THE CHARACTERISTICS OF LIVING THINGS 15 



it needs no demonstration. There is nothing like animal 

 growth among lifeless things, which increase in size only by 

 addition of particles of substances like themselves. If a 

 saturated solution of common alum (or copper sulphate) be 

 made by dissolving as much as possible in boiling water, 

 and then a stone be dropped into the alum solution, it 

 will be coated by alum crystals, but the mass of stone 

 will not increase. If a lump of alum be dropped into 

 the saturated solution, the mass will increase by the 

 addition of more alum crystals to the surface. Such an 

 experiment with alum, showing that a mass of it can increase 

 in size only by addition of the same substance, illustrates a 

 fact applicable to stones, crystals, minerals, and other in- 

 organic things; they cannot add other substances to them- 

 selves and make them really a part of their own bodies. 

 On the other hand, living animals can live and grow on food 

 derived from other animals or from plants. For example, a 

 frog may eat smaller frogs ; but it may also eat earth- 

 worms or plants, and the substance of these will be changed 

 and built into that of the frog. This power of the living 

 animal to take food unlike itself and to make it over into 

 its own substance is known as assimilation (meaning to make 

 like or identical). 



20. The living frog breathes, and the lifeless object does 

 not. If we watch a frog, or a higher animal, we see muscular 

 movements concerned with pumping air into and out of 

 the lungs; and we call this process breathing. In many 

 simple animals there are no lungs, but there are several 

 ways of proving that they require air (only the oxygen, 

 and not the nitrogen of the air), and that they change the 

 air when they breathe it. That animals require air is shown 

 by the fact that land animals die very quickly if placed 

 in a jar from which the air has been pumped out with an air- 

 pump, and aquatic animals will soon die in water from 

 which the air has been withdrawn by an air-pump or been 



