METALS. 203 



ties. For example, an alloy of copper and tin, in the pro- 

 portions of 90 of the former and 10 of the latter, called 

 speculum metal, is as brittle as glass and almost white. 

 Now if it were merely a mixture, its color should be that 

 of copper lightened by the small proportion of tin, as zinc 

 lightens the copper-color in brass, and the tin should give 

 to it but a slight degree of brittleness. A single example 

 more will suffice. There is an alloy which is sometimes 

 used as a source of amusement, for teaspoons made of it 

 will melt in a cup of very hot tea. It is composed of 8 

 parts of lead, 15 of bismuth, 4 of tin, and 3 of cadmium. If 

 it were only a mixture, the melting point of the alloy would 

 be somewhere between the melting points of its constituents. 

 But in fact it is far below them. Lead must be heated to 

 330 to melt it, bismuth to 258, tin to 235, and cadmium 

 to 315; but this alloy melts at about 70 that is, 30 be- 

 low the boiling point of water. 



280. Ores. The ores of metals are certain compounds 

 from which the metals are usually obtained. These com- 

 pounds are commonly oxides or sulphides. When any 

 metal is found in its uncombined state it is said to be 

 native. Some metals, as gold and platinum, are always 

 found in this state, and therefore, strictly speaking, have no 

 ores, though this word is sometimes loosely applied to them. 

 Such metals are often found alloyed with other metals. 

 Thus gold is usually alloyed with silver, copper, etc. Silver 

 is found in the three conditions, native, alloyed, and com- 

 bined. The word ore is not applied to all combinations of 

 metals, but only to those which are used in obtaining the 

 metals. Thus the carbonate of iron is an ore ; but the car- 

 bonate of calcium, occurring in the different forms of chalk, 

 limestone, marble, etc., is not an ore. So while an oxide 

 of iron is an ore, limestone, the carbonate of calcium, is 

 not. 



