Dr. Edwards next proceeds to inquire into the effects of TRANSPIRATION. A liquid transfusion from the skin of animals is constantly going on, either in the form of vapour or of fluid in a denser state.
The latter constitutes sweat. This phenomenon exhibits great variations, and it is important to know what diminution of weight the body suffers in different circumstances. In the course of an hour remarkable fluctuations occur.
Dr. Edwards suspended frogs, toads, and salamanders, in a calm air, weighed them, and noted the results, which, though very changeable in an hour, were generally uniform in three, and in nine hours they averaged an equal result. The successive diminution in the mass of fluids was evident.
The results were modified by the alternate position of the animals in a body of air in repose, or agitated by a draft. And these results do not appear to depend upon any principle of vitality, for they take place equally in death and in life, and indeed among unorganized bodies, as, for example, lumps of charcoal soaked in water. Therefore the cause of the phenomenon of transpiration seems to be referrible entirely to physical agents. The motion of the air seems to be its exciting cause; for even when, to all appearance, it is calm, it is in reality agitated more or less, and produces a sensible evaporation from the skin. But the difference between the effect of calm and agitated air is remarkable; for in a draft, the animals exposed to it sweated away double the quantity of liquid compared with those confined in a room shut up. The amount lost was proportioned to the intensity of the wind, and reached a triple amount over those animals in stagnant air; and this fact explains the variations noticed from hour to hour among animals exposed to currents of air.
The transpiration which occurs in very moist air, always amounts to a diminution of weight; but in dry air it is five or ten times greater; and when the influence of a moist state of the atmosphere is compared with that of a dry state, the amount of evaporation is equal to that of a dry and calm air.
Transpiration may, therefore, be referred to the agitation of the atmosphere for its exciting cause, beyond any modifications of its density. And, although an elevated temperature be favourable to transpiration, its modifying influence is less than that of other causes.
In comparing the effects of absorption and transpiration, [p152] in water and in air, frogs were found to gain an addition to their weight according to the term of their continuance in the former medium. An absorption of water was rendered evident by the loss of bulk it had sustained, when measured after the experiment.
Thus, when the comparative influence of water and air is estimated, the former appears to be absorbed, and adds to the weight of the body; and the latter tends to diminish the weight, by different and fluctuating degrees of evaporation taking place, and dependent much more on the degree of motion in the air, than on its dryness or humidity: these last conditions modify evaporation in a minor degree, when compared with the influence of a current of air.
The celerity of absorption exceeds that of transpiration six times, in the most rapid cases. It therefore results, that the losses by transpiration in air should be repaid by absorption of water in a much less time than the expenditure occurs. But the decrease of weight is not prolonged; it is sudden, and not continuous, alternating with augmentation of weight, by absorption of liquid going on in a ratio superior to the loss; and thus nature’s provision is manifested for the nutriment of the body.
With this last inquiry Dr. Edwards concludes the first part of his work; and it is observed, that, with regard to transpiration, the losses of weight have been considered without reference to the existence of any other influence than water. The losses by transpiration have been examined generally without regard to the matters lost. What relates to water differs essentially in one respect from that which regards the air. The losses sustained by the body ought to be more particularly examined. Temperature and loss of time require estimation. An excretion of solid matter evidently takes place; for the water, in which animals are submersed, becomes turbid, especially in hot weather, and it sensibly contains animal matters, affecting the weight of the body in water.