The word " alloy was originally applied to mixtures of gold and silver with other metals; it now signifies any mixture or compounds of metals with each other; alloys of mercury are, however, termed " amalgams." When two metals are melted together, they always mix, more or less. Some may be mixed in any desired proportion, such as lead and tin; others are partially soluble in each other; zinc, for example, dissolves 1.6 per cent, of lead, and lead 1.2 per cent, of zinc ; but on stirring up the metals together, there is always a layer at the top of the lighter alloy of zinc with lead, and below it the heavier alloy, consisting chiefly of lead. By raising the temperature the mutual solubility of the metals increases, and at a sufficiently high temperature it is probable that they would mix completely.
Alloys in general may be classified under two heads: (I) definite compounds, in which the elements are present in atomic proportions; and (2) mixtures in which combination has not taken place. To these classes may be added a third-mixtures of definite compounds with one or other of the components of the alloy. As such mixtures are usually homogeneous, it is often a matter of great difficulty to identify the definite compounds. In many cases, too, it would appear that one of the metals in the alloy is present in an allotropic form ; for example, on treatment of-one of the alloys of rhodium and zinc with dilute hydrochloric acid, after solution of the zinc, the rhodium is left in an allotropic form.
The constitution of alloys can be deciphered by several processes. One depends on measurement of the electromotive force of a battery consisting of the alloy and a plate of some resistant metal-for instance, platinum - compared with that of a similar cell made with one of the constituents of the alloy. To take a concrete example. Suppose a cell were constructed of a plate of copper and a plate of platinum dipping in some appropriate liquid, a certain electromotive force would result. Imagine a plate of tin riveted to the face of the copper plate, the electromotive force would now be that of the more electropositive metal, tin. If a plate of bronze be substituted, supposing it to contain free tin not in chemical combination with the copper, then the electromotive force will still be that of tin against platinum. A chemical compound of tin and copper, however, would have a less electromotive force than free tin ; and as the tin in the alloy mentioned is dissolved away, the electromotive force will suddenly fall when the excess of tin has been dissolved, until it is equal to that of the chemical compound against platinum. An analysis of the alloy at this stage will reveal the composition of the compound. In this way the existence of a compound of the formula Cu3Sn was detected.
The second method of determining whether an alloy contains a definite compound is to compare the freezing-points of various alloys of the metals. -The presence of a small amount of one metal in another in general lowers the freezing-point; and the freezing-point is continuously lowered by successive additions, until the lowering reaches a maximum. The mixture which has the lowest possible melting-point is termed the " eutectic " alloy. The composition of this alloy does not necessarily coincide with that of a definite compound; indeed, metals which form no compound with each other exhibit this phenomenon. If a compound is formed, however, the melting-point rises to a maximum on continuous addition of the second metal, and that compound which has the highest melting-point corresponds with a definite formula. Further addition of the second metal causes a lowering of the freezing-point of the definite compound; and this lowering increases on addition of the second metal, until a second eutectic alloy is formed, one consisting of a mixture of the compound with excess of the second metal. Further addition of the second metal now causes the melting-point to rise, it may be to the melting-point of the second metal; in that case only one compound of the two metals is capable of existence. It may happen, however, that, after rising to a certain temperature, the temperature again falls on addition of more of the second metal; in that case, the highest temperature reached corresponds to the existence of a second compound; a similar change may even denote the existence of a third. By such means it is possible to detect the existence of definite compounds between any two metals. With ternary alloys, i.e. with alloys containing three metals, although the phenomenon is more complicated, this method has led to the discovery of several definite compounds.
While alloys have generally been prepared by melting the metals together, or by melting one of them and adding the other, some alloys have been produced by submitting mixtures of the metallic powders to enormous pressure.