To understand this method of refrigeration, the limit has been calculated at different values of p, in cases where the temperature would be 30° C.; at the same time, the influence of vaporised water on the breathing has been ascertained; and finally the relative humidity of the internal air, on which the cutaneous transpiration depends, has been determined by supposing that the temperature of the fresh air, on its introduction to the lungs, was raised some 5°C.
The results arrived at are thus tabulated: -
Weight of the Vapour of Water per cubic metre of Air.
Lowering of Temperature
Temperature of the Air.
Humidity of the Air.
Introduced by evaporation, co.
Newly cooled, t - y.
Interior, t - y + 5°.
Relation (11) & (9)
The examination of this table leads to the following consequences, which are at once evident: - 1. The temperature decreases in proportion to the augmentation of humidity in the exterior atmosphere. 2. The relative humidity of the interior air is almost constant and equal at 0 74, which corresponds to 87° of the hair hygrometer, according to the tables of Gay Lussac, and to 82 according to those of August. The temperature of the fresh air does not rise in the interior in a mean quantity of 5° as supposed; it will be greater during greater refrigeration; but during slighter refrigeration, the relative humidity of the interior air, instead of being constant, then augments with the quantity of humidity contained in the exterior atmosphere. In summer, the humidity of the external air being always sufficiently great, the supposition which had been adopted tends rather to reduce the relative humidity of the interior air than to augment it. Lastly, an account has been taken of the individuals within the area who, by double perspiration, introduce new volumes of watery vapour into the surrounding air.
But it also leads to the unexpected result that the humidity introduced by the evaporation reduces pulmonary transpiration for an equal temperature of external air, no matter what its humidity, in an important proportion, sensibly constant, which, when that temperature is 30°, would be from 30 to 31 per cent., or nearly one-third. The action of interior air upon the animal economy is, in consequence of these diverse circumstances, different from that of external air.
The waste of heat increases by direct refrigeration, by contact and radiation; on the other hand, it diminishes by double perspiration. Are these variations in an inverse sense of the principal elements of this waste, taken together, advantageous ? Will the calorific equilibrium be more easy? and would it more closely approach the physical equilibrium? Let us take one of the cases in the table - that, for instance, where the moisture contained in the exterior air is 10 grm. per cub. metre. The temperature of the interior air will be lowered to 23.4° C, and thus will be 6 6° C. less than that of the external air. The direct refrigeration of 2.5 cals. per degree is composed of two terms almost equal in intensity, contact, and radiation; the first depends upon the temperature of the interior air, but the second specially upon that of the walls of the area, which is more elevated. Taking for the latter a mean between the temperature of the exterior and interior airs, the direct refrigeration will be greater in the interior than in the exterior by 12.5 cals. per hour.
Pulmonary transpiration being weaker by 3.43, the waste in this case would be diminished by 2 calories, and there would remain 10.5 cals., from which there is again to be deducted the heat corresponding to the reduction found in cutaneous transpiration. A reduction of 18 grm. an hour would compensate for these 10* 5 cals., and thus, in respect of waste, the same conditions would exist as in the exterior atmosphere. Can it attain this figure ? No exact experiments on the point exist.
It should be considerable, as all at once the temperature diminishes 6.6°, and the relative moisture rises from 0.32 to 0.74; in degrees of the hair hygrometer from 55° to 87°. The air, which is very dry, causing an abundant perspiration, becomes very moist, which only admits of very weak perspiration.
Skin perspiration depends much upon the quantity of watery vapour which the surrounding air can yet contain before arriving at a point of saturation. If it were proportionate to that quantity, it would vary, in the two cases, as 21.4 to 5 • 5; it would then be 4 times weaker in the interior air than in the exterior air. On the other hand, it is known that this perspiration very widely varies; different experimentalists have found the mean expressed in numbers from 30 grm. to 100 grm. an hour, according to circumstances. It is also known that in air at 15° C, half-saturated, and therefore capable of containing 6 •3 grm. of watery vapour per cub. metre more, the mean is 35 grm. per hour.
From these various considerations, it may be concluded that for a like play of the organs, the reduction of skin perspiration in the interior would at most be 18 grm. The calorific equilibrium (standard of heat) would require an additional effort from them to be established; the equally considerable reduction of pulmonary perspiration would be a further cause of trouble and discomfort. In short, the interior air, despite its being cooler, would be far more unfavourable than the exterior. The exterior air, just now taken at 23 4° C, to rise at 30° C, would require per cubic metre a volume of heat represented by 1.22 X 0.24 X 6.6 = 1.93 cal. The 3.43 grm, of watery vapour to be added to the exterior air at 23*4° C.t to form the interior air would give in a latent condition a volume of heat equal to two calories. The interior air also contains, although at a much lower temperature, a much more considerable volume of heat than the exterior air, which leads to the conclusion that the action of the surrounding atmosphere on the calorific equilibrium depends less upon its temperature than upon the quantity of heat contained in it. In summer the heat is far more insupportable and overwhelming when the air is very moist than in higher temperatures when it is dry.