hard, ductile, of a yellow-pink colour, and susceptible of a high polish. An alloy of equal parts by weight of copper and platinum, according to Clarke, is yellow, having the colour and specific gravity of gold, extensible, easily worked by the file, and tarnished by exposure to air. An alloy of 4 parts platinum and 96 parts copper is malleable, rose-coloured, and exhibits a fine-grained fracture. An alloy of 3 parts platinum and 2 parts copper is nearly white, very hard, and brittle. § 171. The following alloys have a golden-yellow colour. No. IV, known as Cooper's gold, is malleable, ductile, and closely resembles 18-carat gold : Platinum Zinc Silver Brass Nickel. I II III IV V VI VII VIII 18.2 5 29.3 18.75 57.7 66.7 29.1 19 5 18.2 60 Cooper's Mirror Metal.-Copper 57.85, platinum 9.49, zinc 3.51, tin 27·49, arsenic 1.66. The inventor claims for this alloy that it is indifferent to the weather, and takes a beautiful polish. Cooper's Pen Metal.—The above alloy is said to be suitable for pens. Another alloy consists of copper 13 parts, platinum 50 parts, and silver 36 parts. The hardness and non-corrosive character of Cooper's alloys render them suitable for the manufacture of mathematical instruments and for chronometer wheels. § 172. Platinum and Iridium.-These metals unite in different proportions, but the intense heat of the oxyhydrogen blowpipe is necessary to melt them and bring them into union. The alloy consisting of 9 parts platinum and 1 part iridium is used as a standard metal bar for the metric system. It is extremely hard, as elastic as steel, more difficultly fusible than platinum, perfectly unalterable in air, and capable of taking an exceedingly beautiful polish. In the year 1870 Messrs. Johnson, Matthey, and Co. prepared a standard bar of the above alloy for the Parisian Commission for the International Metrical System, and after it had been subjected to every possible test which could be suggested in competition with other materials, it was, after two years' trial, pronounced the best, and adopted as the material for the manufacture of all the standard weights and measures. The following alloys have been prepared by Deville and Debray, and their specific gravities determined: Specific gravity of platinum = 21.504; of iridium= 22-421. According to Deville and Debray, an alloy of 90 per cent platinum and 10 iridium has the same coefficient of expansion as the original metre preserved in the French Archives, which is known to have been made of impure platinum. The 90 per cent platinum alloy is not attacked by aquaregia. Alloys with 20 per cent iridium are malleable and capable of being worked. An alloy of equal parts of the two metals is brittle, but capable of welding to some extent. An alloy of 1 part iridium and 10 parts platinum, when laid on copper, serves for metallic mirrors. A native alloy of platinum and iridium is found in the Ural Mountains and in Brazil. Platinum vessels for chemical operations generally contain iridium, which makes them stronger and harder. § 173. Alloys of Platinum with easily fusible Metals. Platinum readily unites with arsenic and antimony, the combination being attended with vivid incandescence, forming brittle and easily fusible alloys. Tin unites with platinum when the metals are fused together in equal parts, forming a hard, dark-coloured, somewhat fusible, brittle, and coarse-grained alloy. Zinc unites with platinum forming an alloy of similar properties to the above-mentioned tin alloy. Platinum and Lead readily unite, and very little lead makes platinum brittle. When molten lead is poured upon platinum, a portion of the latter is fused and dissolved in the lead. The alloys are hard, brittle, and granular. Platinum and Bismuth form brittle alloys. Mr. Lewis found that the alloys ranging from 1 to 24 parts of bismuth to 1 of platinum are brittle, easily fusible, and have a laminar fracture. By contact with air they acquire a purple or violet tint. When moderately heated some of the alloys undergo liquation, the bismuth partially separating out. When strongly heated in air the bismuth largely burns off, forming bismuth oxide. Platinum heated with Cadmium till the excess of the latter is volatilised forms a silver-white, very brittle, finegrained alloy, refractory in the fire, and containing 46 per cent of platinum. $174. Platinum and Nickel.-According to Lampadius, equal parts of nickel and platinum unite to form a pale yellowish-white alloy, perfectly malleable, susceptible of a high polish, equal to copper in fusibility and to nickel in magnetic power. § 175. Platinor.-This is a name given to certain alloys containing platinum, of a golden-yellow colour, and consisting of platinum, copper, silver, zinc, and nickel. An alloy of the colour of gold, and said to be quite constant in air, is prepared as follows: Melt 10 parts of silver with 45 parts of copper, then add 18 parts of brass and 9 parts of nickel. The temperature must then be raised to the highest pitch, and 18 parts of platinum-black added. § 176. Platinum-Bronze.-Several alloys of platinum, of a comparatively inexpensive nature, have been manufactured under the above name; and it has been claimed for them that they are indifferent to the action of the air and water. They admit of a high polish, and retain their lustre for a long time. The following table shows their composition CHAPTER XII IRON AND STEEL ALLOYS § 177. The general impression in the past has been that alloys of iron are of little importance, which was due to an imperfect acquaintance with their nature and properties; but at the present time there is more light being thrown on the subject, and if we expand the idea of an alloy so as to include those compounds in which very small quantities of other metals, such as aluminium, are present, then the alloys of iron may be considered of very great importance. As a rule, iron may be alloyed with most metals; but the combination is somewhat difficult to effect, and, in the majority of cases, only those alloys with a small quantity of iron, or iron with a small quantity of other metals, have been found to have useful applications. Iron, added to other metals or alloys, imparts new and sometimes important properties, such as increased hardness, elasticity, and tenacity. Many of the more recently discovered metals have of late years been added to iron with more or less marked alteration in its qualities, and such combinations are now ordinary commercial articles. In the following description of iron alloys, the metals known as "malleable-iron," "cast-iron," and "steel" are not treated as separate metals, but considered as different varieties of iron. § 178. Iron and Manganese.-Our knowledge of the |