ENERGY AS A FACTOR IN AGRICULTURE. 95 



able form must be provided as an essential condition of the mani- 

 festations of life. It must not, on the other hand, be assumed that 

 the potential energy of foods may be considered as a reliable in- 

 dex of their physiological value. Biological processes are exceed- 

 ingly complex, and, in calling attention to energy as a dominant 

 factor in vital activities, we do not lose sight of other important 

 considerations which can not here be noticed. 



Protean transformations of energy are constantly carried on 

 in all the metabolic tissues. The energy expended in building 

 organic substance in animals, as in plants, is stored up in the 

 form of potential energy as an essential condition of its constitu- 

 tion, and it is again liberated in the form of heat in the correla- 

 tive processes of destructive metabolism which are taking place 

 without cessation in the work performed in every operation of 

 the system. 



Dr. Foster tells us that what is really meant by the phrase, 

 " living substance, is not a thing, or body, of a particular chemical 

 composition, but matter undergoing a series of changes." These 

 metabolic changes are brought about, in the main, at the expense 

 of energy, and they represent in fact successive transformations 

 of energy from the active to the potential form, and a final recon- 

 version to heat, which leaves the body in various ways. 



The animal machine is in effect a heat-engine that is con- 

 stantly being worn out by the work performed, and as constantly 

 repaired by its own processes of nutrition, and the heat leaving 

 the body (animal heat) represents the energy that has been used 

 in internal work, and finally liberated through the agency of de- 

 structive metabolism. 



We must not, however, carry the analogy of the heat-engine 

 so far as to assume that the food consumed by animals is disposed 

 of by a process of combustion, like the fuel burned under a steam- 

 boiler. There is no evidence that anything like a combustive 

 oxidation of the food constituents, or of the tissues, takes place 

 in the animal economy. The building of organic substance and 

 storing of potential energy (constructive metabolism) is accom- 

 panied by parallel processes of disintegration (destructive metab- 

 olism), in which the stored potential energy is changed to heat ; 

 and these alternate, or possibly simultaneous, transformations of 

 energy which take place in living tissues must be regarded as 

 manifestations of vital activities that differ widely in their char- 

 acteristic features from the processes of combustive oxidation 

 that take place in non-living matter. 



From what is now known in regard to animal physics it will 

 be safe to assume that from four fifths to five sixths of the poten- 

 tial energy of the food consumed and digested by working 

 animals is expended in vaporizing the water thrown off by the 



