200 



ether secretion, is in smaller quantity during sleep, than while we are 

 awake; in old age than during infancy, in weak persons, and in damp 

 weather, than under the opposite circumstances. 



The perspiration may be said to be in a compound ratio of the force 

 with which the heart propels the blood into the minute capillary arteries, 

 of the vital energy of the cutaneous organ, and of the solvent powers of 

 the atmosphere. The strongest and most robust men perspire most ; 

 some parts of the skin perspire more than others, as the palms of the 

 hands, the soles of the feet, the arm-pits, &c. When the air is warm, 

 dry, and frequently renewed, cutaneous perspiration is greater, and the 

 necessity of taking liquid aliment is more urgent, and more frequently ex- 

 perienced ; in summer, as every body knows, a profuse perspiration is 

 brought on by passing from the heat of the sun into the shade; and, on 

 no occasion is a copious sweat more easily brought on, than by taking 

 exercise in summer, when on the approach of a storm, the atmosphere, 

 containing a small quantity of vapours, and warm from the rays of the sun, 

 which shows itself, now and then, surrounded by the clouds, is little capa- 

 ble of dissolving the insensible perspiration. 



The skin may be covered with sweat, without any increase of the cuta- 

 neows perspiration; this may happen from dampness in the air, or from 

 its being imperfectly renewed. It must be owned, however, that sweat- 

 ing is more frequently occasioned by an increase of the insensible perspi- 

 ration, and that the warmth of the bed which excites it, acts by increasing 

 the power of the organs of circulation and the energy of the cutaneous 

 system. The body is weakened by sweating, which is seldom the case 

 with the insensible perspiration. A profuse sweat is attended with a 

 very speedy exhaustion ; thus, in hectic fever, in the suelte (sudor angli- 

 cus) and other affections, equally dangerous, it is the cause of a wasting 

 almost universally fatal. 



The matter of the insensible perspiration and of the sweat, is, in great 

 measure, aqueous. Like the urine, it holds in solution several salts, also 

 the volatilized recrementitious matter of animal substances, sometimes 

 even acids, as in the case in which Berthollet detected the phosphoric 

 acid in children affected with worms, in pregnant women, in nurses, from 

 whom there exhales an odour manifestly acid. It may contain ammonia, 

 and, on certain occasions, the smell enables us to discover that alkali, in 

 the sweat or perspiration. 



The air which constantly surrounds our body, does not merely dissolve 

 the aqueous vapour which arises from it, but several physiologists very 

 reasonably conjecture, that the oxygen of the atmosphere may combine 

 with the carbon of the blood brought to the skin by the numerous vessels 

 which are sent to it, and likewise with the gelatine forming the substance 

 of the rete mucosum of Malpighi. 



The experiments of Jurine, of Tin gray, and of several other naturalists, 

 show that carbonic acid is constantly formed on the surface of the skin, so 

 that the skin may be considered as a supplementary organ to that of re- 

 spiration 5 and in that point of view, one may compare to it, the mucous 

 membranes which are in contact with the atmospherical air in the nasal 

 fossae, and in the intestinal canal which they line. 



The cutaneous perspiration is, likewise as was before mentioned, a 

 powerful means of cooling the body, and of keeping it while living, in an 

 uniform temperature. The water which is exhaled from the whole sur- 

 face of the body, carries offfrom it, in passing into a vapour, aconsidera- 



