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intercept the flame of a spirit lamp, but notthat of hydrogene; and when stronglyheated, it will no longer arrest the flame ofthe spirit lamp. A tissue which will notinterrupt the flame of hydrogene when redhot, will still intercept that of olefiant gas,and a heated tissue which would com-municate explosion from a mixture of ole-fiant gas and air, will stop an explosion froma mixture of fire-damp, or carburetted hy-drogene.

The ratio of the combustibility of thedifferent gaseous matters are likewise to acertain extent as the masses of heatedmatter required to inflame them.* Thusan iron wire of one-fortieth of an inchheated cherry red, will not inflame olefiantgas, but it will inflame hydrogene gas; anda wire of one-eighth, heated to the samedegree, will inflame olefiant gas; but awire of one-five-hundredth must be heatedto whiteness to inflame hydrogene, though

* It appeared to me in these experiments, that theworst conducting and best radiating substances requiredto be heated higher for equal masses to produce thesame effect upon the gases; thus red hot charcoal hadevidently less power of inflammation than red hot iron.