70 SCIENTIFIC FEEDING OF ANIMALS 



Fats, like protein and amides, undergo no marsh 

 gas fermentation. Only those parts of the crude fat 

 (p. 12) which are true fats undergo digestion; the 

 wax-like substances, chlorophyll, resins, etc., are 

 excreted undigested. The heat, or energy, content 

 of the fats is unsurpassed by any other digestible 

 food material, the vegetable fat of oily seeds or oil 

 cakes yielding 9300-9500 cal. from 1 gr., whilst the 

 fat from coarse fodder has an average value of 

 8200 cal. It is this that accounts for the high food 

 value of fat. 



From the nitrogen-free extract and crude fibre 

 of the food only those materials are digested which, 

 inclusive of the pentosans, possess the same percent- 

 age composition and heat value as carbohydrates 

 of the class of pure starch and pure cellulose. This 

 fact has been proved by most thorough investiga- 

 tions and it makes the estimate of the feeding 

 values of these substances considerably easier. 

 Cane, fruit, grape and milk sugars have a somewhat 

 different nature. They contain a smaller quantity 

 of carbon and possess a lower heat value than the 

 digestible portion of the other nitrogen-free extract 

 substances and crude fibre. These two latter yield 

 on combustion 4200 cal. for each gram burnt ; milk 

 and cane sugars yield 3950 cal., and grape sugar 

 3740 cal. ; similar differences being also shown in 

 the feeding value of the substances. 



The other nitrogen-free compounds, the organic 



