The reader is already familiar with the effect of heat in promoting perspiration. Why this power should have been given to it, will be rendered obvious by a moment's consideration. Heat above the normal standard is injurious to the system, and must be abated. Perspiration, through the evaporation of the liquid upon the surface, at the moment of elimination, and the consequent absorption of free heat in the vapour, has the effect of reducing or keeping down the temperature of the surface, and secondarily of the system at large. Now, the perspiratory function has been made such, in its susceptibilities, as to feel and respond to the influence of heat, whether applied directly to the skin, or operating on it through the nervous centres, from any other point of accumulation, especially the stomach. Hot water, therefore, taken into the stomach, immediately brings on perspiration through this sympathetic action. When employed, however, as a diaphoretic agent, it is important not to use it in excess, as otherwise we may overshoot the mark, and entirely suppress the function of the skin by over-excitement. in a state of health, there is scarcely any supportable degree of heat which will not cause perspiration; but, when the skin is already hot and dry, as in fever, the addition of heat will often only serve to aggravate the affection.

Exercise ordinarily promotes perspiration. This happens in part because the current of blood is transmitted more rapidly through the skin; but the chief cause is probably the generation of heat, which, if in excess, promotes perspiration in order that it may be reduced to the just point.

Dry heat will generally produce a diaphoretic effect; but this is much increased by the conjoint agency of moisture. Water favours diaphoresis by its relaxing influence upon the coats of the vessels, and, when taken internally, by adding to the volume of the blood. it, therefore, brings quite different agencies in aid of the heat, which operates only by increasing the current through the vessels, and immediately stimulating the function of the sudoriferous follicles. There are consequently few influences more powerfully diaphoretic than that exercised by a combination of heat and water.

Of the different modes of applying heat, whether moist or dry, internally or externally, sufficient has been said either above, or under the head of the diffusible stimulants, to which the reader is referred. (See vol. i. p. 485 to p. 500.)* in chronic inflammation, and especially in chronic rheumatism, the sudorific influence of either dry or moist heat, externally employed, is often extremely serviceable. it may be used also in chronic gout and chronic paralysis; and, in chronic skin affections, it is among the most efficacious remedies.

* Much attention has recently been paid in Great Britain to the dry hot-air bath as a remedy in disease. When the air is perfectly dry, a much higher temperature can be borne than when it is mixed with steam; and, at these high temperatures, perspiration is powerfully excited, with the effect, it is asserted, of eliminating morbid substances from the system, thus operating favourably in a long list of diseases. For an account of what has been done in this way, the reader is referred to a work on the Turkish bath and heat as a mode of cure by Sir John Fife, M.D., drawn from the writings of Mr. Urquhart, and published in London, a.d. 1865. (See also B. and F. Medico-chir. Rev., Jan. 1866, p. 91.) - Note to the third edition.

Therapeutic Application

Moist heat is often highly advantageous as a diaphoretic, at the very commencement of various inflammations, before the disease has become firmly fixed, or the febrile phenomena fully developed. This remark is especially true of catarrhal and anginose affections, and acute or subacute rheumatism. it is usually employed in such cases internally, in the form of hot teas, given at bedtime; but is equally effectual when externally used, as by a vapour bath. Care, however, is always necessary, in these cases, to avoid exposure to the cold next morning. I have known serious internal inflammation result from a neglect of this caution.

It may be employed also to hasten the appearance of eruptions, and to invite the return of those which have been repelled.

In chronic enteritis, diarrhoea, and dysentery, chronic calculous affections connected with the deposition of uric acid, obstinate dropsies especially when dependent on disease of the kidneys, and all cases of obscure disease, in which the existence of a poison in the blood may be suspected, which there may be a hope of eliminating by the skin, indications are offered for the use of this remedy.

The diaphoretics may be arranged in three divisions, each characterized by distinctive properties; 1. the nauseating, 2. the refrigerant, and 3. the stimulating or alterative.