3. Gold is rarely employed in a state of perfect purity, but is generally used in combination with some other metal, which renders it harder, and consequently more capable of enduring the friction to which it is exposed. The metal used for this purpose is called an _alloy_, and generally consists of silver or copper. 4. For convenience in commerce, this precious metal is supposed to be divided into twenty-four equal parts, called _carats_. If perfectly pure, it is denominated gold 24 carats fine; if alloyed with one part of any other metal or mixture of metals, it is said to be 23 carats fine. The standard gold coin of the United States and Great Britain is 22 carats fine; or, in other words, it contains one-twelfth part of alloy. Gold, made standard by equal parts of copper and silver, approaches in color more nearly to pure gold than when alloyed in any other manner. 5. Gold is found in veins in mountains, most usually associated with ores of silver, sulphurets of iron, copper, lead, and other metals. It is often so minutely distributed, that its presence is detected only by pounding and washing the ores in which it exists. But the greatest part of the gold in the possession of mankind, has been found in the form of grains and small detached masses, amid the sands of rivers and in alluvial lands, where it had been deposited by means of water, which had detached it from its original position in the mountains. 6. To separate or extract gold from the foreign matters with which it may be combined, the whole is first pounded fine, and then washed by putting it in a stream of water, which carries off the stony particles, while the gold, by its specific gravity, sinks to the bottom. To render the separation still more perfect, this sediment is mixed with ten times its weight of quicksilver, and put into a leather bag, in which it is submitted to a pressure that forces the fluid part through its pores; while the more solid part of the amalgam, which contains most of the gold, remains. 7. To separate the quicksilver from the gold, the mass is subjected to the process of _sublimation_ in earthen retorts, which, as applied to metals, is similar in its effects to distillation, as applied to liquids. When gold is contained in the ores of other metals, they are roasted, in order to drive off the volatile parts, and to oxydize the other metals. The gold is then extracted by amalgamation, by liquefaction with lead, by the aid of nitric acid, or by other methods adapted to the nature of the ore.