ON THE HISTORY 

 OF A 



GREAT PHYSIOLOGICAL DISCOVERY 



AND ITS BEARING ON AGRICULTURE AND ECONOMICS 



BY 



PROFESSOR J. ALLEN HARKER 



(ROYAL AGRICULTURAL COLLEGE, CIRENCESTER) 



READ 22nd NOVEMBER, 1892 



The most important of the indispensable constituents of 

 plant food, (neglecting carbon and water, which under 

 ordinary conditions Nature supplies gratis) are Nitrogen, 

 Potash, and Phosphoric Acid. If we average in a very 

 rough and general way all food plants, the proportions in 

 which these three substances are required are to each 

 other as 10, 5, and 4. In continuous husbandry they have 

 largely to be artificially supplied to the land. Of these 

 Nitrogen is the most costly. Forming, as this element does, 

 about four-fifths of our atmosphere, surrounded as plants 

 are by milUons of cubic miles of it, why cannot they 

 obtain it as they do the carbon dioxide they need, or as 

 they, and animals too, obtain their needful oxygen, from 



the air ? 



Fifty years ago it was generally supposed that they did 

 so, but this view of the matter, a mere supposition, gave 

 place on the institution of direct experiments, to the 

 opposite opinion, to wit, that Nitrogen was not in any 

 case obtained by plants from the free Nitrogen of the air. 

 The experiments leading to this end are classical. They 



