SOURCE OF NITROGEN 239 



the ash, and he considered that the function of the fixed bases 

 was to act as carriers of nitric acid. These bases potash, 

 soda, lime and magnesia, were not mutually replaceable, but 

 the predominance of one or the other affected the produce. 

 Luxuriance of growth was associated with the amount of 

 nitrogen available and assimilated, and in the presence of this 

 sufficiency of nitrogen the formation of carbohydrates depended 

 on the amount of potash available." The possibility that the 

 free nitrogen of the air might supply the nitrogenous needs of 

 plants was disproved by growing plants in calcined soil and 

 removing all traces of ammonia from the air before it was 

 admitted into the glass case in which the plants were growing. 

 Determinations were made of the nitrogen in the seed and soil 

 at the beginning of the experiments, and in the plants and soil 

 at their conclusion. 



The work on the assimilation of nitrogen by plants extended 

 over three years and was made the subject of a communication 

 to the Royal Society in 1861. The paper, entitled, "The Sources 

 of the Nitrogen of Vegetation ; with special reference to the 

 question whether Plants assimilate free or combined Nitrogen," 

 occupies 144 pages of the Philosophical Transactions, and is a 

 brilliant example of the scrupulous accuracy and attention to 

 detail which characterised all Gilbert's work. It is divided into 

 two parts I. " The General History and Statement of the 

 question." II. "The Experimental Results obtained at Rotham- 

 sted during the years 1857, 1858 and 1859." The authors state 

 in the summary of conclusions that "in our experiments with 

 graminaceous plants, grown both with and without a supply 

 of combined nitrogen beyond that contained in the seed sown, 

 in which there was great variation in the amount of combined 

 nitrogen involved and a wide range in the conditions, character 

 and amount of growth, we have in no case found any evidence 

 of an assimilation of free or uncombined nitrogen. 



"In our experiments with leguminous plants the growth was 

 less satisfactory, and the range of conditions possibly favourable 

 for the assimilation of free nitrogen was, therefore, more limited. 

 But the results recorded with these plants, so far as they go, do 

 not indicate any assimilation of free nitrogen. Since, however, 



