As cold water may be usefully applied in local compresses, hot water is often of the greatest service applied in fomentation, i.e., when a thickly folded flannel, or any thick absorbent substance, such as spongio-piline, is completely wrung out of hot water, and placed on the affected part, covered with dry flannels, oil-silk, or mackintosh outside, to prevent evaporation and retain heat, and changed again frequently, the process being continued for half an hour, or even for several hours if necessary. This stimulates the external skin much more strongly and suddenly than any cold compress, for every degree above the normal skin-temperature is felt much more acutely than every degree below it (Gully), and it acts much better than the more equable heat given by hot salt, hot bran, or tins, or caoutchouc bags of hot water, because the high temperature is more constantly renewed.

If there be congestion, or even inflammation of an internal organ, it may be relieved by such external application which is especially indicated when the patient is too feeble to react to cold, or when the pain and irritation are very severe, and "of the mingled nervous and inflammatory kind, with, if anything, an excess of the former." The direct application of quite hot water causes contraction of small vessels and also of the uterus.