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content themselves with laying succefsive layers of the paste, while yet soft, above one another in the wall, and there moulding it into the proper form, and allowing it to harden till it be sufficiently firm to carry another layer. It is then covered with a roof, that prevents water from soaking into it; and although some of the clay is washed off whenever the wind is so strong as to beat the rain violently against the wall, yet with proper care to keep the roof tight, some of these walls. will stand a very long time. This is perhaps the rudest and most imperfect kind of bricks.
But as clay never loses its quality of becoming soft with water, till it has been subjected to a ve◄ ry intense degree of heat, it becomes necessary to mix it with some substance that is capable of resisting that degree of heat, without being consumed, if we hope to have such bricks as fhall be capable of resisting the weather for any length of time uncovered.
The substance universally employed in modern times for mixing with clay in the manufacture of bricks intended to be burnt in a furnace is sand; and although this is, on many accounts, an improper substance to be employed for that purpose, it seems hardly to have ever entered into the head of any manufacturer of bricks, that its place could be supplied by any other substance whatever. This I imagine is the real cause of the badness of modern
*In the neighbourhood of London, coal ashes, usually called dust, have been for some time past successfully employed for that purpose; but these bricks also contain a large proportion of sand.
bricks, as will, I hope, appear to be the case from the following considerations.
Sand, by itself, requires a very intense heat to bring it to fusion; but when mixed with any other earthy substance, it not only becomes itself easily fusible, but endows the whole mafs with the same quality; so that no earthy mafs, in which sand abounds, can resist an intense fire without being vitrified. Hence it necessarily follows, that if bricks, in which it abounds, are subjected to an intense degree of heat, the whole mafs would be melted, and the bricks would lose their fhape. To prevent this inconvenience, therefore, it becomes necefsary to expose these bricks to a degree of heat in baking, that shall not be so intense as to vitrify the mafs. But this moderate degree of heat is not sufficient to deprive the clay entirely of its quality of absorbing, and of being softened by water; so that these bricks, when exposed to te eather, absorb some part of the water that falls upon them, which in time softens the clay, and makes it crumble to dust.
Thus it appears that it is in vain to hope for good and durable bricks of any kind, so long as we continue to employ clay, for that purpose, that is mixed with sand in almost any proportion. Hence I would afume the following postulatum.
Good bricks can only be formed of a clay that is naturally pure; or at least that is unmixed with sand of any sort.
It is as vain, however, to expect that good bricks can be made of pure unmixed clay, in its native
state, as that it could be made of the same clay mixed with sand; some addition is always ne cefsary to prevent it from cracking in drying or burning. We must therefore find a substance to be added to clay, which is not only itself refractory in the fire, but which also allows the clay to be equally refractory after being mixed with it, as it was by itself.
And such a substance, without attending to others that come at too high a price for ordinary use, we meet with in the clay itself. We have already seen that if pure clay is burnt in an intense fire, it loses all its former qualities, while it acquires those that are wanted on the present occasion. It becomes in that state equally insoluble in water as sand; equally capable of resisting the fire as any other substance 9 and is capable of being reduced to a very fine powder, which may be mixed with the paste of native clay in almost any proportion, without destroying its ductility. To form bricks therefore that fhall be equally hard, and equally capable of resisting every variation of the atmosphere with the finest native stone, all that seems to be necessary, is to make choice of a clay that is naturally free of sand or metallic substances in any considerable proportion; to bake some part of that clay without any mixture, till it is reduced to a stony consistence; to pound that baken clay till it fhall be reduced to a fine powder, and afterwards mix that powder with the paste of native clay, in due proportion to prevent it from cracking, before it is moulded into the proper form. This paste VOL. Xiv,
March 13: will then admit of such an intense degree of heat, without being fused, as will reduce all the clay in its composition to the state of a stone, on which wa ter will never make the smallest impression, while at the same time it will retain its original form unimpaired.
This substance will no doubt come at a somewhat higher price than common bricks; because it will require at least double the quantity of fuel to burn it; but this, with the pounding the burnt clay, (which in large works, and for ordinary purposes, could be done at a very small expence,) are the only additions to the charge at present. This might make them come perhaps at nearly double the price (exclusive of carriage) which they cost at present. But as the price of carriage would be the same now, this small original advance of price would be inconsiderable, considering that one brick só formed, would stand at least ten times as long in an outer wall, as the ordinary bricks at present in
A composition of this kind has been long succefsfully employed as a luting for chemical vefsels, which • affords a sufficient proof that it would answer the purpose required above very effectually.
To be continued.
To see the world, and to know the just value that ought to be set on human favour, is only to be learnt in the school of adversity; a lefson which is taught in no other academy upon earth.
THE PLAID, OR JUPITER'S REPLY TO SCOTIA'S REQUEST.
Thus spake great Jove, when he survey'd our isle §
Or scudding o'er the deep, fhall commerce guide;
A lovely female, clad in mean attire,
Afsu me this garb, with varied fhades adorn'd,