The alloys which the metals obtained by the methods above described form with copper have been made the subject of careful study.  An alloy containing 10 per cent. of aluminum and 90 per cent. of copper forms the so-called aluminum bronze with a fine golden color, which it retains for a long time.  The tensile strength of this alloy is usually given as 100,000 pounds to the square inch; but castings of our ten per cent. bronze have stood a strain of 109,000 pounds.  It is a very hard, tough alloy, with a capacity to withstand wear far in excess of any other alloy in use.  All grades of aluminum bronze make fine castings, taking very exact impressions, and there is no loss in remelting, as in the case of alloys containing zinc.  The 5 per cent. aluminum alloy is a close approximation in color to 18 carat gold, and does not tarnish readily.  Its tensile strength in the form of castings is equivalent to a strain of 68,000 pounds to the square inch.  An alloy containing 2 or 3 per cent. aluminum is stronger than brass, possesses greater permanency of color, and would make an excellent substitute for that metal.  When the percentage of aluminum reaches 13, an exceedingly hard, brittle alloy of a reddish color is obtained, and higher percentages increase the brittleness, and the color becomes grayish-black.  Above 25 per cent. the strength again increases.

The effect of silicon in small proportions upon copper is to greatly increase its tensile strength.  When more than 5 per cent. is present, the product is exceedingly brittle and grayish-black in color.  It is probable that silicon acts to a certain extent as a fluxing material upon the oxides present in the copper, thereby making the metal more homogeneous.  On account of its superior strength and high conductivity for electrical currents, silicon bronze is the best material known for telegraph and telephone wire.

The element boron seems to have almost as marked an effect upon copper as carbon does upon iron.  A small percentage in copper increases its strength to 50,000 or 60,000 pounds per square inch without diminishing to any large extent its conductivity.

Aluminum increases very considerably the strength of all metals with which it is alloyed.  An alloy of copper and nickel containing a small percentage of aluminum, called Hercules metal, withstood a strain of 105,000 pounds, and broke without elongation.  Another grade of this metal broke under a strain of 111,000 pounds, with an elongation equivalent to 33 per cent.  It must be remembered that these tests were all made upon castings of the alloys.  The strength of common brass is doubled by the addition of 2 or 3 per cent. of aluminum.  Alloys of aluminum and iron are obtained without difficulty; one product was analyzed, containing 40 per cent. of aluminum.  In the furnace iron does not seem to be absorbed readily by the reduced aluminum when copper is present; but in one experiment a mixture composed of old files, 60 per cent.; nickel, 5 per cent.; and of 10 per cent. aluminum bronze 35 per cent., was melted together, and it gave a malleable product that stood a strain of 69,000 pounds.