For silvering a convex surface

—A concave plaster mould is made on the convex gla , and their points of coincidence are defined by marks. This mould is to be lined with tin-foil, with the precautions above described; and the tin surface being first brightened with a little mercury, the mould is then filled with the liquid metal. The gla is to be well cleaned, and immersed in the quicksilver bath, which will expel the greater part of the metal. A sand-bag is now to be laid on the gla , and the whole is to be inverted as in the former case on a support; when weights are to be applied to the mould, and the mercury is left to drain off for several days. If the gla be of large dimensions, 30 or 40 inches, for example, another method is adopted. A circular frame or hollow ring of wood or iron is prepared, of twice the diameter of the mirror, supported on three feet. A circular piece of new linen cloth of close texture is cut out, of equal diameter to the ring, which is hemmed stoutly at the border, and furnished round the edge with a row of small holes, for lacing the cloth to the ring, so as to leave no folds in it, but without bracing it so tightly as to deprive it of the elasticity nece ary for making it into a mould. This apparatus being set horizontally, a leaf of tin-foil is spread over it, of sufficient size to cover the surface of the gla ; the tin is first brightened with mercury, and then as much of the liquid metal is poured on as a plane mirror requires. The convex gla , well cleaned, is now set down on the cloth, and its own weight, joined to some additional weights, gradually pre es down the cloth, and causes it to a ume the form of the gla which thus comes into close contact with the tin submersed under the quicksilver. The redundant quicksilver is afterwards drained off by inversion, as in common cases. The following recipe has been given for silvering the inside of gla globes. Melt in an iron ladle or a crucible, equal parts of tin and lead, adding to the fused alloy one part of bruised bismuth. Stir the mixture well and pour into it as it cools two parts of dry mercury; agitating anew and skimming off the dro y film from the surface of the amalgam. The inside of the gla globe being freed from all adhering dust and humidity, is to be gently heated, while a little of the semi-fluid amalgam is introduced. The liquidity being increased by the slight degree of heat, the metallic coating is applied to all the points of the gla , by turning round the globe in every direction, but so slowly as to favour the adhesion of the alloy. This silvering is not so substantial as that of plane mirrors: but the form of the ve el, whether a globe, an ovoid, or a cylinder, conceals or palliates the defects by counter reflection from the opposite surfaces.