SLA 



Drawing-flate frequently accompanies alum-flate ; it is 

 much fofter than common flate, and contains, like alum- 

 flate, a confiderablc portion of carbon : its colour is a 

 greyifh-black : it is known by the property which it pofTefles 

 of leaving a dark line when rubbed on paper. It is foft, 

 and fometimts rather unAuous : fome varieties have a fmall 

 degree of luftre. The frafture, in fmall fragments, is 

 fcarcely flaty, and fometimes approaches the conchoidal. 

 Drawing-flate is eafily cut with the knife. Under the 

 blowpipe it turns white or yellow. It fometimes efflorefces 

 like alum-flate. According to Wiegleb, it contains 



Drawing-flate is employed by mafons, carpenters, &c. to 

 mark with. When fine and pure, it is ufed by artifts for 

 defigns. In France it is called pierre d'ltalie, in England, 

 French chalk. It is found in France, near Seer, in the de- 

 partment of I'Ome, and in the environs of Cherbourgh. 

 It is found alfo in Spain and Italy. 



Whet-flate, or hone, the novaculile of Kirwan, occurs 

 imbedded in clay-flate : its moft common colour is greenifh- 

 grey, inclining to yellow : it is much harder than common 

 (late : its texture is fine-grained, nearly compaft, and the 

 frafture of the fmall pieces fplintery or conchoidal, refem- 

 bling flinty flate. Its fpecific gravity is about 2.72. Whet- 

 flate is tranflucent on the edges ; it does not effervefce with 

 acids, and it melts into a brown enamel under the blowpipe. 

 From its green colour, and rather greafy feel, it may be 

 confidered as intermediate between hard talcous-flate and 

 clay-flate. Though it yields to the point of a knife, or 

 even of a copper tool, it afts upon the flattened or round 

 furfaces of metals, and is ufed for ftiarpening and polifhing 

 the finer kinds of cutlery. It is of confiderable value on 

 account of this property, and was firft brought from the 

 Levant. We have no whet-flate of a fine quality in Eng- 

 land. An inferior kind is procured from Chamwood foreft, 

 in Leicefterlhire. It exifts of a finer quality in the promon- 

 tory of Howth, near Dublin. The common whet-flate of 

 commerce is procured from Saxony. 



ShATE-Spar, Schtefer-fpath, Werner, occurs in lime-ftone 

 beds, in mountains called primitive. Its colour is milk- 

 white, or greenifli and reddifli-white, its luftre fliining and 

 pearly. Slate-fpar is tranflucent and foft, its ftrufture 

 coarfely lamellar, paffing into flaty, and fometimes curved 

 or undulating : it is infufible, and effervefces with violence 

 in acids. 



Slate, in Rural Economy, a well-known, neat, conve- 

 nient, and durable material, for the covering of the roofs 

 of buildings. There are great varieties of this fubftance ; 

 and it likewife differs very greatly in its qualities and colours. 

 In forae places it is found in thick laminse, or flakes ; while 

 in others it is thin and light. The colours are white, brown, 

 and blue. 



It is fo durable, in fome cafes, as to have been known to 

 continue found and good for centuries. However, unlefs it 

 fliould be brought from a quarry of well reputed goodnefs, 

 it is neceffary to try its properties, which may be done by 

 ftriking the flate ftiarply againft a large ftone, and if it pro- 

 duce a complete found, it is a mark of goodnefs ; but if in 

 hewing it does not fliatter before the edge of ihefeB, or 

 inftrument commonly ufed for that purpofe, the criterion is 

 decifive. The goodnefs of flate may be farther eftimated 

 by its colour : the deep black-blue is apt to imbibe moifture, 



SLA 



but the lighter blue is always the lead penetrable : the 

 touch alfo may be in fome degree a guide, for a good firm 

 ftone feels fomewhat hard and rough, whereas an open flate 

 feels very fmooth, and as it were greafy. And another 

 method of trying the goodnefs of flate, is to place the flate- 

 ftone lengthwife, and perpendicularly in a tub of water, 

 about half a foot deep, care being taken that the upper or 

 unimmerfed part of the flate be not accidentally wetted by 

 the hand, or otherwife ; let it remain in this ftate twenty- 

 four hours ; if good and firm ftone, it will not draw water 

 more than half an inch above the furface of the water, and 

 that perhaps at the edges only, thofe parts having been a 

 little loofened in the hewing ; but a fpongy defeftive ftone 

 will draw water to the very top. There is ftill another 

 mode, held to be infallible. Firft, weigh two or three of 

 the moft fufpecled flates, noting the weight ; then immerge 

 them in a veflel of water twelve hours ; take them out, and 

 wipe them as clean as poflible with a linen cloth ; and if 

 they weigh more than at firft, it denotes that quality of 

 flate which imbibes water : a drachm is allowable in a dozen 

 pounds, and no more. 



It may be noticed, that in laying of this material, a bufliel 

 and a half of lime, and three bufliels of frefli-water fand, 

 will be fufficient for a fquare of work ; but if it be pin 

 plaftered, it will take above as much more : but good flate, 

 well laid and plaftered to the pin, will lie an hundred years ; 

 and on good timber a much longer time. It has been com- 

 mon to lay the flates dry, or on mofs only, but they are 

 much better when laid with plafter. When they are to be 

 plaftered to the pin, then about the firft quantity of lime and 

 fand will be fufiicient for the purpofe, when well mixed and 

 blended together, by properly working them. 



Slates differ very much in thicknefs as well as colour, 

 which fuits them for different fituations and purpofes. A 

 great deal of good flate of various kinds is raifed in different 

 parts of Wales, and much excellent blue and other coloured 

 forts is procured from the northern parts of Lancaftiire, and 

 other neighbouring places, as well as from different other 

 counties throughout the kingdom. In fome parts the flate 

 is diftributed into three kinds, as the beft, the middling, 

 and the wafte or common fort. See Quarry. 



SLATE-^xf, provincially a mattock with a fliort axe-end, 

 ufed in flating, &c. 



Slate Mount, in Geography, a mountain of Virginia ; 

 6 miles W. of Richmond. N. lat. 37° 35'. W. long. 72°. 



Slate River, a river of Virginia, which runs into 

 James river, N. lat. 37° 40'. E. long. 78° 34'. 



SLATEE, an African appellation of free black mer- 

 chants, often traders in flaves. 



SLATERS, in Entomology. See Millepedes and Onis- 

 cus. 



Slaters anJ Tilers, are thofe artificers who are employed 

 in flating or tiling houfes. In calculating and eftimating the 

 value of their work, they find the content of a roof by 

 multiplying the length of the ridge by the girt over from 

 eaves to eaves ; making allowance in this girt for the double 

 row of flates at the bottom, or for the quantity or fpace by 

 which one row of flates or tiles is laid over another. In an- 

 gles formed in a roof, running from the ridge to the eaves, 

 when the angle bends inwards, it is called a valley ; but 

 when outwards, it is called a hip. And in tihng and flating, 

 it is common to add the length of the valley or hip to the 

 content in feet. Deduftions are feldom made for chimney- 

 fliafts, or fmall window-holes. E. gr. Required the con- 

 tent of a flated roof, the length being 45 feet 9 inches, and 

 whole girt 34 feet 3 inches. 



By 



