FOOD OF ANIMALS. 237 



to decompose, especially when mixed with other forms 

 of organic matter. 



610. Urea has been made artificially, by chemical 

 means, but uric acid has not yet been so obtained : 

 its only source is the animal system. When urine is 

 kept in a moderately w\arm temperature, it soon be- 

 gins to decompose ; when fresh, it is generally slight- 

 ly acid, but after a short time, it becomes alkaline 

 from the decomposition of urea, and formation of 

 ammonia. The half solid urine of birds and serpents 

 contains a large quantity of uric acid, in combination 

 with ammonia. Guano, the decomposed excrement 

 of sea-fowl, likewise contains a considerable portion 

 of this salt. 



611. It is evident that food of all kinds may be 

 classed under two great divisions ; according as it 

 yields the elements of flesh, or contains substances 

 capable of being at once transferred to the blood, 

 and so carried to all parts of the body ; or as it 

 merely contains substances capable of undergoing 

 transformations, or of supplying the waste caused by 

 respiration. 



612. The quantity of food required by an animal 

 for either of these objects, varies greatly, and de- 

 pends entirely on circumstances : when the waste 

 going on in the system is great, a large supply of 

 blood-making food, that which is rich in the elements 

 of nutrition, will be required. When, however, the 

 body is exposed to cold, or to violent exercise, the 

 loss must be met by a proportionate increase in food 



