410 VITAL HEAT. 



gram of urea oxidized gives 2523 calories; the oxidation of 1 

 gram of proteid produces ^ gram of urea ; hence from the 5778 

 calories produced by the complete oxidation of the proteid we must 

 deduct 841 calories, which gives 4937 calories as the actual heat- 

 value of 1 gram of proteid. 



If now we recall the adequate diet of Moleschott (p. 130), we 

 shall find that the amount of heat-production in twenty-four hours 

 with such a diet is 2,801,148 calories, calculated as follows : 



Grams. Calories. Calories. 



Proteids 120 x 4937 = 592,440 



Fats 90 x 9312 = 838,080 



Carbohydrates 333 x 4116 = 1,370,628 



2,801,148 



It must not be inferred from the above statement that all the 

 food taken into the body is oxidized and reappears in the form 

 of energy, for, as we have seen, a not inconsiderable part passes 

 out from the body without having undergone the digestive process. 



Although oxidation is the great source of the heat produced in 

 the body, there are doubtless contributory causes, such as (2) the 

 various movements which take place, producing heat by friction; 

 also (3) electricity generated in muscles and nerves ; but of the 

 amount produced by these and other physical causes we know 

 but little. 



Channels through which Vital Heat is I/ost. Helm- 

 holtz estimates that 7 per cent, of the total heat produced in the 

 body is expended in the form of mechanical work ; that 78 per 

 cent, is discharged through the skin by evaporation and radia- 

 tion ; and 15 per cent, by the lungs, urine, and feces. 



Vierordt calculates that the heat discharged from the body is 

 distributed as follows : 



Calories. 



1.8 per cent, in urine and feces = 47,500 



3.5 per cent, in expired air = 84,500 



7.2 percent, in evaporation of water from lungs = 182,120 



14.5 per cent, in evaporation of water from skin = 364,120 



73.0 per cent, in radiation and conduction from skin --= 1,791,820 



2,470,060 



Calorimetry. A calorimeter is an apparatus for determining 

 the amount of heat dissipated or disengaged from any substance 

 or from a living animal. Those used for animals consist of a 

 chamber adapted to hold the animal, surrounded by some medium 

 which will absorb the heat, such as ice, air, or water. 



Dulong's calorimeter consists of a chamber in which the animal 

 is placed ; this is contained in a larger chamber holding water. 

 Outside of this is a layer of some non-conducting material, like 

 wool, and outside of all is a box. Air from a gasometer is ad- 

 mitted on one side, and the expired air passes out on the other. 



Reichert's water calorimeter (Fig. 229) is described by its in- 



