tures, and it is essential to know at what temperatures certain solders melt, in order to employ on any given job a solder, the melting point of which is well below that of the material which has to be united. Coke or charcoal fires, jets of gas, and copper bits are used to fuse the various solders employed.

Specific Gravity. The specific gravity of a metal is estimated relatively to that of a given equal bulk of pure water at a temperature of 62 degrees Fahrenheit. Beyond the commercial classification of sheets by weight, the relative weights of metals do not concern the sheet-metal worker much.

The manner in which the physical properties of the alloys is affected by small variations in the proportions of their constituents is often remarkable. Malleability, ductility, fusing points, even appearances are often radically modified. Some metals are more readily influenced in this way than others. Among familiar examples may be noted the effect which very minute percentages of carbon, phosphorous, and silicon exercise on steel.

Taking very common examples, it is remarkable that the union of two soft and malleable metals, as copper and tin, results in alloys ranging from the tough yellow gun metal to the brittle bell and speculum metals of silvery whiteness. So, too, copper alloyed with the very brittle and crystalline zinc forms the soft, yellow brass, which is bent and cut with so much ease. Or, copper with lead forms an alloy so soft as to be hardly workable. Again, tin and lead alloyed together fuse at a temperature lower than that of either of the constituents a fact which renders them valuable as solders. And by adopting different proportions, various fusing

points higher and lower are obtained, suitable for soldering different qualities of metal or alloy.

Copper Alloys. Copper is not only highly valuable in the pure state, but its value is even perhaps greater when alloyed in various proportions with tin, lead, zinc, or other metals. It is only necessary to instance gun metal, brass, bell metal, and the solders. The subject of alloys is one of so great interest and value that volumes might be devoted to them. But, in strictness, the subject is of greater interest to the founder than to the sheet-metal worker. Still, there is very much of interest in it to the latter, since all brass sheets and wires are alloys. All tinning of copper vessels is effected by a union of the surfaces of dissimilar metals, the difference in qualities of sheets and wires depend mainly on the proportions in which certain elements occur. All solders, whether hard or soft, are alloys. So that for these and for other reasons a knowledge of the principles which underlie the union of dissimilar metals to form alloys is desirable.

Whether alloys are true chemical compounds has been doubted. At least, they are not recognized as such in science. The reason is, that there is no fixed and definite proportion in which, and in which alone, combination of the metallic elements occurs. In a true chemical compound such is the case. They invariably combine in definite proportions known as their combining weights, or in multiples of those combining weights. But true alloys are formed apart from any such definite combinations, so that one or other of the elements in one alloy shall be in excess by comparison with another alloy of the same metals. It seems, however, as though true chemical combination

must take place, but that the compound is mechanically associated with an excess of one or more of the elements. The reason for assuming the existence of a true compound is, that an alloy usually possesses physical characteristics very different from those possessed by its separate elements-a feature in which it closely resembles most true chemical compounds. The strength, tenacity, hardness, and fusing points of alloys are generally higher than those of their constituent elements, in some cases very much higher-effects which do not seem possible by a mere mechanical mixture of ele

ments.

Copper is alloyed with tin, lead, and zinc in various proportions. When alloyed with tin alone it forms the gun metals, bronzes, bell metals, and speculum metal. When alloyed with zine only it forms various brasses and spelter solders. Alloyed with lead only, it forms the very common pot metals. Alloyed with tin, zinc, and lead, it forms various gun metals and bronzes.

Alloys of copper with zinc alone are used chiefly to form spelter solder and brass. Copper and zinc mix in all proportions, but exact proportions are difficult to determine, because zinc volatilizes readily. The fusibility of copper-zinc alloys increases with the proportion of zinc. The color ranges, with the successive additions of zinc, from the red of copper to silvery white, and the malleability decreases until a crystalline character prevails.

An alloy of about 1 part of zinc to 16 parts of copper is used for jewelry, one of 3 to 4 parts of zinc to 16 parts of copper for sundry alloys once known as pinchbeck, about 6 to 8 parts of zine to 16 parts of copper

form common brass, the latter being slightly more fusible than the former. Equal parts of zinc and copper form soft spelter solder, or 12 or 14 parts of zinc to 16 parts of copper would probably be the ultimate proportions after volatilization.

Copper and tin also mix in all proportions, successive additions of tin increase the fusibility of the alloy, the malleability diminishes, and the color gradually changes from red to white.

Copper-tin or gun metal alloys range from about 1 part of tin to 16 parts of copper in the softest, to 2 or 21⁄2 parts of tin to 16 parts of copper in the hardest. Beyond the last proportion, up to 5 parts of tin to 16 parts of copper, range the bell metal alloys, from 714 to 814 parts of tin to 16 parts of copper form speculum metal.

The alloys of copper with lead alone are used in the cheap pot metals. The fusibility is increased with successive additions of lead, the malleability is soon lost, and the red color of copper gives place to a leaden hue. About 6 parts of lead to 16 parts of copper is the limit at which a true alloy can be formed, with an increase in the proportion of lead the latter separates in cooling.

Alloys of copper with zinc, tin, and lead are largely used under the names of brasses, bronzes, gun metals, and pot metal. There is practically no limit to the range of these alloys.

Generally, those alloys are not proportioned separately, but the copper is added to a brass alloy. In many mixtures lead is not used at all, but copper, tin, and zinc only. Antimony is also sometimes used. A little iron added to yellow brass hardens it. Lead, on

the contrary, makes it more malleable. Zinc added. to a pure mixture of copper and tin makes it mix better, and increases the malleability. Pot metal is improved by the addition of a little tin, and also of anti

mony.

Aluminum. This metal when of 98.5 per cent purity is bright white in color, somewhat resembling silver, though its appearance depends much on the temperature at which it has been worked. It is capable of taking a high polish. Its fusing point is about 1,050° Fahrenheit, but this may be increased to 1,832° Fahrenheit if impurities are present or if it is alloyed with another metal. Aluminium is only slightly elastic, it is, however, fairly malleable and ductile, but these latter properties are impaired by the presence of its chief two impurities, silicate and iron. If of more than 99 per cent purity, it can be rolled into leaves 1-40,000th part of an inch in thickness, in this respect being inferior only to gold. Aluminium has a tensile strength of 12,000 pounds to the square inch. When pure, it is non-corrosive and resists the oxidizing action of the atmosphere, but this advantage has to be partly sacrificed to obtain increased hardness and elasticity by adding small quantities of copper, nickel, or zinc. It dissolves in hydrochloric acid and in most solutions of the alkalies, but is only slightly affected by dilute sulphuric acid, and not at all by nitric acid. The rolled or forged metal breaks with a fine silky fracture. Aluminium is not found in a metallic state, but when in combination with oxygen, various alkalies, fluorine, silicon, and acids, it is the base of many clays. and soils. Frequent compounds of aluminium are felspar, mica, gneiss, and trachyte, whilst other aluminium