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THOMSON'S THEORY OF COMBUSTION

powder, called yellow oxid of lead. If this yellow oxid be againexposed to heat, it absorbs still more oxigen, and becomes con-verted into red oxid of lead. All these oxids are therefore sup-porters. When the supporters, thus formed by the combinationof oxigen with products, are made to support combustion, theydo not lose all their oxigen but only the additional dose whichconstituted them supporters. Of course they are again reducedto their original state of products of combustion. Hence it fol-lows, that they owe their properties as supporters, not to thewhole of the oxigen which they contain, but to the additionaldose which constituted them supporters. We may therefore callthem partial supporters, indicating by the term, that part onlyof their oxigen is capable of supporting combustion and not thewhole.*

All the partial supporters with which we are acquainted, con-tain a metallic basis ; for metallic oxids are the only products atpresent known capable of combining with an additional dose ofoxigen. It is a circumstance highly deserving attention thatwhen metals are capable of combining with several doses ofoxigen, the product or oxid formed by combustion is seldom ornc\ er that which contains a maximum of oxigen.

Thus it is evident that several of the products of combustionare capable of combining with oxigen. The incombustibility ofproducts, therefore, is not owing to their want of ufpnity for ox-igen, but to some other cause.

5. So product of combustion is capable of supporting combus-tion. This is not occasioned by any want of affinity for combus-tible. bodies; for several of them are capable of combining withan additional dose of their basis. But by this combination theylose their properties as products, and are converted into combits-

* Dr. Thomfon suppofes that it is very possible that both nitrogen and muriaticacid may be products of combustion ; and in that case both the compound andpartial fupporters would agree with each other in every refpeft. In the prefentstate of our knowledge, however, it is neceflary to distinguish them.