16 HISTORICAL INTRODUCTION. 



be fitly described at the present hour as still the burn- 

 ing question of agricultural chemistry. 1 



As soon as it was discovered that nitrogen was a 

 constituent of the plant's substance, speculations as to 

 its source were indulged in. The fact that the air 

 furnished an unlimited storehouse of this valuable 

 element, and the analogy of the absorption of carbon 

 (from the same source by plant-leaves), naturally sug- 

 gested to the minds of early inquirers that the free 

 nitrogen of the air was the source of the plant's 

 nitrogen. As, however, no direct experiments could 

 be adduced to prove this theory, and as, moreover, 

 nitrogen was found in the soil, and seemed to be a 

 necessary ingredient of all fertile soils, the opinion 

 that the soil was the only source gradually supplanted 

 the older theory. Little value, however, must be 

 attached to these early theories, as they can scarcely 

 be said to have been based on experiments of serious 

 value. Indeed it may be safely affirmed, in the light 

 of subsequent experiments, that it was impossible for 

 this question to be decided at this early period, from 

 the fact that analytical apparatus, of a sufficiently 

 delicate nature, was then wholly unknown. Indeed 

 it is only within the last few years that it has been 

 possible to carry out experiments which may be re- 

 garded as at all crucial. A short sketch of the 

 development of our knowledge of the relation of 

 nitrogen to the plant will be given further on. 

 1 See p. 40 to 45. 



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