Copper is sometimes found native; but commonly mixed with sulphur, in ores of a red, green, or blue colour. Copper being an earlier discovery than that of iron, was formerly used for weapons and the shoeing of horses; and the ancients had a method, with which we are not well acquainted, of giving it a considerable degree of hardness, so that a sword made of it might have a pretty good edge. LECTURE XXV. _Of Iron._ Iron is a metal of a bluish colour, of the greatest hardness, the most variable in its properties, and the most useful of all the metals; so that without it it is hardly possible for any people to make great advances in arts and civilization. This metal readily parts with its phlogiston, so as to be very subject to calcine, or rust, by exposure to the air. The same is evident by the colours which appear on its surface when exposed to heat, and also when it is struck with flint; the particles that fly from it being iron partially calcined. In consequence of its readily parting with its phlogiston, it is capable of burning, like wood or other fuel, in pure air. Iron and platina have the property of _welding_ when very hot, so that two pieces may be joined without any solder. When iron is heated in contact with steam, part of the water takes the place of the phlogiston, while the rest unites with it, and makes inflammable air. By this means the iron acquires one third more weight, and becomes what is called _finery cinder_. This substance, heated in inflammable air, imbibes it, parts with its water, and becomes perfect iron again. If the iron be heated in pure air, it also imbibes the water, of which that air chiefly consists, and also a portion of the peculiar element of the pure air. The solution of iron in spirit of vitriol produces _green copperas_; which being calcined, becomes a red substance, called _colcothar_. The precipitate of iron, by an infusion of galls, is the colouring matter in _ink_, which is kept suspended by means of gum. The precipitate from the same solution by phlogisticated alkali, is _Prussian blue_. Water saturated with fixed air dissolves iron, and makes a pleasant chalybeat. The calx of iron gives a green colour to glass. Iron readily combines with sulphur. When they are found combined by nature, the substance is called _pyrites_. The union of phosphoric acid with iron makes it brittle when cold, commonly called _cold short_; and the union of arsenic makes it brittle when hot, thence called _red short_. Iron unites with gold, silver, and platina, and plunged in a white heat into mercury, it becomes coated with it; and if the process be frequently repeated, it will become brittle, which shews that there is some mutual action between them. Iron readily unites with tin; and by dipping thin plates of iron into melted tin, they get a complete coating of it, and make the _tinned plates_ in common use.