212 TILLERS OF THE GROUND CHAP. 



and fix it in the soil in a form in which it can be 

 used by other plants. 



We know now, therefore, that while most green 

 plants must take their nitrogen from the soil in 

 the form of a salt called a nitrate, plants like clover 

 and peas live in partnership with bacteria which 

 can take nitrogen from the air, and hand it on to 

 the green plant, and also leave part of it stored in 

 the soil for the plants of another season. 



Later investigations have taught us many new 

 facts about the soil and the use of manures. It is 

 clear now that we do not need to fear that the 

 soil will grow poorer, for not only are there bacteria 

 ready to enrich it for us, but the chemist also has 

 shown how the nitrogen of the air may be fixed, 

 with the help of lime, in a form which can be used 

 by the higher plants, an artificial manure being 

 thus produced. 



CHAPTER XVIII 



THE GAINS THAT KNOWLEDGE HAS BROUGHT 



BEFORE proceeding to sum up generally the gains 

 that the slowly acquired knowledge of the ages 

 has brought to mankind gains in wealth, gains 

 in security, gains in power to turn the stubborn 



