394 TEXT-BOOK OF PHYSIOLOGY. 



radiation. The quality of clothing is also an important factor. 

 Wool is a poor conductor of heat but a good absorber and retainer of 

 moisture, and hence is adapted for cold weather. Linen and cotton 

 possess the opposite qualities, and hence are adapted for warm 

 weather. Radiation from the skin is somewhat interfered with by 

 subcutaneous fat, the extent of the interference being dependent on its 

 amount. 



The foregoing estimates as to the amounts of heat produced have 

 reference only to the body in repose. When the body passes into a 

 state of muscle activity, there is at once a notable increase in heat- 

 production in consequence of the increase in the activity of the chemic 

 changes which underlie body activity, as shown by the increase in the 

 consumption ofjDxygen and the production of carbon dioxid. Not 

 all of the potential energy set free, however, appears as heat; for 

 if the muscles are engaged in doing work a part of the energy which 

 would otherwise manifest itself as heat is converted into mechanic 

 motion. From the work done during a period of eight hours it has 

 been estimated that about 500 calories are so transformed or utilized. 

 Hirn calculated from an average of five experiments that a man 

 weighing 67 kilos in repose produced 154.4 calories per hour and 

 absorbed 30.7 grams of oxygen per hour; but when engaged in active 

 muscle movements produced 271.2 calories and absorbed 119.84 

 grams of oxygen per hour. The increase in heat-production per hour 

 during activity was thus almost doubled, though the sum total pro- 

 duced daily in which there was a working period of eight or ten hours 

 was only about one-third more than during a day of repose. During 

 sleep there is a greatly diminished heat-production, not more than 

 40 calories per hour being produced. The preceding data may be 

 tabulated as follows (Martin) : 



Day of Rest. Day of Work. 



Heat units (calor- "I Re st 1 6 hrs. Sleep 8 hrs. Rest 8 hrs. Work 8 hrs. Sleep 8 hrs. 

 ies) produced. _ j 2470.4 320 1235.2 2169.6 320 



