158 THE CHEMISTRY OF THE FARM 



of the diet. We may, however, safely assert that 

 the amount of heat generated by the combustion of 

 the digestible constituents of any food, after making 

 the deductions already referred to, will form a true 

 guide to its nutritive value whenever the diet of which 

 it forms a part supplies a sufficient amount of digestible 

 albuminoids, and this will be the case whenever foods 

 are skilfully employed. 



We have already seen (p. 123) that the value of any 

 food to an animal may be quite different according as 

 it is employed for the purpose of maintaining the con- 

 dition of the animal when at rest, or when employed 

 for the production of increase or the performance of 

 external work ; and that this is especially true in the 

 case of fibrous foods, as hay or straw. The heat which 

 is the final outcome of the mechanical labour em- 

 ployed in the digestion of these fibrous foods is quite 

 capable of warming the animal, but the energy thus 

 developed is not generated in the muscles of the Hmbs 

 and is thus useless for performing external work, and 

 it is equally useless for the production of animal 

 increase. As soon, however, as the animal is supplied 

 with a liberal diet in order to perform work or yield 

 increase, the quantity of waste heat available for 

 warming the animal is so greatly increased, that the 

 heat produced by the digestion labour ceases to be of 

 any use to the animal. Foods are thus to be valued 

 at the full heat value of their digestible constituents, 

 including fibre, when used in limited quantities for 

 maintenance only, deduction being simply made for 

 the unburnt matter contained in the urine and intes- 

 tinal gases (table, p. 120). 



The valuation of food for the production of increase 

 is less easy. Kellner's average results (table, p. 125) 



