?2 COOLING BY EVAPORATION. 



tion, is greater than might be supposed, yet it corresponds with the ex- 

 tent of the provision. The length of the water-secreting tubing in the 

 skin of a man is about twenty-eight miles. 



Thus by the action of the kidneys and the skin large quantities of wa- 

 ter are dismissed, either under the liquid or vaporous form. A third or- 

 gan is concerned in this important duty. It is the lungs. These, how- 

 ever, are limited in their operation to its exhalation as vapor or steam. 

 That water abundantly escapes from them is plainly shown when the days 

 are cold, the moisture as it comes from the respiratory passages condens- 

 ing into a visible cloud when it encounters the air. It is estimated that 

 the loss of water by the skin and lungs conjointly is about 18 grains in 

 a minute, of which 11 pass off from the skin and 7 from the lungs. Mak- 

 ing due allowance for the variable action of the skin as dependent on the 

 dew-point and other such causes, we can scarcely set down the entire 

 quantity at less than 1000 pounds a year. In the same period the quan- 

 tity of water lost as urine may be taken at 900 pounds. It may perhaps 

 be remarked, that here we are assuming a loss of 1900 pounds, when the 

 quantity of water annually taken is only 1500 pounds. But it is to be 

 recollected that not only does water form a very prominent constituent of 

 the solid food, whether vegetable or animal, but also that much arises 

 from the oxidation of hydrogen in the interior of the system. 



2d. Water also exerts a cooling influence, arising from its evapora- 

 Cooling influ- tion from the surface of the skin and the cells of the lungs, 

 ence of water, rpj^ diff erence between water in the state of an invisible va- 

 por and in the liquid condition consists in this, that the vapor contains 

 1114 degrees of heat which the liquid does not. When, therefore, it 

 evaporates from a surface of any kind, as from the skin, it obtains there- 

 from that large amount of latent heat, and so tends to cause the tempera- 

 ture to decline. Not that this is the only cooling agency at work. * Ka- 

 diation might also be mentioned ; for, just as a warm inorganic body cools 

 by the escape of radiant heat from it, so too does a living being. 



These considerations explain how an equilibrium of temperature is es- 

 Equiiibrium of tablished. By the process of respiration there is a constant 

 heat in man. tendency to increase the heat ; but by evaporation of water, 

 radiation, and other cooling causes, tllere is a constant-tendency to dimin- 

 ish it. A balance is struck between the two processes, and in man a 

 temperature of 98 degrees is kept up. 



This average temperature is, however, easily departed from. Through 

 some trivial cause the cooling agencies may be interfered with, and then, 

 the heating processes getting the superiority, a high temperature or fe- 

 ver comes on. Or the reverse may ensue. In Asiatic cholera, the con- 

 stitution of the blood is so changed that its cells can no longer carry ox- 

 ygen into the system, the heat-making processes are put a stop to, and> 



