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bum in highly rarefied air, or under qon-siderable cooling agencies.

By heating strongly gases that burn withdifficulty, their continued inflammation be-comes easy, in consequence of increments ofheat occasioned by combustion of smallquantities, which under any other circum-stances would not produce continued com-bustion. Hence if mixtures of fire dampare burnt from systems of tubes or canals,or metallic plates, which have small radi-ating and cooling surfaces: though thesesystems are safe at first, they become dan-gerous as they are heated.* Where cur-rents are occasioned which concentrate ex-plosive mixtures by the air feeders in lampsbeing below,* and made in thick metallicplates or canals; there being an incrementof heat within, and a very small radiatingsurface without, as the heat increases, thecombustion of the explosive mixture willgradually extend further, and at last com-

* I warn the coal miner against any pretended safetylamps made in this manner, and which, to superficialobservers, may appear to be constructed upon principlesof security, but in which these principles cannot reallyexist.

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