136 METABOLISM 



other only need be referred to at the present juncture, viz. the oxidation 

 of nitrogen into nitric and nitrous acids, which takes place under the influence 

 of electric discharge in the atmosphere m^re especially in thunderstorms. 

 Rain, mist, and snow carry this nitric acid in solution down to the soil in 

 considerable quantities, as Boussingault has clearly shown (1861, 325). The 

 greatest quantity which he found in rain was 6 mg. of nitric acid per litre, 

 but for the most part the amounts were 3, 2, or i mg., or even less. It cannot 

 be said that any marked relation exists between the frequency of thunderstorms 

 and the amount of nitric acid present in rain-water, for there is a relatively 

 large quantity of combined nitrogen present in rain even at times when 

 thunderstorms are entirely absent. It is possible that silent electric discharges, 

 which are always taking place in the atmosphere, may account for this fixing of 

 free nitrogen ; perhaps, also, some of the nitric acid present in the air and 

 carried down to the soil may have its origin in the dust which has arisen from 

 the soil itself. 



Altogether only a small amount of freshly combined nitrogen is added to the 

 soil, not more than a kilogram per hectare per annum according to Ad. Mayer 

 (1901, I, 205) [in tropical countries as much as 5 to 6 kg. (Marcano and Muntz, 

 1889)], while the plants which grow on a surface of that extent, according to 

 Boussingault, use up about 50 kg. of nitrogen per annum. It is therefore 

 essential that, in order that plants may continue to exist, the nitrogen derived 

 from dead organisms should be returned to the soil to be used in the construc- 

 tion of subsequent generations. The nitrogenous compounds arising from dead 

 animals and plants are decomposed by micro-organisms and, in general, changed 

 into ammonia (Lecture XVII), This substance is greedily absorbed by the soil, 

 and thus part, at least, of the ammonia arising from putrefaction will be fixed 

 in the earth. Further, owing to the influence of micro-organisms the ammonia 

 undergoes oxidation into nitrous and nitric acids (Nitrification, Lecture XVIII). 

 In this way every soil contains in varying proportions nitrates, nitrites, and 

 ammonia. 



Losses of nitrogen in nature may, in the first place, be quite local. The 

 ammonia arising from putrefactive decomposition — in so far as it remains un- 

 altered — is only in part absorbed ; a not inconsiderable amount passes off into 

 the air in the gaseous form where it is oxidized into nitrous or nitric acids, or 

 united with carbon-dioxide. This ammonia is returned to the soil in the course 

 of atmospheric precipitation. According to Ad. Mayer (1901, i, 205), on an 

 average about 2 kg. of nitrogen in the form of ammonia falls on a hectare of 

 land annually, in addition to the single kg. of nitric and nitrous acids already 

 referred to as brought down by rain. Volatile ammonia is not entirely lost 

 to the plant ; on the other hand, it may be transferred from one place to another ; 

 ultimately it may be removed out of reach of land plants, if it be carried by rain 

 into the sea. The case is the same with nitric acid ; when, owing to nitri- 

 fication of ammonia, this substance arises in the soil it is then of service 

 to the plants on the spot, only if it be at once absorbed by the roots. 

 Since the soil cannot retain nitric acid, all of it not at once absorbed by the root 

 will be washed away by rain and carried into rivers and, finally, into the sea. 

 The development of nitrogen in the gaseous form is of far greater importance 

 than the phenomena mentioned, which, on closer observation, resolve themselves 

 into translocations and transformations of combined nitrogen, and not into actual 

 loss of such. This evolution of free nitrogen takes place in the course of many 

 decompositions (Lecture XVII), and also in certain combustion processes. If 

 there were no organisms capable of making use of this free nitrogen, these losses 

 would be irretrievable. As a matter of fact such organisms are well known to 

 occur (Lecture XIX), and the power they possess is obviously of the greatest 

 importance in the circulation of nitrogen. 



We shall return to such vital phenomena later, meanwhile we may note 



