CH. XIl] THE CIRCULATION OF NITROGEN 293 



effect that nitrogen compounds were given off from the sea to the 

 atmosphere. He shewed that animal metabolism led to an 

 apparent predominance of ammonia salts in the sea, and nitrates 

 upon the land. In some way or other ammonia was volatilised 

 from the sea into the atmosphere. It is true that ammonia, if 

 present in solution in the sea in considerable proportion, would be 

 given off, to some extent, to the air. But it is present in such 

 minute proportions that we must regard this as improbable. 

 Again carbon dioxide also results from the metabolism of marine 

 animals, and we should expect that the free ammonia in the sea 

 water would be neutralised by it, forming a carbonate of ammonium 

 which would not be a volatile substance. Altogether Natterer's 

 hypothesis appears to be an improbable one. 



Denitrification in the sea. There remains therefore the 

 hypothesis that the excess of nitrogen compounds in the sea 

 is destroyed by bacterial activity, the nitrogen being returned to 

 the atmosphere as the elementary gas. This was first suggested 

 by Brandt^ in 1899 before it was definitely known that denitrifying 

 micro-organisms existed in the sea. It was obviously impossible 

 to suppose that a greater mass of such salts as nitrates of calcium, 

 sodium or potassium could be added to the sea than was taken 

 from it. Otherwise the sea water would in the course of time 

 become so surcharged with these substances as to form a medium 

 in which it is impossible that animals could live. Then it was 

 known that bacteria capable of converting nitric into nitrous acid, 

 nitrous acid into ammonia, and the latter compound into free 

 nitrogen, existed upon the land. We have seen that Baur and 

 Gran were able to isolate allied bacteria, capable of carrying out 

 the same reactions, from sea water. Here then was a very probable 

 explanation of the disappearance of the excess of nitrogen 

 compounds carried down from the land to the ocean. These 

 substances are reduced by micro-organisms, so that their nitrogen 

 reappears as the free elementary gas. This diffuses into the sea 

 water, and from the latter it is given off to the atmosphere. 



Density of marine life and temperature. The presence of 

 bacteria possessing this power of reducing the nitrogen compounds 

 ^ Stoffioechsel im Meeres, 1899, loc. cit. 



