90 THE CHEMISTRY OF THE FARM. 



with dry food in addition is obvious from the facts just 

 stated ; by so doing the quantity of water consumed by 

 the animal is diminished, and its proportion in the diet 

 brought more nearly to a normal ratio. 



2. Capacity for producing Heat and Work. — The 



only basis on which the nutritive value of foods of different 

 composition can be compared is in respect to their 

 capacity for producing heat. The production of heat and 

 mechanical work is the principal result which food accom- 

 plishes in the animal body; the capacity for producing 

 heat also stands in a near relation to the capacity for 

 producing fat. On the other hand, the amount of heat 

 which any food is capable of producing stands in no rela- 

 tion to its power of increasing or renewing the nitrogenous 

 tissues of the body. We may, however, safely assert that 

 the amount of heat generated by the combustion of the 

 digestible constituents of any food will be a fair guide to 

 its nutritive value, when 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 used. 



According to Frankland's actual determinations of the 

 heat-producing power of fat, albumin, and starch, their 

 comparative values in this respect are 100, 47'4, and 43-1. 

 The albumin is here reckoned as minus its equivalent 

 quantity of urea, as this product of the decomposition of 

 albumin is not burnt, but excreted by the kidneys. If, 

 now, we take the proportions of digestible fat, albu- 

 minoids, carbo-hydrates and cellulose, supplied by any 

 food, and multiply them by the heat coefficients just given, 

 the sum of the products will represent the heat-producing 

 capacity of the food when consumed in the animal body. 



