186 ORIGIN OF ORGANIC MATTER 



found in them. Whence, then, do forest trees derive their nitro- 

 gen, as they can never have been manured with animal dung? 



Until Liebig demonstrated the fact, it was not known that a 

 constant quantity of ammonia was always present in the atmo- 

 sphere, and that rain and snow contained determinable quantities 

 of salts of ammonia. There can be no doubt, however, that 

 nitrogen is supplied to plants as food in the form of salts of 

 ammonia ; but, on the other hand, Liebig's view, that the atmo- 

 sphere is the sole source from whence plants extract their salts of 

 ammonia, has met with considerable opposition. Boussingault 

 and Liebig have endeavoured to prove from a calculation of the 

 quantities of ammonia present in rain-water, and of the annual 

 amount of rain, that the ammonia extracted from the atmosphere 

 by plants is quite sufficient to form those nitrogenous compounds 

 which we discover in the products of our annual harvests ; and 

 they have also drawn attention to the fact, that there are present 

 in the humus, in dung in short, in every fruitful soil sub- 

 stances, which have the power not only of fixing the ammonia of 

 the water, but also of absorbing ammoniacal vapour directly from 

 the atmosphere. Bouchardat's observation, that salts of ammonia 

 exert a poisonous action upon plants, even when diluted 1000 or 

 1500 times, may perhaps depend upon their unsuitable form, 

 and probably upon their insufficient degree of dilution ; but it in 

 no way refutes the general hypothesis that plants derive their 

 nitrogen from ammonia ; perhaps Mulder's* conjecture may also 

 be correct, that the ammonia passes into plants in combination 

 with organic acids, and that in this form it exerts no deleterious 

 action on these organisms. It would appear from most of the 

 experiments which have been made in reference to the absorption 

 of ammonia by plants, that the roots are designed for the assimi- 

 lation of salts of ammonia to the same extent at least as the 

 green parts serve for the absorption of carbonic acid. 



In the putrefaction of nitrogenous substances, there is a 

 development of carbonate of ammonia from the beginning to the 

 end of the process; its great volatility causes it speedily to be 

 given off to the atmosphere, from whence it is again precipitated 

 with the water in the form of rain and snow, and is thus returned 

 to the vegetable kingdom. If we assume that every pound of rain- 

 water contains only half a grain of ammonia, there must be a 

 sufficient quantity of this substance in the atmosphere to supply 



* Versuch ciuer physiol. Chcm. S. 715-752 [or English translation, pp. 651- 

 601]. 



