VII.] FIXATION OF NITROGEN 131 



itself. The soil bacteria we have been discussing 

 similarly only cause the circulation of nitrogen from one 

 form of combination to another ; one type sets nitrogen 

 free, but none that we have yet examined do anything 

 to bring it back into combination. But if the combined 

 nitrogen of the world had only been suffering loss by 

 change into nitrogen gas ever since the beginning of 

 things, however small those losses may appear to be, we 

 should probably have arrived at some evidence of a 

 diminution of the stock and signs of its ultimate 

 exhaustioa No indications of the sort, however, exist, 

 from which it might be concluded that some agency 

 must be at work replenishing the stock by "fixing" 

 nitrogen gas and bringing it back into combination. 



The discovery of such an agency arose out of the 

 long-debated question of whether our ordinary plants, 

 which are always in contact with a boundless store of 

 nitrogen gas, do not possess in their leaves the power 

 of thus "fixing" the nitrogen they require. Many 

 of the earlier investigators were firmly convinced that 

 plants had such a power, arguing from the difficulty 

 of understanding how there could be combined nitrogen 

 in the world from any other source. Moreover, great 

 refinement of experiment is necessary to demonstrate 

 that the nitrogen gained by a plant is exactly balanced 

 by the amount lost by the soil, hence the controversy 

 lasted for a considerable time, even if it can be regarded 

 as ended now. However, the weight of evidence, and 

 notably a series of experiments carried out by Dr Evan 

 Pugh of Pennsylvania at Rothamsted in 1857-8, coupled 

 with the experience gained by field trials of the great 

 value to the crop of a supply of manurial nitrogen, have 

 led people ever since the middle of last century to accept 

 as a general principle the opinion that plants do not fix 

 nitrogea In 1886, however, two German investigators, 



