Seventhly. When circumstances are most favourable, the greatest loss of weight, caused by insensible transpiration, was, according to their observations, 32 grains per minute; consequently 3 ounces, 2 drachms and 48 grains, poids de marc, per hour; and 5 pounds in twenty-four hours; under the calculation that the loss is alike at all hours of the day, which is not the fact.

Eighthly. When all the accessory circumstances are least favourable, provided only that digestion is properly accomplished, the smallest loss of weight is 11 grains per minute; consequently, 1 ounce, 1 drachm and 12 grains per hour; and 1 pound, 11 ounces and 4 drachms in the twenty-four hours.

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Ninthly. Immediately after eating, the loss of weight, caused by the insensible perspiration, is 10 grains per minute, during the time at which all the extraneous causes are most unfavourable to transpiration; and 19 grains per minute, when these causes are most favourable and the internal causes are alike. "These differences," says M. Seguin, "in the transpiration after a meal, according as the causes, influencing it, are more or less favourable, are not in the same ratio with the differences, observed at any other time, when the other circumstances are equal; but we know not how to account for the phenomenon.'

Tenthly. The cutaneous transpiration is immediately dependent both on the solvent virtue of the circumambient air, and on the power possessed by the exhalants of conveying the perspirable fluid as far as the surface of the skin.

Eleventhly. From the average of all the experiments it seems, that the loss of weight caused by the insensible transpiration is 18 grains per minute; and that of these 18 grains, 11, on the average, belong to the cutaneous transpiration, and 7 to the pulmonary.

Twelfthly. The pulmonary transpiration, compared with the volume of the lungs, is much more considerable than the cutaneous, compared with the surface of the skin.

Thirteenthly. When every other circumstance is equal, the pulmonary transpiration is nearly the same before and immediately after a meal; and if, as an average, the pulmonary transpiration be 17 grains per minute before dinner, it is 1776 grains after dinner. Lastly. Every other intrinsic circumstance being equal, the weight of the solid excrements is least during winter.

Although these results are probably fairly deduced from the experiments; and the experiments themselves were as well conceived as the subject admits of, we cannot regard the estimates as more than approximations. Independently of the fact, that the envelope of taffeta must necessarily have retarded the exhalation, and caused more to pass off by pulmonary transpiration; the perspiration must incessantly vary according to circumstances within and without the system; some individuals, too, perspire more readily than others; and its extent is dependent, as we have seen, upon climate and season, and likewise upon the quantity of fluid received into the

digestive organs. From all these and other causes, Bichat is led to observe, that the attempt to determine the quantity of the cutaneous transpiration is as vain as to endeavour to specify what quantity of water is evaporated every hour, by a fire, the intensity of which is varying every instant.

Since the time of Lavoisier and Seguin, Dr. Edwards has made some experiments, for the purpose of illustrating the effect produced upon cutaneous transpiration by various circumstances, to which the body is subjected. His first trials were made on coldblooded animals, in which the cutaneous transpiration can be readily separated from the pulmonary, owing to the length of time, that they are capable of living without respiring. All that is necessary is to weigh the animal before and after the experiment, and to make allowance for the ingesta and egesta.

In this way he discovered, that the body loses successively less and less in equal portions of time; that the transpiration proceeds more rapidly in dry than in moist air; in the extreme states nearly in the proportion of 10 to 1; that temperature has, also, considerable influence, the transpiration, at 68° of Fahrenheit, being twice as much; and, at 104°, seven times as much as at 32°. He likewise found, that frogs transpire, whilst they are in water, as is shown by the diminution, which they experience while immersed in that fluid, and by the appearance of the water itself, which becomes perceptibly impregnated by the matter excreted by the skin.

In warm-blooded animals, he found, as in the cold-blooded, the transpiration become less and less in proportion to the quantity of fluid evaporated from the body; and he observed the same difference between the effects of moist and dry air, and between a high and a low temperature. The effects of these agents were essentially the same on man as on other animals. He found, that the transpiration was more copious during the early than the latter part of the day; that it is greater after taking food; and, on the whole, appeared to be increased during sleep.

Whenever the fluid, which constitutes the insensible transpiration, does not evaporate, owing to the causes referred to at the commencement of this article, it appears on the surface in the form of insensible perspiration or sweat. It has been supposed by some physiologists, that the insensible and sensible perspiration are two distinct functions. Such appears to be the opinion of Haller and of Edwards, but no sufficient reason seems to exist why we should not regard them as different degrees of the same function. It is, indeed, affirmed, that the sweat is generally less charged with carbonic acid than the vapour of transpiration, but that it is richer in salts, which are deposited on the skin, and are sometimes seen in the form of white flocculi; but our knowledge on this matter is extremely vague.

Particular parts of the body perspire more freely, and sweat more

readily, than others. The forehead, armpits, groins, hands, feet, &c. exhibit the evidences most frequently; some of these, perhaps, owing to the fluid, when exhaled, not evaporating readily,—the contact of air being impeded. It is presumed, likewise, that the sweat has not every where the same composition. Its odour certainly varies in different parts of the body. In the armpits and feet it is more acid; in the violent sweats, accompanying acute rheumatism, this acidity always attracts attention; in the groins, its odour is strong and rank. It differs too greatly in individuals, and especially in the races. In the red-haired, it is said to be unusually strong; and in the negro, during the heat of summer, it is alliaceous and overwhelming. By cleanliness, the red-haired can obviate the unpleasant effects, in a great measure, by preventing undue accumulation in the axillæ, groins, &c.; but no ablution can remove the odour of the negro, although cleanliness can detract from its intensity. Each race appears to have its characteristic scent; and, according to Humboldt, the Peruvian Indian, whose smell is highly developed by education, can distinguish the European, the American Indian, and the negro, in the middle of the night, by the sole evidence of this sense.

Some physiologists have doubted whether the odorous matter of the skin belongs properly to the perspiration, and have presumed it to be the product of specific organs. This is, however, conjectural; and the experiments of Thenard, as well as the facts we have just mentioned, would rather seem to show, that the matter of sweat itself has, within it, the peculiar odour. The fact of the dog tracing its master to an immense distance, and discovering him, perhaps, in a crowd, has induced a belief, that the scent may be distinct from the matter of sweat; but the supposition is not necessary, if we admit the matter of perspiration to be itself odorous.

Besides the causes before referred to, the quantity of perspiration is greatly augmented by running or by violent exertion of any kind; especially if the temperature of the air be elevated. Warm fluids favour it greatly, and hence their use, alone or combined with sudorifics, where this class of medicines is indicated. Magendie conceives, that being readily absorbed, they are also readily exhaled. This is true; but, in hot climates, ice-cold drinks are as rapidly followed by sensible perspiration, owing, probably, to the copious exhalation which is constantly going on diminishing the quantity of fluid circulating in the vessels; and, we know that, under such circumstances, the activity of absorption is largely augmented.

With regard to the uses of the insensible transpiration, it has been supposed to preserve the surface supple, and thus to favour the exercise of touch; and, also, by undergoing evaporation, to aid in the refrigeration of the body. It is probable, however, that these are quite secondary uses, under ordinary circumstances and that the great office, performed by it, is to remove a certain quantity of fluid from the blood: hence it has been termed by Broussais the cutaneous depuration. In this respect, consequently, it bears a VOL. II.

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striking analogy to the urine, which is the only other depuratory secretion, with the exception of the pulmonary transpiration, which, we shall find, essentially resembles the cutaneous.

It can, therefore, be readily conceived, that any interruption to this necessary exhalation should be attended with equally serious effects as in the case of the urinary depuration. Most diseases are, indeed, produced probably by irregularly impeded cutaneous transpiration. By exposure to currents of air, or to the irregular action of cold in any manner to the surface, the depuration of the part is morbidly modified; and, owing to the extensive sympathy existing, as we have elsewhere seen, between every part of the capillary surface, any organ, which may be at the time particularly predisposed to irritation, is affected with disease. Sudden atmospheric vicissitudes are not so liable to excite such partial disease, as when the check to perspiration is more local and irregular; the exhalation from the lungs taking place in greater abundance, so as in some measure to compensate for the diminished cutaneous exhalation.

As the sensible transpiration or sweat is merely the insensible perspiration in increased quantity, its uses demand no special notice.

The pulmonary transpiration, to which we have so often alluded, bears a striking analogy to the cutaneous. At one time, it was universally believed to be owing to the combustion of the air with the hydrogen and carbon given off from the lungs; but we have elsewhere shown, that no such combustion occurs; and besides the exhalation occurs when gases, containing no oxygen, have been respired by animals.

It is now universally admitted to be exhaled into the air-cells of the lungs from the pulmonary artery chiefly, but partly from the bronchial arteries, distributed to the mucous membrane of the airpassages.

Several interesting experiments have been made on this exhalation, by Magendie, Milne Edwards, Breschet, and others. If water be injected into the pulmonary artery, it passes into the aircells, in an innumerable quantity of almost imperceptible drops, and mixes with the air contained in them.

Magendie found, that its quantity might be augmented at pleasure on living animals, by injecting distilled water, at a temperature approaching that of the body, into the venous system. He injected into the veins of a small dog, a considerable amount of water. The animal was at first in a state of real plethora, the vessels being so much distended that it could scarcely move; but, in a few minutes, the respiration became manifestly hurried, and a large quantity of fluid was discharged from the mouth, the source of which appeared evidently to be in the pulmonary transpiration considerably augmented.

Not only, however, is the aqueous portion of the blood exhaled

in this manner. Experiment shows that many substances, when introduced into the veins by absorption, or by direct injection, issue by the lungs. Weak alcohol, a solution of camphor, ether and other odorous substances, when thrown into the cavity of the peritoneum or elsewhere, were found, by Magendie, to be speedily absorbed by the veins and conveyed to the lungs, where they transuded into the bronchial cells, and were recognised by the smell in the expired air.

Phosphorus, when injected, exhibited this transmission in a singular and evident manner.

Magendie, on the suggestion of M. Armand de Montgarny, "a young physician," he remarks, " of much merit," now no more, injected into the crural vein of a dog, half an ounce of oil, in which phosphorus had been dissolved; and, scarcely had he finished the injection, before the animal sent through the nostrils clouds of a thick, white vapour, which was phosphorous acid. When the experiment was made in the dark, these clouds were luminous.

More lately, MM. Breschet and Milne Edwards have made several experiments, for the purpose of discovering why the pulmonary transpiration expels so promptly the different gaseous and liquid substances received into the blood.

Considering properly, that exhalation differs only from absorption in taking place in an inverse direction, these gentlemen conjectured, that it ought to be accelerated by every force that would attract the fluids from within to without; and such a force they conceive inspiration to be, which, in their view, solicits the fluids of the economy to the lungs, in the same mechanical manner as it occasions the entrance of the air into the air-cells. In support of this view, they adduce the following experiments.

1. To the trachea of a dog, a pipe, communicating with a bellows, was adapted, and the thorax was largely opened. Natural respiration was immediately suspended; but artificial respiration was kept up by means of the bellows. The surface of the air-cells was, in this way, constantly subjected to the same pressure; there being no longer diminished pressure during expiration, as when the thorax is sound, and the animal breathing naturally.

Six grains of camphorated spirit were now injected into the peritoneum of the animal; and, at the same time, a similar quantity was injected into another dog, whose respiration was natural. In the course of from three to six minutes, the odorous substance was detected in the pulmonary transpiration of the latter; but in the other it was never manifested.

In the first animal, they now exposed a part of the muscles of the abdomen, and applied a cupping-glass to it; when the smell of the camphor speedily appeared at the cupped surface. The conclusion was obvious, that the pulmonary surface, having ceased to be subjected to the suction force of the chest, during inspiration, the exhalation was arrested, whilst that of the skin was developed as