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sect. If very hard, the frequent cleaning of it from themarks, by rubbing it with tripoli or a piece-of charcoalwetted with water, gives the surface too great a smooth-ness ; and if very soft, it is liable to be scratched in thecleaning. In want of the proper kind of stone, mode-rately smooth pieces of flint are the best substitutes: themore these approach in colour to the other, the better.

The piece of gold, to be examined, being well cleanedin some convenient part of its surface, a stroke is to bemade with it on the stone ; and another, close by it, withsuch of the touch-needles as appears to come the nearestto it in colour. If the colour of both, upon the stone,is exactly the same, it is judged that the given mass isof the same fineness with the needle : if different, anotherand another needle must be tried, till such a one is foundas exactly corresponds to it. To do this readily, practiceonly can teach.

In making the strokes, both the given piece, and theneedle of comparison, are to be rubbed several times back-wards and forwards upon the stone, that the marks maybe strong and full, not less than a quarter of an inch long ,and about a tenth or an eighth of an inch broad : bothmarks are to be wetted before the examination of them,their colours being thus rendered more distinct. A stroke,which has been drawn some days, is never to be com-pared with a fresh one, as the colour may have suffered analteration from the air; the fine atoms, left upon thetouchstone, being much more susceptible of such altera-tions than the metal in the mass. If the piece is supposedto be superficially heightened by art in its colour, thatpart of it, which the stroke is designed to be made with,should be previously rubbed on another part of the stone,or rather on a rougher kind of stone than the commontouchstones, that a fresh surface of the metal may be

exposed.