EFFECT OF. EXTERNAL TEMPERATURE 515 



The influence of climate upon the respiratory exchange is due 

 not to the temperature alone, but also to other factors, such as 

 moisture and wind. The capacity of dry air to take up heat is 

 much less than that of moist air ; hence it happens that in dry, 

 calm air several degrees below zero the sensation of cold may be 

 much less than that felt in moist air with a temperature a few 

 degrees above the freezing point. On the other hand, a hot moist 

 atmosphere, by preventing the evaporation of sweat, produces a 

 more marked feeling of heat than dry air at the same temperature. 

 The more stationary the cool air is, the less the loss of heat, for 

 the body becomes surrounded with a layer of air which is warmer 

 than that of the atmosphere. Between the clothing of a man and 

 the fur or feathers of an animal strata of air are enclosed, the 

 temperature of which gradually increases above the outside air, 

 until the layer in actual contact with the greater part of the body 

 is 24 to 30. A cold wind quickly reduces the temperature of 

 these different layers. The sailors of Parry's expedition to the 

 Polar Seas found that when the air was perfectly calm they could 

 bear better intense cold, 40, which would freeze mercury, than 

 a temperature of 12* 2, when there was a wind. 



The depressing effects of hot moist climates are well known. 

 European children languish in hot humid climates, but rapidly 

 improve in health in the cooler hill-country or in England. Cold 

 increases their respiratory exchange, general metabolism, and 

 activity, and is favourable to a vigorous and sturdy growth. The 

 success also of the open-air treatment of phthisical patients de- 

 pends, it would seem, not so much upon any greater purity of the 

 air, as upon free exposure ; the open air and exercise increase the 

 combustion and respiratory exchange, improve the appetite, and 

 augment the metabolism. A slightly greater pressure of oxygen 

 in the atmosphere is not the important factor, for in many of the 

 health resorts it is, owing to the altitude, less than the normal, and 

 actual experiments upon men and animals show that small changes 

 have little or no effect : a high percentage of carbon dioxide is 

 in itself no strict indication of an unhealthy city or dwelling, it is 

 a sign that there is no free ventilation, and therefore a tendency 

 for the air to become stagnant, humid, and warm. It would even 

 appear that the number of bacteria affords no certain test of the 

 purity of air. Evidence steadily accumulates to show that the 

 bad effects of overcrowded rooms are due, not to diminished 



