their transpiration as affected by external agents." The transpiration of man was likewise found subject to the same general laws, but probably, in consequence of the greater delicacy of the human frame, aud the more complicated nature of his functions, it is more subject to be disturbed by external agents. A circumstance is noticed by Dr. Edwards, which had not been sufficiently attended to by his predecessors, that in endeavouring to ascertain the comparative amount of the transpiration under different circumstances, it is necessary to take intervals of considerable length, in order that the results may not be influenced by the effect of the fluctuations which are always occurring, and which constituted one principal source of the apparent irregularities, that produced so many anomalies in the older experiments. A period of six hours was thought to be necessary, and this he accordingly employed in all his researches. With respect to the different states of the air, its effects upon the cutaneous transpiration were essentially the same on man as on other animals; among other observations 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 although most of the vital functions appear to be diminished during sleep, it appeared upon the whole, that the transpiration was increased during this state.1

9 Ibid. part 3. C. 7.

1 De l'Influence, &c. part. 4. C. 11. Where the data are confessedly so insufficient, it may appear to little purpose to found

The author next proceeds to investigate the nature and source of the matter which is transpired. He

be desirable to form the

any calculation upon them; still it may best estimate that we have it in our power to deduce from them.' The numbers will be as follows:

Ingesta in 24 hours.

Food, according to Rye...

Oz.

96

Oxygen retained in the system, vide supra, p. 111....... 4.

Urine, according to

Egesta in 24 hours.

Alvine discharge, Bye, Rogers, p. 310, 1.

Do.

100

{40

Various other excretions, see Haller, El. Phys. xii. 2. 11. 3 Carbon discharged from the lungs, vide supra, p.111..... 11 Water expired, according to Menzies, vide supra, p. 107. 6

-

66

The ingesta will therefore exceed the visible egesta by 34 oz. which may therefore be assumed as the average amount of the transpiration in 24 hours. Perhaps, a farther addition should be made to this quantity, in order to compensate for the water which is absorbed by the skin; for, as I shall have occasion to observe hereafter, it is probable that an action of this kind takes place, although it appears quite impossible to ascertain its amount. An objection, which is not without considerable weight, has been urged against the supposition of so large a quantity of water being discharged from the skin, that many nations are in the habit of smearing the surface of the body with substances, which it is supposed must prevent any thing from being discharged from it, Haller, El. Phys. xii. 2. 19; and many individuals fall under our observation, in whom, from various accidental circumstances, the skin is frequently so covered with extraneous substances, as apparently to obstruct any discharge from its pores. The only method of obviating this difficulty is to suppose that the pulmonary transpiration is in this case proportionally increased to supply the deficiency.

begins by making a distinction between what is carried off from the body by evaporation, and what is removed from it by transudation; the first depending upon a mere physical operation, in which a substance is converted into vapour, by the addition of heat, while transudation is a vital process, of the nature of secretion or excretion. He observes that the terms evaporation and transudation are not synonymous with the insensible and sensible perspiration respectively of the older writers, because a part of what is removed by transpiration is first transuded, and then evaporated. Evaporation may take place from the dead body, while transudation can only take place from the living body; transpiration is, therefore, properly an operation of an intermediate kind, where the fluid is furnished by a vital function, while it is removed from the body by a mere physical process. The older physiologists were much divided respecting the question, whether the matter of the sensible and the insensible perspiration were originally the same substance, the former being in the fluid state, the latter in the form of vapour. Haller was inclined to suppose that they were essentially dif ferent, and Dr. Edwards appears to be of the same opinion, although it is not very clear, whether he considers that the whole, or only a part of the matter

I am not aware, however, that any experiments were ever performed on these individuals, and until this be done, it would be improper to speculate upon the subject.

2 El. Phys. xii. 2, 9,

of the insensible transpiration is not derived from the sensible transpiration.

The two operations of evaporation and transuda

tion being considered as, to a certain extent, indetion being considered as, to pendent of each other, it became an interesting object to endeavour to ascertain the degree in which they are each of them respectively exercised. For this purpose Dr. Edwards had recourse to cold-blooded animals, in which we can easily suppress the evaporation, by placing them in air saturated with moisture, and which will of course be nearly of the same temperature with themselves; in this case therefore we can obtain the loss by transpiration alone. By performing this experiment on frogs at a medium temperature not exceeding 68° (20 C.) the evaporation was found to be to the transpiration as 6 to 1; and as the transudation in this animal is very copious, we may infer that in man the proportional quantity of the evaporation is still more considerable. With respect to the comparative action of the skin and the lungs, it is supposed that what is lost by the lungs must be entirely due to evaporation, as nothing can be removed from them except what is carried off in the state of vapour, mixed with, or dissolved in the air of expiration, so that strictly speaking we have no pulmonary transudation. On this account we may presume that the loss by the skin will be greater than that by the lungs, although it must be expected that the former will be much more variable.

De l'Influence, &c. § 6. p. 331. et seq.

♦ Ibid. p. 334. et seq.

The distinction upon which Dr. Edwards so much insists between evaporation and transudation, although one of great importance, had been but little attended to by preceding physiologists. But on some occasions, I conceive it has been carried by him too far, as where it is maintained that the lungs can transpire only by means of evaporation and not by transudation, and that, in this respect, their action differs from that of the surface of the body. I should suppose, on the contrary, that in both cases, the matter to be transpired must leave the mouths of the vessels in the fluid form, and that the fluid is subsequently evaporated by means of the air, so that, except in degree, I do not perceive any essential difference between the operations.

There is another function that is nearly allied to the one which we have been now considering, and which has been made the subject of experiment by some of the modern physiologists; the chemical action which the skin has been supposed to exercise upon the air contiguous to it. Shortly after the discovery of the production of carbonic acid by the lungs, an inquiry was instituted whether the same kind of change was effected at the surface of the body; a circumstance which seemed in itself not improbable, the only difference between the relative situation of the air and the blood, in the lungs and in the skin, being the different thickness of the membrane by which they are separated. The first experiments on this subject appear to have been those of Millet, who, while the body was immersed in the