CH. XIl] THE CIRCULATION OF NITROGEN 291 



combining elementary nitrogen with other elements to form 

 proteid. Of the two known genera Clostridium is anaerobic, while 

 Azotohacter is aerobic. Between them they can make use of a 

 variety of carbonaceous food-materials. They appear to be widely 

 distributed, and their life-conditions are such as, apparently, enable 

 them to carry on the nitrogen-fixing function under diverse 

 circumstances. The elementary gas is present in solution in sea 

 water, and as fast as it is taken out of solution more can be 

 dissolved from the atmosphere. We cannot, of course, attempt any 

 estimate of the mass of nitrogen compounds thus added annually 

 to the sea ; but it is evident that it cannot be neglected. 



Let us take 30 millions of kilogrammes as the mass of nitrogen 

 that is annually taken from the North Sea. Probably this is a 

 maximum estimate, being greater than the actual mass. Again 

 let us take 390 millions of kilos, as the mass of nitrogen that 

 is annually added to the North Sea. Probably this is a minimum 

 estimate, being less than the mass really added. Therefore we 

 find that : — 



(1) About 30 millions of kilos, of nitrogen at the most are 

 annually taken from the North Sea, whereas 



(2) About 390 millions of kilos, of nitrogen, at the least, are 

 annually added to the North Sea. So that, even if we make every 

 allowance for errors in the deduction of these estimates, it is still 

 apparently the case that much more nitrogen, in a form easily 

 assimilable by plants, is added to this area than is taken from it. 

 Probably ten times as much combined nitrogen enters the area 

 than is taken from it. What becomes of the excess ? 



Destruction of nitrogen compounds in the sea. The North 

 Sea is, of course, in open communication with the Atlantic and 

 Norwegian Seas, and much of the nitrogen compounds which enter 

 it may be widely dispersed throughout the larger area. But this, 

 obviously, does not afford any explanation, for the reverse is also 

 the case, many of the food-fishes captured in the North Sea being 

 migratory animals that enter it from without. Plankton also may 

 be transported into the smaller sea. And we must remember that 

 the same influx of nitrogen compounds goes on all round the shores 

 of the Atlantic. Of course the length of coast-line bears a smaller 



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