©8 RETROSPECT. 



not correspond to more than the quantity in the manure. If the 

 crops contained more than this quantity, the excess must have 

 been obtained from other sources, and these could only be in the 

 atmosphere ; such were the suppositions on which Boussingault 

 proceeded. According to his estimation, the three rotations* 

 yielded : — 



1st Rotation. 2d Rotation. 3d Rotation. In 16 years. 

 Nitrogen in lbs. - - - 501*4 503'4 707*2 17170 

 in stable manure - 406*4 406*4 487*6 1300-4 



Excess of nitrogen obtained, lbs. 95 102 2196 416*6 



In the first and second rotation, the excess of nitrogen obtained 

 was nearly equal ; in the third it was twice as much. 



Now, asked Boussingault — did each of these plants possess the 

 power of absorbing and appropriating to their organism nitrogen 

 from the air, or was this power confined only to one of them ; and 

 was the excess of nitrogen due to all the various kinds of plants, 

 or was it yielded by only one of them l A new experiment 

 seemed to him to decide the question. Two successive crops of 

 corn were taken from a fallow-field, well manured, and the pro 

 duce amounted to : — 



Nitrogen in the crops ... 174*8 lbs. 

 the manure - 165*6 



An excess of nitrogen - - 9*2 lbs. 



But this excess in the crop is too small not to be liable to errors 

 in the experiment. Boussingault concluded from it, that cereals 

 do not absorb nitrogen from the air, and that the amount of nitro- 

 gen yielded in crops is only equivalent to that contained in the 

 manures. 



Now, as it had been found that the quantity of nitrogen ob- 



