COMBINATION OF OXIGEN WITH IRON.

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In like manner every other metal, as we have seen already*,combines with certain proportions of oxigen, and forms eithefrtwo or more oxids, according to its nature.

Whenever metals are capable of forming more than one oxid,it is only the first oxid, or that which contains a minimum ofoxigen, which is formed by the direct combination of oxigenwith the metal, for oxigen is capable of uniting with metals, orwith any other substance for which it has an affinity, only inone determinate proportion.

Iron, for instance, and oxigen can only combine in the pro-portion of parts of iron, and 27 of oxigen. These twoquantities saturate each other, and form a compound which isincapable of receiving into it any more oxigen or iron : this com-pound is the Hack or green oxid of iron.

How comes it then, it will be asked, that there is anotheroxid of iron, the brown, or red oxid, which contains 52 parts ofiron, and 4S of oxigen, proportions certainly very different from73 and 27 ? Because there is an affinity between the green andblack oxid of iron and oxigen. They are capable of combining to-gether, and of saturating each other in the proportion of about 71 - 5parts of black oxid, and 28.5 of oxigen, and the compound whichthey form is the brown, or red oxid, which of course contains 52parts of iron, and 48 of oxigen: But then it is not formed bythe combination of these two substances directly, but by thecombination of the black oxid and oxigen. In like manner leadand oxigen combined form grey oxid of lead, (page 110) ; greyoxid of lead and oxigen combined form white oxid ; white oxidof lead and oxigen combined form red oxid; which last oxid,when combined with oxigen, forms the brown oxid of lead.

That this is the case, cannot be doubted, if we consider that,in general, every one of these successive doses of oxigen isretained by a weaker affinity than the dose which precedes it.Thus the brown.oxid of iron is composed of iron, and two doses

Vol. 11. I