HOW PLANTS DRINK. 59 



dead and lifeless. Protoplasm, in short, is the 

 only living material we hiow ; and its life constitutes 

 the larger life of the wholes compounded of it. 



Well, now you are in a position to see why the 

 farmer and the gardener attach so much impor- 

 tance to the soil, and so little, apparently, to the 

 air and the sunlight. The reason is that the air 

 is everywhere; you get it for nothing; but the 

 soil costs money, and, when cultivated, it requires 

 to be supplied from time to time with fresh stores 

 of the particular materials the plants take from it. 



Let me give two simple parallel cases. A fire 

 is made by the combination of two sorts of fuel — - 

 coal and oxygen. One is just as necessary for 

 fire-making as the other. But we buy coal dear, 

 and we neglect to take oxygen into consideration 

 accordingly. The reason is that oxygen exists 

 in abundance everywhere; so we don't have to 

 buy it. If we paid a pound a ton for it, as we do 

 with coal, we should very soon remember how 

 necessary a part it is of every fire. Even at pres- 

 ent we are obliged to provide for its free admis- 

 sion by the bars of the grate, and by checking or 

 regulating its ingress we can slacken or quicken 

 the burning of the fire. 



Or, to take another analogy, oxygen is just as 

 necessary to human beings and other animals as 

 food and drink are. But, as a rule, we get oxygen 

 everywhere in such great abundance that we never 

 think of taking it into practical consideration. 

 Still, in the Black Hole of Calcutta, the unhappy 

 prisoners thoroughly realised the full value of 

 oxygen, and would gladly have paid its weight in 

 gold for the life-giving element. 



Now, carbonic acid, on which plants mainly 



