252 METALS AND THEIR COMBINATIONS. 



Properties of metals. All metals have a peculiar lustre krrown 

 as metallic lustre, and all are more or less good conductors of heat 

 and electricity. The color of most metals is white, grayish, or 

 bluish-white, or dark gray ; a few metals show a distinct color, as, 

 for instance, gold (yellow) and copper (red). 



At ordinary temperatures metals are solids with the exception of 

 mercury, all are fusible, and some are so volatile that they may be 

 distilled. Most, probably all, metals may be obtained in a crystal- 

 lized condition. 



Metals show a wide difference in the properties of malleability, ductility, and 

 tenacity. Gold is both the most malleable and most ductile metal, while lead 

 possesses comparatively little of these qualities. In many cases heat increases 

 or develops malleability and ductility, but diminishes tenacity ; however, the 

 tenacity of iron, which surpasses that of any other metal, is not lessened by 

 heating. 



The term annealing denotes the process of restoring the malleability and 

 ductility of some metals after these properties have been diminished, by caus- 

 ing a change in the molecular structure of the metals through hammering, 

 rolling, or sudden cooling. Annealing consists in heating the metal and per- 

 mitting it to cool slowly (in a few cases quickly) in order to allow the cohesive 

 force to produce the most stable arrangement of the molecules. 



Tempering, which term at times is used analogously with annealing, consists 

 in heating the metal and chilling it suddenly. The result of annealing is the 

 highest development of softness and in case of some metals the restoration of 

 cohesiveness ; the object of tempering is the attainment of a certain degree of 

 hardness and elasticity. 



Elasticity, i. e., the power of recovering original form when twisted or bent, 

 and sonorousness, i. e., the property of yielding a musical sound when struck, 

 are possessed only by the harder metals, and to a high degree by certain 

 mixtures of metals. 



All metals expand when heated, but the rate, of expansion of the different 

 metals differs. Within certain limits of temperature the expansion of a metal 

 occurs uniformly in direct ratio to the increase in temperature. The great 

 expansibility of zinc is an important property of the metal when used as a die 

 in dental prosthesis. 



Metals do not combine chemically with one another. Their mix- 

 tures (alloys) still exhibit the* metallic nature in their general physical 

 characters. It is different, however, when metals combine with non- 

 metals ; in this case the metallic characters are lost almost invariably. 

 All metals combine with chlorine, fluorine, and oxygen ; most metals 

 also with sulphur, bromine, and iodine ; many also with carbon and 

 phosphorus, forming the respective chlorides, fluorides, oxides, sul- 

 phides, bromides, iodides, carbides, and phosphides. Metals replace 

 hydrogen in acids, forming salts. 



