368 HUMAN PHYSIOLOGY 



capacity to contain water vapor. We have said that 

 this happens when air fully saturated, but cooler than 

 the tissues, is respired. It happens also when such air 

 blows over the surface of the body. High humidity is, 

 accordingly, not a serious obstacle to evaporation from 

 the skin unless the air is exceptionally warm. 



The coincidence of high temperature with high 

 humidity is uncomfortable and even dangerous. We 

 know that the most trying days of the summer are not, 

 as a rule, the ones on which the highest records are 

 made by the thermometer; they are rather the days 

 which we characterize as sticky and lifeless. Loss of 

 heat from the body is impeded by the limitation of 

 evaporation as well as by the actual warmth of the sur- 

 roundings. It has been found that men working in 

 deep mines where it is both warm and moist may have 

 continuous fever temperatures. Their situation is almost 

 intolerable. The miner may be in a much worse position 

 than the stoker of the steamer who endures a much 

 hotter atmosphere. 



The air that is brought into the fireroom of the steamer 

 is fresh from the cool exterior and when it is heated it 

 comes to have a very low relative humidity. It is well 

 adapted to take up the water which is secreted in such 

 abundance by the toilers. We must call attention to the 

 fact that we cannot judge the amount of the perspiration 

 by the appearance of the skin. So long as evaporation 

 fully keeps pace with the production there may be no 

 visible moisture. It is when evaporation lags behind the 

 outpouring that we notice the drops. We are actually 

 most conscious of the perspiration when it is failing to 

 accomplish its purpose. An English student of these 

 problems has asserted that a man cannot work with 

 safety in fully saturated air at 90F., but can be pro- 

 tected by warming the same air by 40. The paradox is 

 easily explained. Heating the air gives it a low relative 

 humidity and the evaporation which was very slight at 

 90 becomes extremely rapid at 130. 



