OXIDE OF MANGANESE, AND SILICA. 19 



of oxygen, it is always the peroxide that is formed : 

 hence we invariably find this oxide on exposed iron 

 surfaces, and in plants. 



The sub&tances hitherto described have all been 

 those that are found quite abundantly; but that which 

 is now to be mentioned, the Oxide of Manganese, is 

 more rare. Many species of our cultivated plants are 

 found to be without it in their ash far more often than 

 with it; and when it is present in the soil, we can 

 not, from any experiments hitherto made, see that 

 their growth is more luxuriant. In some trees it is said 

 to exist abundantly; but for the ash of our cultivated 

 crops generally, I am inclined to think that it can 

 scarcely be considered an indispensable constituent. 

 Manganese is a metal somewhat resembling iron, but 

 much lesi abundant. It also is always found in some 

 compound form, never as a pure metal. It forms 

 oxides with oxygen; and one of these, the black 

 oxide, is of much value in certain manufacturing pro- 

 cesses. For these purposes, it is mined whenever it is 

 found in large quantity. This black oxide may easily 

 be obtained and shown to a class. As it is now large- 

 ly used in some manufactures, it is a cheap article. 



Silica is a substance that exists abundantly in almost 

 all plants, often forming more than half of the whole 

 ash. a. We see a nearly pure form of it in the com- 

 mon quartz crystals, or agate, or cornelian, or flint : 

 these all consist almost entirely of silica. Specimens 

 of silica, in some form, may be found in almost every 

 neighborhood, as it is one of the most common minerals. 

 When perfectly pure, it is a very hard, white sub- 

 stance, tasteless, and quite difficult to melt. The fine 

 grains in ordinary sandstones are particles of silica. 

 b. It is not dissolved in water, and even strong acids 

 produce little effect : how singular then that it should 

 be found so abundantly in the interior of plants! 



