DISTINCTION BETWEEN ANIMALS AND PLANTS, g 



mineral salts, namely, and combining these witli the 

 elements of water and with carbon derived from the 

 carbonic acid of the air, builds up the complex com- 

 pounds' that the animal organism requires for its food. 

 Moreover, the two groups play into each other's 

 hands, so to speak, as regards the waste products 

 which result respectively from their chemical activity, 

 or metahoUsyn^ as it is called. Animal life may be 

 compared to a fii-e : the final outcome of its chemical 

 processes, fed by a constant supply of air, is a slow 

 oxidation, which gives off heat, and sets free carbonic 

 acid gas, to escape chiefly through the channel of the 

 lungs or equivalent structures. Moreover, the proto- 

 plasm not only undergoes a constant waste and re- 

 newal of its carbon, but also at the same time of its 

 nitrogen. The result is a residue of waste material, 

 which is carried away from the animal body, dissolved 

 in water, in the liquid excreta; its principal con- 

 stituent is known as urea; it is a compound of the 

 four elements, carbon, oxygen, hydrogen, and nitro- 

 gen, indicated by the formula CON2H4, and may be 

 artificially made ; indeed it was the first organic 

 substance that was ever prepared artificially by the 

 synthesis of inorganic materials. In decomposition it 

 forms ammonia ; this, dissolved in rain water and 

 filtering through the soil, supplies the various nitrates, 



' This term (Greek /iera|3oXij, change) is used to denote 

 the series of chemical changes which take place in a living 

 body, and result in the transformation of the material of the 

 body into new chemical compounds. 



