a word applied to denote that property of some bodies, by which they resist the most violent fire without any sensible alteration. Apyrus bodies ought to be distinguished from those which are refractory. Refractory substances are those which cannot by violent heat be fused, whatever other alteration they may sustain. But a body, properly speaking, apyrus, can neither be fused by heat, nor undergo any other change. Diamonds were long thought to be possessed of this property; but experiments have shown that diamonds may be entirely dissipated or evaporated by heat, and are therefore not entitled to be ranked among apyrus substances. Perhaps there is no body in nature essentially and rigorously apyrus. But it is sufficient that there be bodies apyrus relatively to the degree of fire which art can produce, to entitle them to that name.