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4. Magnesia exists in a pure crystallized state* 
constituting a mineral like talc found in North 
America. In its common form it is the magnesia 
usta, or calcined magnesia of druggists. It gener- 
ally exists in soils combined with carbonic acid. 
It is soluble in all the mineral acids ; but not in 
alkaline lixivia. It is distinguished from the other 
earths found in soils by its ready solubility in 
solutions of alkaline carbonates, saturated with 
carbonic acid. It appears to consist of 38 mag- 
nesium and 15 oxygene. 
5. There are two well-known oxides of iron, 
the black and the brown. The black is the sub- 
stance that flies off when red-hot iron is ham- 
mered. The brown oxide may be formed by 
keeping the black oxide red-hot, for a long time 
in contact with air. The first seems to consist of 
one proportion of iron 103, and two of oxygene 30 ; 
and the second of one proportion of iron 103, and 
three proportions of oxygene 45. The oxides of 
iron sometimes exist in soils combined with car- 
bonic acid. They are easily distinguished from 
other substances by their giving, when dissolved in 
acids, a black colour to solution of galls, and a 
bright blue precipitate to solution of prussiate of 
potassa and iron. 
6. The oxide of manganesum is the substance 
commonly called manganese, and used in bleach- 
ing. It appears to be composed of one proportion 
of manganesum 113, and three of oxygene 45. It 
is distinguished from the other substances found 
in soils, by its property of decomposing muriatic 
acid, and converting it into chlorine. 
