ANIMAL HEAT. 437 



Radiation from the skin is modified to some extent by clothing. An 

 excess of clothing decreases, a diminution of clothing increases 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 quali- 

 ties, 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 refer- 

 ence 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 con- 

 sequence of the increase in the activity of the chemic changes which underlie 

 body activity, as shown by the increase in the consumption of oxygen 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 pro- 

 duced 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 produced 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 (Calories) pro- \ Rest 16 hrs. Sleep 8 hrs. Rest 8 hrs. Work 8 hrs. Sleep 8 hrs. 

 duced / 2470.4 320 1235.2 2169.6 320 



2790.4 



