CH. XLTL] HEAT-VALUE OF FOODS 635 



weighing 16 kilos ; this also has to be raised 25 C., and so to be 

 multiplied by 25 ; it has further to be multiplied by the relative heat 

 of air (0'24). The 630 grammes of water evaporated in the lungs 

 must be multiplied by the potential or latent heat of steam at 37 C. 

 (582) ; the portion of heat lost by radiation, conduction, and evapora- 

 tion from the skin constitutes about four-fifths of the whole, and is 

 obtained by deducting the three previous amounts from the total. 

 This table does not take into account the small quantities of heat lost 

 with urine and faeces. If the man does external work the amount of 

 energy dissipated is increased, and he would, in consequence, require 

 more to be supplied in the form of food. Very few men in active 

 work get on well with a smaller supply than 3000 large calories 

 ( = 3,000,000 small calories) in their diet. A man, however, at rest 

 is always doing what is called internal work, that is, maintaining 

 the circulation, respiration, etc. 



From experiments of this nature, it has been found that the 

 principle of the conservation of energy holds in the living body. 

 The results may be stated as follows : 



1. If an animal is doing no external work, and is neither gaining 

 nor losing substance, the potential energy of the food (expressed as 

 its heat of combustion) will be equal to that of the excreta, plus that 

 given off as heat, plus that of internal work. 



2. If an animal is doing external work, and is neither gaining 

 nor losing substance, the potential energy of the food will be equal to 

 the potential energy of the excreta, plus that given off as heat, plus 

 that of the internal work, plus that of the external work. 



3. If an animal is doing no external work, but gaining or 

 losing body-substance, the potential energy of the food will equal 

 the potential energy of the excreta, plus that given off as heat, plus 

 that of the internal work, plus that of the gain by the body- 

 substance (a loss by the body being regarded as a negative gain). 



4. In an animal doing external work, and gaining or losing body- 

 substance, the potential energy of the food will equal the potential 

 energy of the excreta, plus that given off as heat, plus that of the 

 internal and external work, plus that of the gain (positive or 

 negative) of the body-substance. 



