It was formerly the custom to accumulate all these stimuli for particular purposes, thinking that the greater the heat, the more copious would be the discharge. Physicians, in this plan, were frequently disappointed by their own eagerness. They could produce burning heat, and a clammy fluid on the skin, which appears to be the serosity, with a large proportion of the gluten of the blood; but they soon found that this was not of the nature of sweat, nor so efficacious as the more fluid discharge. We were brought nearer the truth by Dr. Alexander, who found, by his experiments, that the temperature of the body must be often reduced before the proper discharge takes place. The temperature, at which sweating most freely occurs, he fixes at 108° of Fahrenheit. We have found, by some experiments, that this heat is too high, and suspect the accuracy of his thermometer. We are confident that the highest point is 102°; and that generally the heat, when the sweat is most free and salutary, does not exceed 100°.
To this reduction of temperature, rather than to its increase, we must attribute the effects of many diaphoretics. A draught of cold water will often, in this way, excite a free diaphoresis; and cold affusions, during the height of the febrile paroxysm, will have the same effect. We have striking instances of this kind in the practice of Dr. Currie and Dr. Gregory in scarlatina. Acid drinks have a similar power; and to this cause we attribute the diaphoretic effects of vinegar whey, the whey of milk, and similar drinks.
There is, however, a class of stimulant diaphoretics which act more gradually, and almost insensibly, by mixing with the blood, and stimulating, in the course of their circulation, the extreme vessels. From what has been observed under the article of Argentum 4 B vivum, it will be obvious that mercurials are remedies of this kind, and we are inclined to refer all the metallic tonics to the same head. Arsenic will probably be found a similar remedy. There are many vegetable substances which may be referred to this class, particularly the serpentaria contrayerva, the mezereon, and the guaiacum. The seneka, which may appear to belong to it, owes, more probably, its diaphoretic effects to the action of vomiting which it excites.
To these more moderate stimuli of the extreme vessels, some of the gentler exercises maybe added. Sailing, riding in a carriage, and swinging, require bodily exertion in the same order; but are apparently similar in their effects of determining to the surface. Riding on horseback, walking, dancing, running, digging, tennis, and ringing, require still more violent exertion, and at last excite copious sweat.
External stimulating diaphoretics are, friction, rubefacients,warm plasters of Burgundy pitch, euphorbium, and cummin seeds; and blisters, which excite copious partial perspiration, previous to their vesication.
The relaxing diaphoretics are much more powerful in their operation; and at the head of this list is opium. There are few ancient sudorifics which have not this medicine as an ingredient in a greater or less proportion; and of whatever nature the diaphoretic is, the addition of opium makes it more active and successful. Antimonials are equally useful, and ipecacuanha scarcely yields to them in this power. It has been doubted, whether these two last medicines act independently of the nausea they produce. We have little hesitation in thinking that they do so, for their effect is not in proportion to the degree of nausea, and is sometimes considerable, when no such previous power is obvious. Squills seem to connect the nauseating medicines with those which act on different principles. No diaphoretic effects are strikingly produced by them in moderate doses; but it may be presumed that such exist, by the balance observable between the skin and kidneys; for whatever may excite the discharge from one organ, if its operation is prevented, stimulates the other. Vomiting constantly relaxes the vessels on the surface, and every medicine which produces it is a diaphoretic. The whole tribe of narcotic vegetables, with their deleterious effects, produce cold sweats. Yet these we should not employ as salutary medicines; though we suspect that the aconite, and some others, employed in chronic rheumatism, or that hybrid disease which seems to connect gout and rheumatism, act in no other way.
Such are the remedies employed as diaphoretics, and such, in general, the principles on which they act. We must now attend to their effects on the animal economy.
The connection between the state of the extreme vessels, the system in general, and the stomach, has been already noticed. This has been attributed to a nervous sympathy, and probably is owing to such a connection, since the effect is more sudden than can be explained in any other manner. To keep up the action of these vessels must consequently be always of the greatest importance, since health is inconsistent with a contrary state; and, in the whole circle of acute and chronic com-plaiuts,no single circumstance requires greater attention.
When we reflect, however, on the extent of the surface of the body, we shall find, that to fill the extreme vessels will require no inconsiderable proportion of the fluids. To prevent, therefore, as well as to relieve, internal congestions, this class of remedies is of the greatest utility. If, however, carried to excess, no evacuation so greatly debilitates; and though all secretory organs,after their action has been violently excited, sink into a torpid state, the vessels of the skin seem peculiarly disposed to this alternation of inactivity after exertion. If then it is intended to relieve congestion, we must be peculiarly cautious to excite no greater action than we can constantly keep up. In all such cases, therefore, the slightest diaphoresis is only admissible. In fact, we should fill the vessels, instead of promoting any considerable evacuation.