OXIDE OF MANGANESE, AKD SILICA. 19 



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

 hence we invariably find thi^ v- ' on exposed iron 

 surfaces, and in plants. 



The substances hitherto described have all been 

 those that are found quite abundantly; but that which 

 is now to be mentioned, the Oxide of Manganesty is 

 more rare. M t our cultivated plants are 



finrnd to be wi: ir ash far more of^en than 



with it; and whtii ii i t in the soil, we eaa 



not, from any experirn therto made, see tliat 



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 

 ■careely be considered an indispensablt- '*' lent 



li « n g ^n«><i c is a metal so m e w i ia t reaembli: but 



■ueii leas abundant. It also is always found iu some 

 teflB|x>und form, never as a pure metal. It forms 

 oxides with oxyG:en; and one of these, the black 

 oxide, is of much value in certain manufacturing pro- 

 cesMa. For tht-st- purposi's, it is mined whenever it is 

 found in large quantity. This black oxide may easily 

 be obtained and shown to a class. .\s 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, oflen 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 : 

 thes«> all consist almost entirely of silica. SpecioMOt 

 of silit .1. in some foTO, may be found in alnKtst every 

 :. as it is one of the most common minerals. 

 Wl.ii. jr, ;! tly pure, it is a very hard, white sub- 

 stance, tasti I< ss, and quite difficult to melt The fine 

 riins in ordinary sandstones are particles of silica. 

 It is not dissolved in wat«>r, and evoi strong acids 

 nro«)> rfrtH:;t: how si ' hen that it should 



be f<' . . liiundantly in tt; n>rof plants! 



