HEAT AND CLOTHING 237 



with the greatest comfort while the temperature of the 

 air is about 70 degrees, which is but little more than half- 

 way between the temperature of freezing and the heat of 

 the blood. A temperature of 80 degrees feels too warm, 

 while 90 degrees is hot, and 98 J, or the temperature of the 

 body, is oppressive. 



In winter a temperature of 70 degrees in a living room 

 feels neither warm nor cold, and the change between it 

 and the outside cold air is less noticeable than at any other 

 temperature. A temperature of 75 or 80 degrees feels too 

 warm, and when the person goes out of doors the cold air 

 produces a sudden contraction of the arteries and a chill, 

 which often results in taking cold. A sleeping room should 

 be at a lower temperature than a living room. 



418. Clothing. Man protects his body against the loss 

 of heat by covering it with clothes. Some kinds of sub- 

 stances readily permit heat to pass through them, and are 

 called good Jieat conductors, while others carry heat poorly 

 and are called poor conductors. Linen is a good conductor 

 of heat. It is a poor protection against cold, for it lets 

 out the heat of the body, but it makes good summer cloth- 

 ing. When the linen clothing is adjusted to one tempera- 

 ture, a change to cooler air is quickly and suddenly felt. 

 Thus it is an undesirable clothing material in changeable 

 climates or in cold weather. 



Cotton also conducts heat readily, but if it is loosely 

 woven, the air in its meshes makes it a poor conductor of 

 heat. Then it makes warm clothing. 



Wool is a poor conductor of heat. When the tempera- 

 ture is suddenly lowered, it permits the heat of the body 

 to pass off but slowly, and thus gives the skin time to ad- 

 just itself to the change. In summer it retains too much 

 heat, and does not make so good summer clothing as cot- 



