INTRODUCTORY 3 



plant or animal. Linseed oil, olive oil, and castor oil are all 

 different ; so also are butter fat, beef fat, and mutton fat. 

 They exhibit well marked differences in their properties and 

 are easily distinguished one from the other ; but they are all 

 fats. They are similar in composition, possess certain pro- 

 perties in common, and all belong to the same chemical 

 class. 



In like manner, the albuminoids or proteins which are 

 included in the nitrogenous substances of plants are not 

 identical with those of animals. The proteins of one kind 

 of plant or animal" are not identical with the proteins of 

 another plant or animal, but they are similar in composition, 

 exhibit certain common properties, and* are, therefore, all 

 referred to the same chemical class or group. 



Animals require food not only to increase their mass but 

 also to maintain it. If an animal which has reached maturity 

 be deprived of food, a reverse process takes place, the mass, 

 which is measured by weight, diminishes, and in a short time 

 the animal dies of starvation. This reverse process is due to 

 the respiration of the animal which causes the tissues to 

 become oxidised. It has been compared to the burning of a 

 fire. It is actually and in truth a process of slow combustion. 

 The substance of the body is gradually consumed and certain 

 quantities of food must be supplied from time to time in order 

 to repair the waste. If the quantity of food is not sufficient 

 to counterbalance the loss due to oxidation, the mass, i.e. the 

 weight, of the animal is diminished. On the other hand, if 

 more material is absorbed than is lost by oxidation in the 

 same time, the excess may be stored up in the body and 

 increase of weight results. 



When a fire burns, heat is produced. That heat is pro- 

 duced by the slow combustion which goes on in animal bodies 

 is evident from the fact that the bodies are warm. Much of 

 this heat is lost by radiation from the skin and it is essential 

 to maintain the body at the proper temperature. The food 

 serves as fuel for this purpose. 



All work involves the expenditure of energy. When an 

 animal does work, the energy is derived from the oxidation 



