80 THE FEEDING OF ANIMALS 



portion of woody fiber, although the decrease in the 

 relative amount of water in the tissues and the deposition 

 of other substances have more or less effect. 



109. The acids. — Other substances besides those of a 

 carbohydrate character are included in the nitrogen- 

 free extract. Chief among these are the organic acids, 

 compotmds which are found mostly in the fruits, although 

 they appear in certain fermented products, such as silage 

 and sour milk. The most important and well known of 

 these are acetic acid, found in vinegar, citric acid in 

 lemons, lactic acid in sour milk, malic acid in many 

 fruits, such as currants and apples, and oxalic acid in 

 rhubarb. Sometimes these acids are free, that is, not 

 combined with any other compound. In the main they 

 are imited with lime or some other base, forming an 

 acid salt. Excepting the fruits, only fermented foods 

 contain acids to an appreciable extent. When milk sours, 

 the sugar in it is changed to lactic acid under the influence 

 of a ferment. The acids of silage are formed at the expense 

 of the carbohydrates that are in the material which is 

 subjected to fermentation. 



110. .Fat's and oils. — When any finely ground food- 

 stuff, either vegetable or animal, is submitted to the 

 leaching action of ether, chloroform, or certain other 

 solvents, several compounds are taken into solution, the 

 main and important ones being fats or oils. These bodies 

 make up the chief portion of such an extract from seeds, 

 while the ejctract from other vegetable materials also 

 contains a considerable amoimt of wax, chlorophyl, and 

 other substances. Tables that show the composition of 

 foods have a column which is sometimes designated 

 "ether-extract," and sotnetimes "fats or oils." The former 

 is the more accurate term, because the compounds which 



