48 MINERALOGY. 



on the borders of a stream, the material which has been 

 drifted down previously may have much gold in it. To 

 bring it to light, the stream is turned across it, to per- 

 form on a large scale the same operation of washing that 

 is done in a small way with the pan. The processes 

 necessary for separating the gold when it is alloyed with 

 silver or copper I will not stop to describe. 



93. Uses of Gold. The usefulness of this metal results 

 from its malleability, its ductility, its rich color, its sus- 

 ceptibility of a high polish, and its indisposition to tarnish 

 on exposure. A gold-beater can hammer a grain of gold 

 into a leaf covering a space of 50 square inches, and its 

 thickness is only the 282,000th of an inch. Its ductility 

 equals its malleability, as is shown in the wire of gold 

 lace. It is from these two qualities that gilding is so 

 cheap, and therefore so common, thereby making this 

 costly metal a great convenience to the people at large. 

 For ordinary uses, gold is alloyed with silver and copper 

 to give it the requisite hardness. The standard gold of 

 the United States contains nine parts of pure gold to 

 one of alloy. The word carat, used so often in speaking 

 of the purity of gold in the market, means one twenty- 

 fourth ; so that if any specimen is said, for example, to 

 be twenty carats fine, it is meant that the pure gold is to 

 the alloy as 20 to 4, and so of other proportions. 



94. Platinum and its Associate Metals. Platinum is 

 seldom found alone, but is commonly mingled with more 

 or less of certain rare metals iridium, rhodium, palla- 

 dium, and osmium and such common metals as iron 

 and copper. It occurs usually in small grains, but some- 

 times is found in pieces of considerable size. The lar- 

 gest mass that has yet been found weighed 21 pounds 

 Troy. Platinum is often spoken of as being the heavi- 

 est substance known, but there is one metal, iridium, 

 that is a very little heavier. Its color is between that 

 of tin and steel, though in the alloys in which it is usu- 

 ally found it is darker than this. It is very malleable, 



