i68 CHEMISTRY FOR AGRICULTURAL STUDENTS 



done through the agency of muscle, which, since it is constantly under- 

 going oxidation, must be constantly renewed by a fresh supply of albuminoid 

 derived from the peptones of the digested food. When albuminoids are 

 oxidised, in addition to carbonic acid gas and water, urea is one of the 

 products ; and this, being excreted from the blood by the kidneys, is found 

 in urine. Thus the simple siibsta7ices excreted by animals are oxidation 

 p^'oducts of complex organic cotnpounds, heat being produced during the 

 process. 



3. Food. — The constituents of food are classified into (i) heat-producers, 

 viz. carbohydrates, fats, and amides ; (2) flesh - formers and heat- 

 producers, viz. albuminoids ; (3) inorganic salts, including especially 

 phosphate of calcium and chloride of sodium. In addition to these, water 

 is a necessary constituent as the food-carrier, and certain alcohols and 

 ethereal salts in the spices, alkaloids in tea, coffee, and cocoa, and the 

 ethyl alcohol of fermented liquors, may serve a useful purpose as stimulants. 



The value of a food will depend upon (i) composition and (2) digestibility. 

 The value of food constituents as heat-producers depends upon their heat of 

 combustion. Thus the heat of combustion of cane sugar is 97, albumin 

 107, fat 229, and asparagine 49, if that of starch be taken as 100. The 

 albuminoid ratio {i.e. the proportion of albuminoids to the equivalent in 

 starch of the other organic constituents), required in a diet, will depend upon 

 the kind and age of the animal and the functions it is expected to perform, 

 and will vary from 1:2 to 1:14. Digestibility varies with the kind and 

 condition of the food and the kind of animal, ruminating animals having a 

 considerable power of digesting cellulose, while man has none. Cooking 

 materially affects digestibility : for instance, baking of bread and toasting 

 it converts a portion of the starch into dextrin, and therefore increases 

 digestibility ; but the prolonged heating of albuminoids renders them much 

 less digestible. 



XXXVI. CONCLUSIONS 



The carbon compounds dealt with in the foregoing studies, of 

 which the constitution is understood, have been regarded as 

 inorganic hydrogen compounds, in which the hydrogen, or 

 other element, has been replaced by an organic radicle ; and 



