des Agens Physiques siir la Vie, 309' 



stances when neither the temperature of the air, nor their 

 covering, can explain the phenomenon. 



Under certain limitations of temperature, the passage of 

 fluid from the surface of the body is accelerated or retarded 

 as the air is more or less saturated with moisture. When 

 the air did not exceed 20° Reaumur in its temperature, Dr. 

 Edwards found a moderate dryness renders the losses of 

 weight by transpiration six or seven times greater than in 

 cases of extreme moisture. 



The union of vital and physical causes tends to make the 

 phenomena in these experiments appear complex in their 

 conclusions, and render calculations liable to error. If the 

 air be dry, the waste by transpiration, compared with that 

 of moist air, exceeds, on account of the evaporation of water 

 being most abundant in a dry atmosphere. And this occurs 

 equally in the dead and living body as a physical law. But, 

 in the living state, the increase of transpiration will vary from 

 causes withi?i, which modify those acting without the system. 



When the air is agitated, the temperature of strata of air 

 in contact with the body is affected, and so also is the degree 

 of moisture. If the warm and moist air close to the body 

 be replaced with a cooler air, evaporation is indefinitely in- 

 creased. The slightest and most imperceptible agitation in 

 the air produces its effect upon transpiration. 



The pressure of the atmosphere becoming lessened, tlie 

 evaporation of liquids is a necessary physical result. In 

 order to ascertain if this took place also in the living body. 

 Dr. Edwards placed one animal under a bell-glass, from 

 which part of the contained air was removed, while another 

 was exposed to the pressure of the air without. The result 

 was, that the animal in the rarefied air lost more weight than 

 the one under ordinary pressure. 



Besides the cutaneous transpiration, vapour is constantly 

 exhaling from the lungs, as may be seen by breathing on a 

 mirror, or into a glass vessel. Sanctorius and others knew 

 of this; but their deductions from experiments on the relative 

 quantities are incorrect. Lavoisier and Seguin arrived at 

 better conclusions ; for chemistry and physics in general were 

 more understood in their time. These authors estimate the 

 mean daily loss by the lungs and the skin at two pounds 

 thirteen ounces, of which one pound fourteen ounces go off 

 by the skin, and fifteen ounces by the lungs. The loss thus 

 sustained is derived partly from the pulmonic water evapo- 

 rated, ^nd partly from chemical changes effected by respira- 



