This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
By an action contrary to that of cold baths, these attract the blood primarily and directly to the part exposed to their influence, relaxing the vessels and tissues, and leaving them afterward in a condition of lessened tonicity. A similar effect is exerted by all kinds of warm baths, but it differs in degree according to the temperature and duration. By a tepid bath is meant one at from 70° to 80° F., and this is chiefly used for cleansing and moistening the skin - a temperature of 92° to 98° F. gives a warm, and upwards to 112°, a hot bath. With the former there is at first a pleasant sense of soothing and refreshing warmth, the skin reddens, and the pulse quickens in frequency, while it lessens in tension; the respiration is also quickened, and the temperature rises. If the bath be too prolonged, a sense of languor comes on, and after it there is less aptitude for exertion than before. Under favorable conditions, excretion is increased from the skin, the kidneys, and the lungs, at the time of the bath, and oxidation is lessened subsequently.
In a hot bath, 98° to 112° F., the first sense of heat may be painful rather than pleasant, then a general stimulating effect is perceived, the whole surface becomes deep red, and the cutaneous veins distended; relief is thus given to internal congestion, to pain, and muscular spasm and convulsion. Complete muscular relaxation follows, with greatly diminished tension of the pulse, and increased frequency of heart-action; before very long, a sense of oppression and distention may be felt in the chest and head with general languor, giddiness, or faintness from paralysis of vasomotor and cardiac inhibitory nerves. These unpleasant effects occur much sooner in some persons than others.
By a hot bath perspiration is usually, but not always, increased; and sometimes from the high temperature of internal organs a restless, heated condition, similar to that of true pyrexia, may be induced for a time. This may be noticed, especially after a too-prolonged use of the hot strong saline baths (Droitwich, etc.).
The length of stay in a hot bath should depend on the purpose to be accomplished, whether (1) mere excitation of circulation in the skin (which is effected by a short bath with or without the extra stimulus of salt or mustard), or (2) perspiration and relief of pain (which require, perhaps, half an hour), or (3) complete muscular relaxation (which needs a prolonged immersion). In contradistinction to the ultimate tonic effect of the cold bath, decided loss of weight results from a course of warm bathing (A. Steffen).
Since the thoracic organs and the brain become more or less congested during a hot bath, its prescription needs as much caution as that of cold bathing, though for a different reason: pulmonary or cerebral vessels may even give way, especially if the latter be atheromatous. According to Dr. Steffen, hot-water baths are pre-eminently contra-indicated by congestion or oedema of the lung, and Dr. H. Wood has seen under such conditions, "the most frightful dyspnoea result." In such an accident, cold effusion should be freely used.