174 PROOFS OF THE COMBUSTIBILITY OP DIAMOND.

To any disbelief of this matter it must be answered, that, ironexists every where under every form except in the state of per-fect purity or uninsulated : for the existence of pure nativeiron Is still doubtful. The wonder consists therefore only in thedifference between facts and our opinions; it vanishes in pro-portion as we discover and appropriate the powers of nature toproduce the same effects.

The diamond is advantageously used on account of its exces-sive hardness to engrave on hard stones and glass, or to cut theminto convenient pieces. It is also used as an ornament. Theextreme hardness, the labour of cutting and polishing it, toge-ther with its rarity, render it precious without the assistance ofthe caprice of fashion.

Before we proceed farther in the description of simple sub-stances, it will be necessary to illustrate die nature and produc-tion of gases, together with their properties; for the agency ofthese bodies produces so considerable a number of the pheno-mena which distinguish some bodies from others, that without aprevious acquaintance with diem, a great number of die ex-periments illustrative of the properties of simple substanceswould be unintelligible.