X.] ENERGY OF FOOD 187 



of calories present. The difference is that such low- 

 grade heat has lost its effectiveness, and can no longer 

 be transformed into work, light, or electricity. As 

 another illustration we may take a watch : in the act 

 of winding a certain amount of energy is communicated 

 to it, and remains stored in the energy of the coiled-up 

 spring. Gradually this energy is transformed into the 

 energy of motion of the parts of the watch, which is just 

 as continuously rubbed down by friction into heat which 

 leaks away. There is no loss of energy; the world 

 when the watch runs down contains a little more evenly 

 distributed heat, exactly balancing the energy com- 

 municated to the coiled spring at the outset. 



The total energy possessed by any food — that which 

 is often called its fuel value — can be measured in the 

 simplest fashion by actually burning in oxygen a given 

 weight of the food in a vessel surrounded by a known 

 quantity of water and observing the rise of temperature 

 that ensues. This gives the maximum amount of 

 energy available from the food, but the animal does 

 not realise the whole of it except in the case of a pure 

 fat or oil which is as completely burnt in the body 

 as it is in the calorimeter. Most foods are not 

 completely digested, hence the excreta contain some 

 energy and must also be burnt separately in the calo- 

 rimeter, the heat evolved being deducted from the 

 total energy of the food in order to estimate what is 

 available for the animal. The nitrogen of proteins, 

 again, is excreted as urea, which is combustible and 

 contains energy ; this also must be burnt and the heat 

 deducted from the fuel value of the food ; also from all 

 carbohydrates, and particularly from fibre a certain 

 proportion of methane and hydrogen is produced in 

 the intestine, and these unburnt gases represent losses 

 of energy to the animal. For example, with oxen, 



