For this purpose, heat is usually employed in the form of boiling-hot water. This has the recommendation of acting with great rapidity. Rubefaction is speedily produced, followed almost immediately by vesication. But the intense pain, the frequent indisposition of the denuded surface to heal, the danger of carrying the effect so far as to produce gangrene, and the difficulty of precisely limiting the application, are objections which prevent a resort to this measure, except under extraordinary circumstances.

In cases of great and alarming prostration, in which the surface is insensible to the most powerful rubefacients, vesication by boiling water may sometimes be not only justified, but called for. The influence of heat, in torpor and coldness of the surface, seems to restore it in some degree to the normal state of impressibility; and an effect is produced to which no other agent, however stimulating, is adequate. The sudden sinking spells of typhoid or malignant fevers, the prostration amounting to syncope from shocks on the system, the condition approaching apparent death sometimes attendant on intense gastric or other internal spasms, threatened death in angina pectoris when the heart has nearly ceased to beat; these, and other analogous cases sometimes offer the opportunity for the effective application of this remedy. Of course, it is only the first arousing impression that is sought for from it; for the subsequent support of the system, other measures must be depended on.

Boiling-hot water may also sometimes be employed to produce a limited vesication for the endermic application of remedies, when it is of great importance that the application should be promptly made, and other vesicating agents capable of speedy action, as the strong solution of ammonia, or one of the stronger mineral acids, may not be at command.

Hot water may be best limited, by applying it carefully, by means of compresses of linen wet with it, or perhaps preferably by a sponge. its operation should be watched closely, so as to prevent the death of any part of the surface, and a consequent slough.

A plate of metal, heated to 212° by immersion in boiling water, has also been used to produce vesication. (See Escharotics.)