In mixing paints for iron surfacee, it is of the first importance that the best materials only be used. Linseed-oil is the best medium, when free from admixture with turpentine. A volatile oil, like turpentine, can not be used with advantage on a non-absorbent surface like that of iron, for the reason that it leaves the paint a dry scale on the outside, which, haying no cohesion, can be readily crumbled or washed away. Linseed-oil, on the other hand, is peculiarly well adapted for this purpose. It does not evaporate in any perceptible degree, but the large percentage of linolein which it contains combines with the oxygen of the air, and forms a solid, translucent substance, of resinous appearance, which possesses much toughness and elasticity, and will not crack or blister by reason of the expansion and contraction of the iron with variations oft temperature. It is, moreover, remarkably adhesive, is impervious to water, and is very difficult of solution in essential oils, spirits, or naphtha, and even in bisulphide of carbon. Another important advantage of linolein is that it expands in drying, which peculiarly adapts it to iron surfaces; since cracks, however minute, resulting from shrinkage, expose enough of the metal to afford a chance for corrosion, which will spread in all directions, undermining the paint and causing it to scale off. beside discoloring it. In selecting a paint for iron, mechanical adhesion is a consideration of the first importance. Pitchy or bituminous films are especially effective as regards their adhesion to iron; for example, solutions of asphalt or pitch in petroleum or turpentine. These are also very effective as regards continuity, owing to the fact that, in drying, they form plastic films, which yield with the expansion and contraction of the iron, and manifest no tendency to crack. If the surface is rusty, they penetrate the oxide scale, and envelop the particles very effectually, making them a portion of the paint. The solubility of such a film in water may be counteracted by mixing it with linseed-oil. The experiment may easily be tried by mixing about 2 parts of Brunswick black with one of white, red, or stone colored paint, the body of which is composed of red or 'white lead or litharge. Red-lead is the best, for many reasons, if finely ground and thoroughly mixed with linseed-oil. Any one of several kinds of bitumen may be used, either natural mineral asphalt, pine pitch, or artificial asphalt, such as gas-tar or the residuum of petroleum distillation, in cases where the crude oil has been distilled before being treated with acid. This gives a very hard, bright pitch, which is soluble in "once run" paraffine spirit, and which makes the base of an excellent, cheap, and durable paint for iron-work in exposed positions. Paraffine can be recommended for all classes of iron-work which can be treated hot. The most effective method of applying it is to heat the iron in vacuo, in order to expand it and open its pores, when paraffine, raised to the proper temperature, is run upon it. By this means the iron is penetrated to a sufficient depth to afford a very effectual protection against oxidation, especially when a suitable paint is subsequently applied.