SLATE, a well known, neat, convenient and durable material, for the covering of the roofs of buildings. There are great varieties of this substance, and it likewise differs very greatly in its qualities and co lours In some places it is found in thick laminz or flakes, while in others it is thin and light. The colours are white, brown, 'and blue. It is so durable in some cases, as to have been known to continue sound and good for centuries. However, unless it should be brought from a quarry of well reputed goodness, it is necessary to try its properties, which may be done by striking the slate sharply against a large stone, and if it produce a complete sound, it is a mark of goodness; but if, in 'hew ing, it does not shatter before the edge of the sect, or instrument commonly used for that purpose, the criterion is decisive. The goodness of the slate may be further estimated by its colour: the deep black blue is apt to imbibe moisture, but the lighter blue is always the least penetra ble :. the touch also may be in some de gree a guide, for a good firm stone feels somewhat hard and rough, whereas an open slate feels very smooth, and as it were greasy. And another method of trying the goodness of slate is, to place the slate-stone lengthWise, and perpendi cular in a tub of water, about half a foot deep, care being taken that the opperor unimmersed part of the slate be not acci dentally wetted by the hand or other wise: let it remain in this state twenty-four hours ; if good and firm stone, it will not draw water more than half an inch above the surface of the water, and that perhaps at the edges only, those parts having been a little loosened in the hewing ; but a spongy defective stone will draw water to the very top. There is still another mode,
held to be infallible. First weigh two or three of the most suspected plates, noting the weight ; then immerge them in a ves sel of water twelve hours, take them out, and wipe them as clean as possible with a linen cloth, and if they weigh more than at first, it denotes that quality of slate which imbibes water; a drachm is allow able in a dozen pounds, and no more. In the laying of this material, a bushel and a half of lime, and three bushels of fresh water sand will be sufficient for a square of work, but if it be pin plastered, it will take above as much more : but good slate, well laid, and plastered to the pin, will lie an hundred years, and on good timber a much longer tune. It has been common to lay the slates dry, or on moss only. When they are to be plastered to the pin, then about the first quantity of lime and sand will be sufficient for the purpose. See TaoxSC srss.