THE CHEMICAL ELEMENTS 25 



The significance of these facts in their relation to 

 cattle-feeding is that the chemical change which we call 

 combustion is one of the phenomena of animal nutrition. 

 Substances which may suffer either slow or rapid oxida- 

 tion outside the animal body may undergo complete or 

 partial combustion in the animal; or, stated in another 

 way, the part of the plant which "burns up" in the fire- 

 place or crucible is the part which in general undergoes 

 the same change within the animal organism in so far as 

 the food is digested. 



31. Organic and inorganic classes. — ^The terms com- 

 bustible and incombustible are less used, perhaps, than 

 two others which represent practically the same divisions 

 of plant or animal substance, viz., organic and inorganic. 

 In chemical literature, the portion of a plant or animal 

 which suffers combustion is called the organic, and the 

 ash is known as the inorganic part. These terms were 

 evidently based on the erroneous assmnption that the 

 compoimds which bum and break up into simpler ones 

 are peculiarly those which sustain necessary and vital 

 relations to life, and are formed only through the func- 

 tions of living organisms. To be sure, the dry substance 

 of the plant is organized chiefly by building up com- 

 pounds of carbon, oxygen, hydrogen, and nitrogen, which 

 suffer combustion; but compounds of sulfiu", phos- 

 phorus, chlorine, potassium, sodium, and calcium are 

 also constant and essential constituents of the juices and 

 tissues of the plant and animal. They sustain important 

 relations to nutrition and growth. It is true, however, 

 that the portion of a food material which is commonly 

 spoken of as organic embraces those compounds that fur- 

 nish practically all the energy which is utilized by animal 

 life and much the larger part of the building-material. 



