This is prepared by dissolving filings or slips of copper with nitrous acid in a receiver. When the acid is saturated, the slips are to be removed; or, if filings be employed, the solution is to be poured off from what remains undissolved; small bars are then put in, which will precipitate the copper from the saturated acid, in a powder of the peculiar appearance and colour of copper, and the liquid being poured from the powder, this is to be washed clean of the crystals by repeated levigations. 177. COMMON SIZE The size used by painters for most sorts of common work is prepared by boiling in water pieces of parchment, and of the skins of animals and fins of fish, and evaporating the solution to a proper consistency. It only differs, however, from a solution of glue containing fewer foreign ingredients, and in not being so strong. 178. DR. JOHN'S VARNISH FOR PLASTER OF PARIS CASTS Take of white soap and white wax, each half an ounce, of water two pints; boil them together for a short time in a clean vessel. This varnish is to be applied when cold, by means of a soft brush. It does not sink in, it readily dries, and its effect may be heightened by lightly using a silk pocket handkerchief. 179. GENERAL DIRECTIONS FOR BRONZING This art is nothing but a species of painting, but far from being of the most delicate kind. The principal ingredients made use of in it are the true gold powder, the German gold, the aurum mosaicum, and copper powder, (all above described.) The choice of these powders is, of course, to be determined by the degree of brilliancy you wish to obtain. The powder is mixed with strong gum water or isinglass, and laid on with a brush or pencil; and when not so dry as to have still a certain clamminess, a piece of soft leather wrapped round the finger, is dipped in the powder and rubbed over the work; when the work has been all covered with the bronze, it must be left to dry, and any loose powder then cleared away by a hair pencil. 180. BRONZING IN WOOD