The metals, or metallic mixtures, which are employed for the purpose of joining other metals, with the aid of heat, are known by the general name of solders. Their number is considerable, as soldering is done not only with pure metals, but also, and more frequently, indeed, with various alloys.

Solders are classified according to the manner in which they are used, with further distinctions in regard to their fusibility and to the metals which are to be soldered. We will take up first the most general classification referring later to special cases.

The two principal classes may be distinguished as homogeneous and heterogeneous solders. The first consist of the same metals as those which are to be joined to each other, the latter either of a foreign metal or, more frequently, of a mixture of metals.

The homogeneous solders are, of course, the simplest, and in general most to be recommended, since it is possible, with them, to make the soldered article appear as if actually cast in one piece But they are less frequently employed now than formerly, when fewer good solders were known; and this for the reason that this method of soldering is somewhat difficult, and requires in most cases a very high temperature. It has recently been revived, however, for many purposes, since devices have been learned for obtaining a very high degree of heat in a short time.

The heterogeneous solders, consisting of alloys, have the advantage that by proper changes in their composition they can be easily adapted to any special purpose, and thus the labor is lightened. Since the so-called soldering machine has been so far perfected that a good deal of soldering can be done without the use of a soldering iron, simply by letting the melted solder run into the seams, many kinds of solder, even quite hard, are used in this way, and the work of soldering is thereby considerably simplified.

* Copyright 1906 by Munn & Co.