238 Chemistry. [Lecture 32. 



as one of the general effects of mixture upon all 

 bodies ; but it is very remarkable in metals. Dr. 

 Lewis, in remarking on the compounds of pla- 

 tinum with the different metals, says, he found 

 them sometimes to have more weight than ought 

 to have resulted from the combined weight of 

 the two, as when it was combined with iron. In 

 many cases the compound was lighter, the metal 

 rather swelling than contracting itself. 



Few metals are found in a pure state ; gold, 

 silver, and sometimes copper, are exceptions. 

 The other metals are generally found in the state 

 of ores, in which they are mixed and blended 

 with other substances, so as not to have the duc- 

 tility or other qualities of metals : often, indeed, 

 they have the metallic lustre. Sometimes the 

 ore is only a pure oxide, which requires no more 

 than that the oxygen should be drawn from it, by 

 heating it with a combustible substance. Such 

 are all the ferruginous ochres, which are oxides 

 of iron. 



The ores of metals are generally found in 

 the veins -of mountains or rocky strata, and are 

 always separated from the rocks on each side 

 by a quantity of spar, quartz, or sometimes soft 

 clay or earth. The spai is generally of the 

 gypseous kind. These form the matrix of the 

 ore; in England called the rider. In different 

 veins it is of different thickness; the quantity 

 of the ore increasing as that of the matter which 

 surrounds it diminishes. Often the ore is in 



