VARIATIONS IN HEAT-PRODUCTION. 333 



Albumin, 1 grm., .... 4993 heat-units. 



Grape-sugar, 1 grm., . . . 3217 ,, 



Ox fat, 1 grm., .... 9069 



The albumin, however, is only oxidised to the stage of urea, hence the heat-units of urea 

 must be deducted from 4998, which gives 4263 heat-units obtainable from 1 grm. albumin. 

 When we know the number of grammes consumed, a simple multiplication gives the number 

 of heat-units. 



The heat-units will vary, of course, with the nature of the food. J. Ranke gives the fol- 

 lowing : 



With animal diet, .... 2,779,524 heat-units. 



,, food free from N, .... 2,059,506 



mixed diet, .... 2,200,000 



,, during hunger, .... 2,012,816 ,, 



216. VARIATIONS IN HEAT-PRODUCTION. (1) Influence of Bodily Surface. Rubner 



found that the production of heat depended more upon the size of the body and its superficial 

 area than upon the body-weight. Small or young animals have a relatively larger surface than 

 large or older ones, and as the removal of heat takes place chiefly from the external surface, 

 animals with a larger surface must produce more heat. Small animals used relatively more O. 

 Rubner's investigations on dogs of different size gave a heat-production of 1,143,000 calories for 

 every square metre of cutaneous surface. On comparing the body-weight with the cutaneous 

 surface in different animals, he found that for every 1 kilo, of body-weight there was in the rat 

 1650, rabbit 946, man 287 square centimetres of surface. 



(2) Age and Sex. The heat-production is less in infancy and in old age, and it is less in 

 proportion in the female than in the male. 



(3) Daily Variation. The heat-production shows variations in twenty-four hours correspond- 

 ing with the temperature of the body ( 213, 4). 



(4) The heat-production is greater in the waking condition, during physical and mental 

 exertion, and during digestion, than in the opposite conditions. 



217. RELATION OF HEAT-PRODUCTION TO WORK The potential 

 energy supplied to the body may be transformed into heat and kinetic energy 

 (see Introduction). In the resting condition, almost all the potential energy is 

 changed into heat ; the workman, however, transforms potential energy into work 

 mechanical work in addition to heat. These two may be compared by using 

 an equivalent measurement, thus, 1 heat-unit (energy required to raise 1 gramme 

 of water 1 C.) = 425*5 gramme-metres. 



Relation of Heat to Work. The following example may serve to illustrate the relation 

 between heat-production and the production of work: Suppose a small steam-engine to be 

 placed within a capacious calorimeter, and a certain quantity of coal to be burned, then as long 

 as the engine does not perform any mechanical work, heat alone is produced by the burning of 

 the coal. Let this amount of heat be estimated, and a second experiment made by burning the 

 same amount of coal, but allow the engine to do a certain amount of work say, raise a weight 

 by a suitable arrangement. This work must, of course, be accomplished by the potential 

 energy of the heating material. At the end of this experiment, the temperature of the water 

 will be much less than in the first experiment, i.e., fewer heat-units have been transferred to the 

 calorimeter when the engine was heated than when it did no work. Comparative experiments 

 of this nature have shown that in the second experiment, the useful work is very nearly pro- 

 portional to the decrease of the heat (Him). 



Compare this with what happens within the body : A man in a passive 

 condition forms from the potential energy of the food between 1\ to 2f million 

 calories. The work done by a workman is reckoned at 300,000 kilogramme- 

 metres ( 300). 



If the organism were precisely similar to a machine, a smaller amount of heat, 

 corresponding to the work done, would be formed "in the body. As a matter of 

 fact, the organism produces less heat from the same amount of potential energy 

 when mechanical work is done. There is one point of difference between a 

 workman and a working machine. The workman consumes much more potential 

 energy in the same time than a passive person ; much more is transformed in his 

 body ; and hence the increased consumption is not only covered, but even over- 

 compensated. Hence, the workman is warmer than the passive person, owing to 

 the increased muscular activity ( 210, 1, b). Take an example : Hirn remained 



