Cold is directly sedative; but, as it does not for a time lessen power, while the excitability of the depressed part is increased by its comparative rest, the necessary consequence is that, upon the withdrawal of the cold, and, in some degree, without its withdrawal, the ordinary normal excitants produce more than their ordinary effect, and the part is excited beyond its original condition Besides, the sensation of cold has, through a wise provision of nature, an excitant influence upon the nervous centres. causing them to send a stimulant impression to the circulatory system. by which injury is obviated. Through the operation of these principles, reaction follows the first depression produced by the cold; and this reae-tion is not confined to the part first impressed, but extends throughout the system. Thus, cold secondarily elevates the vital functions; and, as this effect is usually moderate, it must take rank among the tonics. The invigorating power of this agent has long been known, as a matter of observation; and it has been much employed in cases of local and general debility. Caution, however, in its use is very important. If too long continued, it at length exhausts excitability, and then produces a steady depression, without reaction. Too intensely applied, it rapidly exhausts the excitability; and the reaction, if produced at all, may be feeble, and soon cease. In proportion to the debility, is this indisposition to reaction; and a degree of cold, which would secondarily stimulate a healthy person, might prostrate still further one already much debilitated. Therefore, in its employment as a tonic, the degree and continuance of the cold should be proportioned to the remaining strength, and the remedy should be abandoned if found to be followed by a feeble reaction, or by none. When the skin is cool, and at the same time relaxed by perspiration, the ability to react is much diminished; and the remedy should never be employed in this state of the surface. When the reaction is insufficient, it should be aided by friction upon the skin, or by muscular exertion.

As a general rule, cold should not be used as a tonic when there is a tendency to dangerous internal congestions, as of the brain or lungs, and should, therefore, be avoided in organic diseases of the heart, which predispose to such congestions. The reason is obvious; namely, that the blood, driven by the cold from the surface, accumulates internally, and thus greatly adds to the pre-existing danger.

Methods of Application. The cold-air bath may sometimes be usefully employed. This is applied by simply stripping the body in a cold room, at a temperature from below the freezing point to 60° F., remaining thus exposed for a short time, then dressing, and aiding reaction by moderate exercise.

The cold-water bath is more efficient. For the purposes of a tonic. this should seldom have a lower temperature than 60°, and may be as high as 70° or 75° F. When too cold, there is the twofold danger, first that it may not be followed by reaction, and secondly that, if reaction do take place, it may rise too high, even so as to amount to a febrile paroxysm. There can, however, be no fixed point of heat applicable under all circumstances; so different are the susceptibilities of individuals even in health, and so much are these susceptibilities affected by habit, by the degree of debility, and by the varying influence of peculiarity in disease. The general rule is, that the temperature should be sufficiently low to produce a decided feeling of coldness, with shivering, paleness and contraction of the surface, and some reduction of the pulse. The duration of immersion should be no longer than sufficient to produce a decided impression, and should cease at once upon the occurrence of headache, pain in the stomach, cramps in the muscles of the extremities, general uneasiness, a purple colour of the lips, ears, fingers, etc., or other symptom indicating any material disturbance of the vital functions. There is usually a shivering on the first immersion; and a repetition of this should be the signal for leaving the bath. The time may be momentary, it may be for a minute or two, or it may extend, when the temperature is but moderately reduced, to fifteen minutes or more. From 70° to 75°, the bath is scarcely applicable as a tonic, except to cases of ready susceptibility, or considerable weakness, with less than the usual tendency to reaction. In health, the reactive influence, at this temperature, would generally be balanced by the sedative power of the water itself; and little tonic effect would be experienced. A decided, but not excessive reaction is the essentia] test of the propriety of the remedy; and trial must determine how great a coldness, and how long an immersion are necessary for this result. With a repetition of the remedy, an increase, in one or both of these respects, is generally necessary in order to sustain the original effect. Should but a feeble reaction or none occur, the remedy must be abandoned. Upon leaving the bath, the patient should be wiped quite dry, and then aid reaction by gentle exercise. If he be too feeble for this, the reaction may be promoted by moderate friction. The signs of a sufficient reaction are a general glow, the return of colour to the surface, a fuller and somewhat more frequent pulse, and a feeling of lightness, exhilaration, and increased muscular strength. The head should also be immersed or wetted, in order to prevent determination to that part. The cold bath should not be employed by women in advanced pregnancy, nor during menstruation. It is applicable, as a tonic, only to cases of chronic debility; and especially those of a nervous character, without serious structural lesion. It may be repeated daily, and should be taken rather on an empty than a full stomach. Probably the most suitable period in the twenty-four hours is early in the morning; for then the excitability of the system, having been recruited by rest, is greatest, and reaction will be most apt to take place. The patient, however, should not pass immediately from the warmth of bed into the bath, especially if perspiring. A little exercise previously is desirable, so as to induce a moderate action of the surface, but without perspiration. An hour or two before dinner is also a suitable period, if the excitability of the patient has not been impaired by physical exertion previously; but exposure to the hot sun is an objection to bathing, at this time of day, in the open air.*

The cold shower bath is often employed with reference to its secondary tonic effects. It is administered by causing water to fall over the body from a greater or less height, in minute streams, formed by passing the liquid through a vessel perforated at bottom by numerous small holes. A common colander may be employed for the purpose extemporaneously. The shower bath acts on the same principles as the cold bath; but the shock is somewhat greater, and the reaction, therefore, more speedy. It may be employed in similar cases, and with the same cautions. The time of continuance, for a given temperature, should be somewhat shorter.

The cold douche is often useful as a local corroborant. It consists in the continuous impinging upon a part of the body of a column of cold water, either falling upon the part, or forcibly impelled against it by mechanical means. It operates upon the same principles locally, as the cold bath does generally; that is, it first depresses, and then secondarily stimulates the part by the reaction. The affections to which it is applicable are old and persistent gouty or rheumatic swellings of the joints, obstinate and indolent tumours, local paralysis, debility of the joints following sprains, nervous deafness, certain conditions of amaurosis, and obstinate weakness of the eyes, sometimes following their acute diseases.

* As to effects produced on the system by simple baths, through the absorption of the water, they do not, from the most recent experiments, appear to be such as materially to modify their remedial influence. M. Frederic Duriau found that, whenever the temperature was below the normal heat of the body, water was absorbed, but never when above it; but the whole amount absorbed was small; not exceeding the average, in three persons, of 561 grains after an immersion of 75 minutes, at the heat of 77° F. (Archives Generales, Fev. 1856, p. 165).

Sea-bathing, or the cold salt-water bath, is still more efficacious than the simple cold bath, in consequence of the stimulant influence of the salt on the surface of the body; while, from the same cause, there is less risk of dangerous prostration. The reaction, under its use, is more speedy and certain, and from a less amount of antecedent depression; and patients can remain in it longer without exhaustion. Hence it may be employed in cases of debility, in which reaction, under the use of the simple cold bath, is imperfect or wanting. While adapted to chronic debility in general, it is peculiarly useful in scrofulous affections, as of the bones, joints, and lymphatic glands, both external and internal. Seabathing has long been considered as among the most efficacious remedies in these affections. In threatened and incipient phthisis, it may be resorted to with hope of benefit, when the air of the sea-shore is not found injuriously to irritate the lungs. Should it do so, the artificial salt bath should be substituted. But particular caution should be observed, in this disease, to abandon the measure if not attended with full reaction. The probability is that sea-water acts, in scrofula, not only as a tonic, but, through its iodine compounds, as an alterative also; and might be expected, therefore, to be more efficacious than mere salt water * The salt-water bath may be made by dissolving common salt in water, in the proportion of four avoirdupois ounces to the gallon. When a strong stimulant impression upon the skin is desired, in reference to a revulsive influence from within, the solution may be much stronger; but simply as a tonic, the strength mentioned, which is about that of sea-water, is probably preferable, at least as a general rule.