Soldering or "sweating" is a difficult operation. Careful investigations show that as much as 90% of the soldering work done is defective. This trouble is due principally to improper and careless cleaning Of the metal surfaces to be joined. Then again an improper flux may be used. The most effective method of soldering is first to fit the parts accurately, to make the joint strong; then to clean the parts to be soldered thoroughly. A solder with as high a melting point as possible should be used and the heat applied so that the temperature of the work to be joined is brought as near as possible to the fusing point of the solder. Thus the solder will be allowed to flow freely and a better union will result.