Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/626

Rh 588 A L L - A L L the tin is in excess after cooling, a metallic ingot is obtained resembling closely the original substance ; but if the mass is treated with strong hydrochloric acid, the excess of tin is dissolved and crystals remain of a definite alloy of tin and the precious metal. These alloys are in soluble in strong hydrochloric acid, which dissolves tin so easily ; but they are soluble in aqua regia. even when the precious metal contained therein (rhodium, ruthenium, iridium) is in the free state absolutely insoluble. This is no proof that the industrial alloys are always the result of one definite combination dissolved in excess of one of the metals, as many combinations are able to co-exist in the same alloy. This may be proved by taking an alloy of tin, lead, and bismuth, which melts below the boiling point of water, heating to a temperature of 25 (1, and observing the rate of cooling by means of a thermometer. The thermometer falls at first regularly as far as a certain degree, where it remains stationary for some time, after which it descends to a lower temperature, where it is again similarly arrested. These two stoppages in the rate of cooling can only be explained by admitting the production of a less fusible alloy in the fluid mass, which solidifies with an evolution of heat, rendering the thermometer stationary for a time. Each successive arrest will therefore correspond to the formation of more fusible combinations. Thus the metals form amongst themselves true chemical combinations ; and alloys are often formed by the mixture of one or more of these compounds with excess of one of the constituents. Recently hydrogen, which, although a gaseous substance, has chemical properties resembling those of the true metals, has been combined with palladium, sodium, and potassium, producing compounds similar in properties to the recognised alloys. PROPERTIES OF ALLOYS. Density. If the density of any alloy is calculated from that of the components assum ing that there is no condensation of volume the resulting number is sometimes greater than, equal to, or less than, the experimental result. Thus the alloys of gold and silver are less&quot;dense than the theoretical mean density ; whereas brass and the alloys of lead and antimony vary in the opposite direction. The former are therefore produced through an expansion, the latter through a condensation of their constituents. In the formation of many alloys there is no alteration of volume, and then the calculated density is correct. Colour. This is generally grey, unless when we have a coloured metal like copper or gold present in sufficient quantity. Hardness, Ductility, and Tenacity. Alloys are for the most part harder and more brittle, and are generally less ductile and possess less tenacity than the constituent metal that has these properties in excess. Aluminium bronze is an exception, as its tenacity is greater than that of either of the components. Fusibility. This is always greater than that of the least fusible metal entering into the composition of the alloy, and is sometimes greater than in any of the components. Thus an alloy, composed of 5 parts of bismuth, 3 of lead, and 2 of tin, melts at 91 C. Alloys of lead and silver, containing a small quantity of the latter, are more fusible than lead, and potassium and sodium form an alloy fluid at the ordi nary temperature of the air. Liquation. The constituents of an alloy heated gradually to near its point of liquefication frequently unite anew in such proportions as to form a mass that is fusible at the given temperature. If the fluid por tion is poured off, there remains a solid alloy less fusible than the original. Copper is separated from silver by this process. Decomposition. When the alloy contains a vola tile metal like zinc or mercury, heat decomposes it, but the temperature required to expel the last trace of th&amp;lt;; volatile metal must be considerably higher than that Coinage of gold,. 1 Gold, 90. Copper, 10. Specula of tele- 1 Copper, G7. Tin 33 Gold jewellery ( Gold, 75 to 92. Pinchbeck, ( Copper, 90. Silver coinage, ] Silver, 90. Brass, ] Copper, 67 to 7 Silver vessels, | Silver jewellery,., { Copper, 10. Silver, 95. Copper, 5. Silver, 80. Copper, 20. German silver, I Zinc, 33 to 28. ( Copper, 50. I Zinc, 25. ( Nickel, 25. ( Lead, 80. Aluminium ( Copper, 90 to 95. ( Antimony, 20. ITin 100 Bronze. Coins, J Medals j Copper, 94 to 9G. Tin, 4 to 6. English metal, Antimony, 8. Bismuth, 1. ( Zinc, 1 to 5. Copper, 90. Copper, 4 ( Tin, 92. Bronze. Bells,.... | Bronze. Cym- ( bals 1 Tin, 10. Copper, 78. Tin, 22. Copper, 80. Tin. 20. Liquid measures, Plumbers solder, I Lead, 8. ( Tin, 82. 1 Lead, IS. ( Tin, 67. I Lead. 33. metal s normal temperature of ebullition. Temper. The alloy of 94 parts of copper and 6 parts of tin forms a bronze so brittle that it may be pulverised with a hammer when it has been slowly cooled; but if, on the contrary, it is cooled rapidly by tempering it in cold water, it becomes malleable. Influence of the Constituent Metals. Mercury, bismuth, tin, and cadmium give fusibility to aUoys into which they enter; tin also gives hardness and tenacity if present in considerable quantity; lead and iron give hard ness; arsenic and antimony render alloys brittle. COMPOSITION or ALLOYS. A statement of the average proportions in which the metals enter into the best known alloys, the composition of which is generally very variable, is given in the following table : PREPARATION OF ALLOYS. The metals are generally fused together under a layer of charcoal to prevent oxida tion, thoroughly mixed by agitating, and the mass left to cool slowly. This process can only be employed when the constituent metals are all non-volatile at the temperature required for combination. If the mixture contains volatile metals, like sodium, potassium, magnesium, or zinc, they are added after the more refractory metal is fused. ALLSTON, WASHINGTON, an eminent American his torical painter and poet, was born 5th November 1779, at Waccamaw, in South Carolina, where his father was a planter. He early displayed a taste for the art to which he afterwards devoted himself. He graduated at Harvard in 1800, and for a short time pursued his artistic studies at Charleston with Malbone and Charles Fraser. He then removed to London, and entered the Royal Academy as a student of Benjamin West, with whom he formed a life long friendship. In 1804 he repaired to Paris, and from that city, after a few months residence, to Rome, where he spent the greater part of the next four years studying Italian art and Italian scenery. During this period he became intimate with Coleridge and Thorwaldsen. From 1809 to 1811 he resided in his native couutry, and from this latter date to 1817 he painted in England. After visiting Paris for a second time, he returned to the United States, and practised his profession at Boston (1818-30), and afterwards at Cambridge, Massachusetts, where he died on the 9th July 1843. He was elected an associate of the Royal Academy in 1819. The paintings of Allston are characterised rather by grandeur of conception than by skilful execution. In colour and the management of light and shade he closely imitated the Venetian school, and he- has hence been styled &quot;the American Titian.&quot; Many of his pictures have biblical subjects, and Allston himself had a profoundly religious nature. His first great painting, &quot;The Dead Man Revived,&quot; executed shortly after his second visit to England, gained a prize of 200 guineas from the British Institution ; in England he also prepared his &quot; St Peter Liberated by the Angel,&quot; &quot;Uriel in the Sun,&quot; &quot;Jacob s Dream,&quot; and &quot;Elijah in the Wilderness.&quot; To the period of