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3. Alumina exists in a pure and crystallized statein tlie white sapphire, and united to a little oxideof iron and silica in the other oriental gems. Inthe state in which it is procured by chemists, itappears as a white powder, soluble in acids andfixed alkaline liquors. From my experiments, itappears that alumina consists of one proportion 33of aluminum, and one 15 of oxygene.

4. Magnesia exists in a pure crystallized state,constituting a mineral like talc found in North Ame­ rica . In its common form it is the magnesia usta,or calcined magnesia of druggists. It generallyexists in soils combined with carbonic acid. It issoluble in all the mineral acids; but not in alkalinelixivia. It is distinguished from the other earthsfound in soils by its ready solubility in solutions ofalkaline carbonates, saturated with carbonic acid.It appears to consist of 38 magnesium and 15 oxy-gene.

5. There are two well known oxides of iron, theblack and the brown. The black is the substancethat flies off’ when red hot iron is hammered. Thebrown oxide may be formed by keeping the blackoxide red hot, for a long time in contact with air.The first seems to consist of one proportion of iron103, and two of oxygene 30; and the second ofone proportion of iron 103, and three proportions ofoxygene 45. The oxides of iron sometimes existin soils combined with carbonic acid. They areeasily distinguished from other substances by their