spreading the plates over, after they are polished, with quicksilver, &c., in order to reflect the image. It is performed thus: A thin blotting paper is spread on the table, and sprinkled with fine chalk; and then a fine lamina or leaf of tin, called foil, is laid over the paper; upon this is poured mercury, which is to be distributed equally over the leaf with a hare's foot or cotton; over this is laid a clean paper, and over that the glass-plate, which is pressed down with the right-hand, and the paper drawn gently out with the left; this being done, the plate is covered with a thicker paper, and loaded with a greater weight, that the superfluous mercury may be driven out and the tin adhere more closely to the glass. When it is dried, the weight is removed, and the looking-glass is complete.

Some add an ounce of marcasite, melted by the fire; and, lest the mercury should evaporate in smoke, they pour it into cold water; and when cooled, squeeze through a cloth, or through leather.

Some add a quarter of an ounce of tin and lead to the marcasite, that the glass may dry the sooner.

Foliating of Globe Looking-glasses, is done as follows: Take five ounces of quicksilver, and one ounce of bismuth; of lead and tin, half an ounce each; first put the lead and tin into fusion, then put in the bismuth; and when you perceive that in fusion too, let it stand till it is almost cold, and pour the quicksilver into it; after this, take the glass-globe, which must be very clean, and the inside free from dust; make a paper-funnel, which put into the hole of the globe, as near the glass as you can, so that the amalgam, when you pour it in, may not splash, and cause the glass to be full of spots; pour it in gently, and move it about, so that the amalgam may touch every where; if you find the amalgam begin to be curdly and fixed, then hold it over a gentle fire, and it will easily flow again; and if you find the amalgam too thin, add a little more lead, tin, and bismuth to it. The finer and clearer your globe is, the better will the looking-glass be.

Dr Shaw observes, that this operation has considerable advantages, as being performable in the cold; and that it is not attended with the danger of poisonous fumes from arsenic, or other unwholesome matters, usually employed for this purpose; besides, how far it is applicable to the more commodious foliating of the common looking-glasses, and other speculums, he thinks, may deserve to be considered.