Heat, in a certain degree of intensity and duration, destroys the life of the part on which it acts. When it produces this effect directly, as it almost always does, it operates by altering the chemical affinities of the constituents of the tissue, and disposing them to form new combinations. The force of the affinities previously in operation, and the vital resistance are at once overcome, and decomposition takes place, with a consequent loss of life in the part.

But heat may also operate dynamically as an escharotic. Without immediately destroying life, it may produce an amount of inflammation greater than the vitality of the part can withstand; and gangrene follows as a direct result of the inflammation, and a secondary effect of the heat. it is never, however, practically employed with this intention. The effect is always uncertain; and the operation would be much more painful than that which aims at the immediate death of the part; as sensibility is lost with life, and the pain soon ceases. indeed, under the operation of an exceedingly intense heat, pain is sometimes scarcely felt. A workman in a furnace, while lading out melted iron, slipped, and allowed the contents of the ladle to fall into his shoe. The liquid metal surrounded his foot, and formed a sort of casing about it. Nevertheless, without appearing to suffer greatly, he walked unaided to his house, at a considerable distance. The whole surface of the foot was of course charred; but the action had been so instantaneous and intense, that sensibility was lost almost before there had been time for sensation.

Many different modes of applying heat, with a view to its escharotic effect, have been employed; but only three are now in use; namely the actual cautery, moxa, and galvanism.