124 ^^^ Great World's Farm 



however abundant, will not be enough for them, or enable 

 them to grow properly, if they be stinted in this respect. 

 Their more usual way of obtaining it, however, is from 

 the soil, or from the air; but in neither case can they take 

 the pure gas itself; it must be in the form of a compound 

 before they can make any use of it. 



By way of trying whether plants could do without 

 nitrogen, other than that by which they are surrounded in 

 the air, three pots were filled with a soil of sand and brick- 

 dust, from which all animal and vegetable matter had been 

 removed. A couple of sunflower-seeds were planted in 

 each, and all three were watered with pure, distilled water, 

 containing no food whatever. 



The plants in the first pot turned out mere dwarfs, as 

 was to be expected; those in the second were not much 

 better, though they had had a small quantity of clover- 

 ashes given them; but those in the third were almost as 

 large as the finest specimens grown in the garden, for they 

 had been supplied with a compound of nitrogen, in the 

 form of potassium nitrate; and while the two first had 

 managed to get only about the thirtieth part of a grain of 

 nitrogen from the air, these had taken sixty-six times as 

 much from the soil. The quantity is still very small, of 

 course, only two grains and a fifth; but it strikingly illus- 

 trates the immense importance of small, and even minute 

 quantities, since it made the whole difference in the growth 

 of the plants. 



The dwarf sunflowers obtained their small fraction of 

 nitrogen from the air; but this is no contradiction to what 

 has been previously said, for they did not take pure nitro- 

 gen, but ammonia, which is a compound of nitrogen and 

 hydrogen. 



