i86 UTILISATION OF FOOD BY THE ANIMAL [chap. 



its energy as heat by the time it is at rest again at the 

 foot of the hilL During the running-down process, also, 

 in both cases a certain proportion of the stored-up 

 energy can be converted into work or other forms of 

 valuable energy ; moreover, both processes will run of 

 themselves if the sluice-gate be opened in one case, or 

 the mixture be ignited in the other. The energy of all 

 bodies, i.e. their capacity for doing work, is best 

 measured in terms of heat, the unit usually employed 

 in this sort of work being the large calorie, which is the 

 amount of heat required to raise the temperature of 

 I kilogram of water by i degree. This calorie has 

 been valued against other kinds of energy ; for instance, 

 if the heat contained in the calorie were transformed 

 into work, it would lift 425 kilograms i metre (or 1-4 

 tons I foot) ; similarly, electricity or light can also be 

 measured in terms of calories. To take an example, we 

 can say that a given weight of coal possesses a certain 

 amount of energy, which would be developed and 

 measured as heat if the coal were burnt and the heat 

 communicated to a known weight of water. But if the 

 coal is burnt in a steam boiler, and the steam made to 

 drive an engine, less than the whole of the energy of 

 the coal will be found in the waste heat from the boiler 

 and steam, because a certain proportion has been 

 transformed into work. A dynamo will transform this 

 moving energy into electricity with but a small loss by 

 friction {i.e. with but a small reconversion into heat), 

 and the electricity may be changed again into work, 

 light, chemical energy, or other forms — the ultimate 

 result being a running down into low-grade heat. The 

 original stock of energy is never lost or destroyed, the 

 sum total of the different forms developed being always 

 that originally present in the coal ; even when finally 

 degraded into heat, there are still the original number 



