METALLURGY 



3751 



METAMORPHISM 



Some metals, sftch as gold, silver and plati- 

 num, are found in nuggets or grains in an almost 

 pure state. The surrounding rock is crushed, 

 . and the metal is separated by picking or wash- 

 ing in machines (see GOLD; MERCURY). In the 

 case of some other ores, a process known as 

 fusing with a flux is employed. A flux is a 

 substance which will combine with the rocky or 

 metallic envelope of the metal and allow it to 

 be readily melted. The resulting slag is lighter 

 than the molten metal ; the latter sinks and can 

 then easily be drawn off at the bottom of the 

 furnace. 



Many metals are found combined with sul- 

 phur or with oxygen. Among the oxide ores 

 are those of the important metals, iron, man- 

 ganese and tin. These are freed by heating the 

 ores with carbon (see IRON). Silver, copper, 

 zinc, lead and mercury are commonly made 

 from sulphide ores. The first operation in 

 treating a sulphide ore is usually "roasting." 

 This consists in heating the ore in a current 

 of air. The sulphur combines with the oxygen 

 of the air and is driven off in the form of sul- 

 phur dioxide gas. In some instances the metal 

 is left free, as is mercury. In others, the metal, 

 as copper and zinc, is converted into an oxide 

 which is then reduced by smelting with carbon. 



Amalgamation. This process is frequently 

 employed in extracting gold and silver. When 

 brought into contact with mercury, the crushed 

 ore readily unites with it, forming an amalgam. 

 The mercury is then removed by distillation, 

 leaving the gold or silver in a free state (see 

 MERCURY; GOLD; SILVER). 



Another so-called wet process consists in dis- 

 solving the gold or silver from the ore by 

 chemical action and then precipitating it (drop- 

 ping it to the bottom of its receptacle), either 

 by using another chemical or by applying elec- 

 tricity. In one process for the extraction of 

 gold, the solvent used is potassium cyanide; in 

 another it is chlorine. Both of these reagents 

 are extremely poisonous. 



Electrolysis. In this process the ore is de- 

 posited in a solution of the metal which it is 

 desired to free. . The ore is attached to the 

 positive electrode and a plate of similar metal 

 to the negative electrode. When the current 

 is turned on the metal is dissolved from the 

 ore and deposited on the metal plate. This 

 process will be found described in the articles 

 ELECTROTYPING and ELECTROLYSIS. It is advan- 

 tageous in freeing copper. J.P.S. 



Consult Hoffman's General Metallurgy; Rob- 

 erts-Austen's Introduction to Study of Metallurgy. 



METALS, met'alz, one of the two great 

 classes of elements (see CHEMISTRY, subhead 

 Elements), or any material resembling these. 

 Metals are usually lustrous, malleable, ductile, 

 strong and good conductors of heat and elec- 

 tricity. The six metals known to the ancients 

 were gold, silver, copper, tin, iron and lead. 

 The alchemists of the Middle Ages recognized 

 mercury as a liquid metal and associated each 

 of the seven metals with 'one of the heavenly 

 bodies, as follows: gold, the sun (Sol); silver, 

 the moon (Luna); copper, Venus; iron, Mars; 

 tin, Jupiter; lead, Saturn; mercury, Mercury. 

 Modern chemists have discovered many addi- 

 tional metals. Some of these, such as lithium, 

 sodium, potassium, calcium, magnesium and 

 aluminum, are much lighter than those pre- 

 viously known. Others, such as platinum and 

 iridium, are heavier than even gold. Alloys of 

 the metallic elements are also commonly called 

 metals. Among such are brass, bronze, magna- 

 tium, bell metal, gun metal, Britannia metal, 

 type metal, etc. 



Related Subjects. The following articles in 

 these volumes, most of them dealing with specific 

 metals but a few with more general topics, will 

 be found helpful : 



Alloy Magnesium 



Aluminum Manganese 



Antimony Mercury 



Arsenic Metallurgy 



Babbitt Metal Minerals and 



Barium Mineralogy 



Bismuth Nickel 



Brass Osmium 



Bronze Palladium 



Calcium Platinum 



Chemistry Potassium 



Chromium Radium 



Cobalt Silver 



Copper Sodium 



Copper Glance Steel 



Galena Strontium 



Gallium Thorium 



Galvanized Iron Tin 



German Silver Titanium 



Gold Tungsten 



Iridium Uranium 



Iron Vanadium 



Lead Zinc 



Lithium 



METAMORPHIC, met.amawr'fik, ROCKS. 

 See METAMORPHISM, below. 



METAMORPHISM, metamawr'fiz'm, from 

 Greek words meaning change in form, is the 

 name given by geologists to changes which 

 rocks have undergone since their formation. 

 The agencies causing these changes are heat, 

 pressure, chemical action and moisture, but 

 heat and pressure are the most important. The 

 changes include: 



