(_b_) _Engineering Uses._—Metallic copper finds application in marine shipbuilding and engine work, as well as in railway and locomotive work, where the metal is particularly employed for steam pipes, and for fire-box plates and stays, sometimes also for boiler tubes, on account of its high conductivity for heat, combined with toughness. The questions of suitable composition, and the other requirements of the metal intended for these purposes, has been a subject for discussion by some of the leading marine and locomotive engineers. Useful information on the subject will be found in the reports of some of these discussions at the Institution of Mechanical Engineers. The following tests are required for copper plate (best quality) intended for locomotive fire-boxes on the Lancashire and Yorkshire Railway, taken from standard specifications given by their Chief Mechanical Engineer at the Institute of Metals:— _Bending Test._—Pieces of the plate shall be tested both cold and at a red heat by being doubled over on themselves— that is, bent through an angle of 180°—without showing either crack or flaw on the outside of the bend. _Flanging._—Plates must not show any defects in flanging. _Tensile Test._—Ultimate breaking load, 14 tons per square inch; Elongation, 35 per cent. in 8 inches. _Analytical Test._—To be made at contractor’s expense. The copper upon analysis to give the following results:—Arsenic, not less than 0·35 per cent. nor more than 0·55 per cent.; other foreign elements, exclusive of combined oxygen, not to exceed 0·25 per cent. Clauses are also inserted as to stamping, inspection, and the giving of testing facilities. Typical analysis of such plates show— Copper, 99·30 per cent. Arsenic, 0·43 to 0·51 per cent. Oxygen, 0·1 per cent. Impurities, chiefly antimony, lead, iron, nickel, tin, and sulphur, not exceeding 0·25 per cent. The average test on a number of plates gave— Tensile strength, 14·66 tons per square inch. Elongation on 8 inches, 43·36 per cent. of original length. Contraction of area, 45·9 per cent. Close bend test, Double. The effect of temperature and the influence of impurities on the mechanical properties of the metal intended for engineering purposes are of very great importance, and much attention has been devoted to researches in this subject, particularly by Milton and Le Chatelier, whose published experience gives important information of much practical value. The main conclusions arrived at from practice have had reference to the general effects of impurities in hardening the metal, and the general tendency of heat to soften it and to increase the ductility. The diverse effects of different impurities on strength and ductility will be reviewed in detail at a later stage.