On the other hand, their malleability, ductility, and power of resisting oxygen is generally diminished. The alloy formed of two brittle metals is always brittle; that of a brittle and a ductile metal, generally so; and even two ductile metals sometimes unite to form a brittle compound. The alloys formed of metals having different fusing-points are usually malleable whilst cold, and brittle whilst hot. The action of the air on alloys is generally less than on their simple metals, unless the former are heated. A mixture of 1 part of tin and 3 parts of lead is scarcely acted on at common temperatures; but at a red heat it readily takes fire, and continues to burn for some time like a piece of bad turf. In like manner, a mixture of tin and zinc, when strongly heated, decomposes both moist air and steam with almost fearful rapidity. The specific gravity of alloys is never the arithmetical mean of that of their constituents, as commonly taught; and in many cases considerable condensation or expansion occurs. When there is a strong affinity between two metals, the density of their alloy is generally greater than the calculated mean; and _vice versâ_, as may be seen in the following Table:—— _Alloys having a density_—— Greater than the mean of Less than the mean their constituents:—— of their constituents:—— Copper and bismuth, Gold and copper, ” palladium, ” iridium, ” tin, ” iron, ” zinc, ” lead, Gold and antimony, ” nickel, ” bismuth, ” silver, ” cobalt, Iron and antimony, ” tin, ” bismuth, ” zinc, ” lead, Lead and antimony, Nickel and arsenic, Palladium and bismuth, Silver and copper, Platinum and molybdenum, Tin and antimony, Silver and antimony, ” lead, ” bismuth, ” palladium, ” lead, Zinc and antimony. ” tin, ” zinc. “Every alloy,” says Dr Ure, “is, in reference to the arts and manufactures, a new metal, on account of its chemical and physical properties. A vast field here remains to be explored. Not above sixty alloys have been studied by chemists, out of many hundreds which may be made, and of these very few have yet been practically employed. Very slight modifications often constitute very valuable improvements upon metallic bodies.” See ANALYSIS, ASSAYING, BRASS, BRONZE, ELECTROTYPE, GERMAN SILVER, GOLD, METALS, SPECIFIC GRAVITY, &c. =ALL′SPICE.= See PIMENTO.