This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Heated air may be employed for the same purposes. The body will support a much higher temperature of the surrounding air when it is dry, than when moist, because in the former state it is a much worse conductor. Persons have even remained, for a short time, in chambers heated considerably above the boiling point, without suffering material inconvenience. According to MM. Berger and Delaroche, when the temperature is between 150° and 190°, a smarting sensation is produced in the surface, particularly in the conjunctiva and nostrils, the veins swell, the skin becomes burning hot, the pulse is very greatly accelerated, even to 100 in a minute, the respiration is laboured, and vertigo, headache, and other disordered cerebral phenomena are experienced; but very soon a copious perspiration breaks out, and tends in a considerable degree to obviate the unpleasant effects. (Forbes, Cyclop, of Pract. Med., Am. ed., i. 286.) It is very seldom necessary, however, in order to obtain any desirable stimulant effect, that the temperature should exceed 150°; and often a much lower heat will answer. The stimulation is greatest when the patient is wholly immersed in the hot air, so as to inhale it into the lungs; but it is a safer and equally effectual plan, for all practical purposes, to allow the hot air to be applied only to the surface of the body, while the patient breathes air of the ordinary temperature. As, in many of the cases to which the remedy is applicable, there is an indication for revulsion from the interior to the surface, the latter plan is preferable on this account For the modes of preparing the hot-air bath, see page 70, in the first part of this work.
The hot-air bath is probably more frequently used with a view to produce perspiration than as a mere stimulant. For the latter purpose, however, it may be resorted to with much benefit in certain cases of prostration, with coldness of the surface, to which more particular reference will be made when the therapeutic applications of the hot-water bath are considered. As this cannot always be commanded, the hot-air bath, which can generally be quickly prepared and applied with facility, may often be substituted for it with advantage. * water; and its excitant influence, even when it is only in very moderate excess, is experienced before the water has fairly begun to operate; so that a short time must elapse before the balance is established. Nor does the exact balance continue long. Under the stimulus of heat, the excitability is impaired, and the excitation consequently gradually ceases, and is at length followed by depression, according to the general law; while the sedative effect of the water is increased, the longer that agent continues to operate. Hence, the short period during which the two are balanced is followed by a period of depression much greater than the antecedent elevation, and increasing with the continuance of the agent. The bath, therefore, of which the temperature is but slightly above that of the surface, can scarcely be considered as a stimulant agent; the excitant effects being moderate and very brief, and the sedation soon predominating. It is, therefore, as a sedative remedy that it is almost uniformly employed; and as such it will be considered hereafter, under the name of the warm bath. The title of hot bath should be confined to that in which the heat is felt at first rather disagreeably, and in which the excitant effect decidedly predominates, and continues to predominate for a considerable time. It is the operation of this variety of the bath that is now to be considered.
* Through the efforts of Mr. Urquhart and others, the use of the hot-air hath has recently received great extension in England, with extraordinary therapeutic results. Of the various public and private baths, erected within the last seven or eight years, one connected with the Newcastle Infirmary, under the supervision of Sir John Fife, has yielded a large experience of this mode of cure, which, in the course of five years, has been employed in more than 20,000 cases. The complaints b. Moist Heat. In moist heat, two quite distinct agents are combined, which must be considered separately before their joint influence can be well understood. Heat is purely stimulant; water, purely sedative; and the result of their combined action will depend on the proportion in which they respectively exist in the combination. The influence, too, of cold, that is, of the diminution of heat below the normal standard of the surface of our bodies, must be taken into account. This is sedative; but differs in this respect from the sedative influence of water, that its depressing effect is always attended with a disposition to reaction, which is wanting in the operation of the latter. The sedative properties of cold and water will be more fully considered in a more appropriate place. They are here alluded to, merely to make more intelligible the properties and effects of heat and moisture combined. Water may be applied to the body, with a view to remedial action, at three different temperatures; below, at, and above that of the surface in health; so as to produce, in the first instance, the sensation of cold; in the second, no sensation either of cold or heat; and in the third, that of heat. It is obvious that, by a bath answering to the first condition, the joint depressing influence of cold and of water would be produced; to the second, the purely sedative influence of water; and to the third, the combined sedative influence of water, and stimulant influence of heat. The first two conditions of the bath may be left out of view in this place, as not belonging to the subject of stimulation. It is to the third only that our attention is to be now directed; that, namely, in which the temperature of the bath is above that of the surface, so as to occasion the sensation of heat.