeholds. Most of them are built upon ground let for a lease of a certain number of years, which seldom exceeds ninety-nine years. After the expiration of this period, the house becomes the property of the landlord who let the ground. Thus it becomes the interest of the builder to construct the house so that it shall last only as long as the lease. Hence the goodness of the bricks becomes only a secondary object. Their cheapness is the principal point. The object, therefore, of the brickmakers is not to furnish durable bricks, but to make them at as cheap a rate as possible. Accordingly, the saving of manual labour, and of fuel, has been carried by the makers of London bricks to very great lengths. We cannot but consider this mode of proceeding as very objectionable, and as entailing a much heavier expense upon London than would have been incurred had twice the original price been laid out upon the bricks when they were first used, and had the houses been constructed to last a thousand instead of a hundred years. No doubt, certain advantages attend these ephemeral structures. The inhabitants are enabled, once every century, to suit their houses to the prevailing taste of the day; and thus, there are no antiquated houses in London. But as the increase of the price of all the materials of building has more than kept pace with the increase of the wealth of individuals, it is to be questioned whether the houses are always improved when they are pulled down and rebuilt.

2. The best material for making brick is what in the English language is called loam, a term usually applied to a natural mixture of sand and clay. Such a mixture may be converted into brick without any addition whatever. Marl likewise answers the purpose of common bricks very well, indeed better than most other mixtures. Marl is a natural mixture of limestone and clay in variable proportions. Now, the more lime it contains, the better does it answer for a manure; and the less lime it contains, the more suitable it is to the brick-maker.

It would be in vain to attempt a particular detail of the constituents of clay, because they vary too much from each other to admit of any correct generalization. We believe, however, that clays very frequently consist of decomposed felspar, in which case we may conceive them as composed of about three parts of silica in the state of a very fine powder, and one part of alumina. This is the case with porcelain clay. Indeed, the porcelain clay of Cornwall appears incontrovertibly to be nothing else than decayed felspar, or perhaps felspar which never had assumed any other form than that of clay. The rock from which it is taken is an agglutinated mixture of quartz and this clay. The quartz is separated by washing. Such a rock might probably be converted into most beautiful brick, merely by cutting it out in the proper shape, and subjecting it to the requisite heat; or rather, by kneading the whole into a paste with the requisite quantity of water, moulding it into bricks, and then drying and burning them.

Potter's clay is a compound of:

| Component | Percentage | |-------------|------------| | Silica | 43.5 | | Alumina | 33.2 | | Lime | 8.5 | | Oxide of iron | 1.0 | | Water | 18.0 |

Loss: 99.2

Total: 100.0

When the clay proceeds from the decomposition of hornblende, as is likewise often the case, it contains about three parts and a half of silica, one of alumina, one of lime, and about one and a half of oxide of iron. Sometimes the grains of sand which exist in clay consist of fragments of felspar. In such cases the clay may be fused by heat.

No mixture of alumina and silica, in any proportions whatever, can be fused by the strongest heat which can be raised in our furnaces. Hence such mixtures are best adapted for making fire-bricks, crucibles, and glass-house pots. Stourbridge clay is such a mixture, blackened by coaly matter. It answers these purposes better than any other clay in England. It is a slate clay belonging to the coal formation, and contains interspersed coaly matter. There is a similar bed of clay upon the banks of the Calder, about ten miles east from Glasgow. Mr Buttray uses it to make the crucibles in which he fuses steel, a process requiring the most intense heat that can be raised in furnaces. Its quality seems fully equal to that of Stourbridge clay. Neither can a mixture of lime and alumina be fused, in whatever proportions the ingredients be mixed. But a mixture of silica, lime, and alumina, is very fusible, and the fusion is most readily effected when we employ two parts of silica to one of lime. The presence of oxide of iron also renders clay fusible, but not unless its proportion be much greater than ever is likely to occur in any clay used for the manufacture of bricks.

For making common bricks, the most durable mixture ought to be common clay and limestone or chalk. Perhaps the best proportions would be three parts of clay, and one part of limestone or chalk in powder. When such a mixture is exposed to heat, it would experience an incipient fusion, and would thereby be rendered much harder and denser than common bricks. The consequence would be, that it would imbibe much less water, and would therefore be much less liable to crack and fall to pieces in winter, than common bricks. For when water has insinuated itself into the pores of a common brick, and is converted into ice, it undergoes an expansion which dislocates the parts of the brick and reduces it to fragments. This is often conspicuously the case with tiles, which, from their exposed situation, are more liable to be soaked with water than common bricks. Hence also covering the surface of the brick with a coating of paint has a great tendency to preserve them from cracking and breaking. This practice is frequently followed in England.

It would be foreign to the object of this article to enter into any long details respecting the chemical investigations and the opinions entertained at different periods respecting the nature of clay. At first it was supposed to be a peculiar species of earth, but Hellot demonstrated that it consisted at least of two constituents; for sulphuric acid had the property of destroying its plastic nature, and of rendering it scarcely more adhesive than sand. The portion that remained behind did not effervesce with acids. It was not therefore of a calcareous nature. Mr Pott went a step farther; he showed, in the continuation of his Lithogeognosia, that sulphuric acid formed, with the por- tion of clay which it dissolved, a salt possessing the properties of alum. In the year 1769 Baume published his Dissertation on Clays, which he had drawn up in consequence of a premium offered by the Academy of Sciences at Bordeaux, for the best solution of the following question—What are the principles and constituents of clay, and the natural changes which it experiences, and what are the methods of rendering it fertile? The academy did not consider Baume's solutions as satisfactory. He published his Memoir, in consequence, as a kind of defiance. He had been employed along with Macquer in making numerous experiments on clay, with a view to the improvement of the porcelain manufacture in France. Guided by these experiments, he drew as a conclusion that clay is a mixture of two different substances: 1. Silica in a state of purity; 2. Silica combined with an underdose of sulphuric acid. It was the second of these constituents that gave to clay its fattish and plastic nature.

Margraaf had long before (in 1756) demonstrated that the ingredient of clay which Baume took for a salt, and which he affirmed was soluble in water, was a peculiar species of earth, different from every other, which constitutes the basis of alum, which dissolves in sulphuric acid, but which does not form alum unless a portion of potash be added to the solution. Thus, by the labours of Hellot, Pott, Baume, and Margraaf, the nature of clay was completely developed. It was ascertained to be a mixture of alumina and silica, in variable proportions. It was shown, also, that it sometimes contained sulphuric acid, and not unfrequently potash. Hence the reason why, in some cases, it could be converted into alum by digestion in sulphuric acid, without the necessity of adding any potash to the solution. Modern chemists have added considerably to these facts. They have shown that chalk, felspar, mica, hornblende, oxide of iron, coal, bitumen, &c., are not unfrequently mixed with it; and that these additions alter its qualities considerably, and render it fit or unfit for the different purposes to which clay is usually applied.

3. Clay intended to be made into bricks ought to be dug out of the earth and exposed to the air and weather for a considerable time before it is employed. The longer this exposure is continued, so much the better will it be fitted for making bricks. This exposure answers a variety of purposes. If the stones, by the decomposition of which the clay has been formed, are not entirely decomposed, this exposure serves to complete the process, by promoting the disintegrating action of the air and rain. The exposure serves likewise to pulverize the clay, which is essential to the making of good bricks. We have little doubt that the same amelioration in the clay would be produced by simply drying it in the open air, and then grinding it to powder in a mill. By such a process the quality of the bricks would be prodigiously improved. Nor do we conceive that such an addition would greatly enhance the expenses of the brickmaker, at least in those districts where the mill could be driven by water.

When the clay has been reduced to powder, the next step is to make it into a stiff paste with water. Too much water should not be employed, because it is injurious to the strength of the bricks; and the utmost care should be taken to mix the whole of the clay as equally as possible with the water. If some parts of the paste be moister than others, it will occasion an inequality in the texture of the bricks formed of it, will render them apt to crack, and will greatly injure both their strength and their beauty. Hence the great importance of working the clay for a considerable length of time before moulding it into bricks. It is in this part of the process that we believe British brickmakers in general are most defective. As far as we have had an opportunity of witnessing the process of kneading the clay, as conducted either in the Brick-neighbourhood of London or Edinburgh, we have always found a great sparing of labour. Hence we believe the reason why so many of the English bricks appear full of cracks, even when sold to the builder. Such bricks ought never to be purchased, as it is perfectly obvious that they cannot make a durable building.

The kneading of the clay is performed in some places by men's feet; in others by the feet of horses, and in others by machinery. The last method is undoubtedly the best; and we conceive likewise that it might be rendered the cheapest. It would be easy to devise machinery for kneading the clay, upon principles similar to those employed in mashing by the London porter brewers. And, if such a machine were driven by water, we conceive that it would not be nearly so expensive as either men or horses.

When the clay is sufficiently kneaded, it is moulded into the form of a brick, by being put into a very simple wooden mould; and the upper part of the brick is made smooth and even by cutting off the superfluous part with a wooden knife. The process is very simple, and is conducted by the workmen with great rapidity. A good brickmaker would mould about 5000 bricks in a day. He disengages the bricks from the mould by a gentle stroke on the back of the mould; and the wet bricks are at first arranged in rows upon long boards. When sufficiently dry to be handled, they are turned, and at last piled up in loose walls, which are thatched with straw to keep off the rain. In this position they are allowed to remain till they have become as dry as they can become in the open air.

In many cases the clay used for brick-making is destitute of the requisite quantity of sand. If such clay were made into bricks, it would shrink so much in their burning, that the bricks would lose their shape, and would probably crack in every direction. To prevent this, it is necessary to add a certain quantity of sand. This sand should not be very fine. It answers best when the particles are of such a size as to be readily distinguished by the naked eye. Even when as large as coriander seeds, it has been found to answer better than very fine sand. The brickmakers in the neighbourhood of London bring their sand from the bottom of the Thames near Woolwich, where it is raised by boats employed for that purpose, and brought up the river for the use of the brick-makers.

4. No general directions can be given respecting the quantity of sand to be mixed with the clay, because that depends upon the nature of the clay and upon the uses for which the bricks are intended. The more sand is added, the more accurately do the bricks retain their shape, and the less apt are they to crack during the burning; but at the same time their strength is diminished. Chemical lutes are often composed of four parts of sand and one part of clay. Such mixtures do not contract much in burning, and, therefore, are not apt to crack and drop off, which is the reason why chemists employ them. But they have not the adhesiveness of brick after being burned, and would not therefore answer the purposes of the brick-maker. In stone-ware the mixture consists of about four parts of clay and one of fine sand. It burns to a hard cohesive substance, capable of striking fire with steel. Such a proportion, then, in many cases would answer the purposes of the brick-maker.

The London brick-makers make another addition to the clay, which we believe is peculiar to them. They add to every three parts of the clay about one part of the ashes from the fire-places of the city of London. These ashes contain some earthy matter; but they consist in a great measure of small coal unburnt and little altered, which has fallen through the interstices of the grate. The conse-