290 THE CIRCULATION OF NITROGEN [PART III 



as manure; and fairly large quantities of starfishes^ are also so 

 taken. Storms and high tides must strand considerable masses of 

 sea-weed, &c. which rots and decays. If, taking these circum- 

 stances into account, . we estimate the annual depletion of the 

 North Sea as about 25 millions of kilogrammes of nitrogen, I think 

 the estimate might be an approximate one. To put it at 30 

 millions would probably be too high. 



Again any figures that may be given for the addition of nitrogen 



compounds to the sea can only be roughly approximate ones. We 



cannot say, with much accuracy, what volume of fresh water enters 



the sea via the great rivers, though just such estimates are 



frequently made use of by the geologists. Neither can we claim 



very great accuracy for the determinations that have been made 



of the amount of dissolved nitrogen compounds in river water; 



though again it may be observed that these estimates are of 



•corresponding value to those of the amount of suspended mineral 



matter, or dissolved saline matter, carried down to the sea in the 



rivers. Such data have been made use of in attempting to 



estimate the rate of detrition of the land, or the age of the crust 



of the earth, and, so far as I know, are generally accepted^. Perhaps 



we may safely assign certain limiting values for the addition of 



nitrogen to the North Sea ; and the annual amount of 390 



millions of kilogrammes appears to me to be such a limiting value. 



For not only are nitrogen compounds carried down to the sea in 



solution in river water, but they are also precipitated upon the 



surface from the atmosphere. Every flash of lightning, or other 



electric discharge in the air, causes some of the elementary nitrogen 



and oxygen to combine, forming oxides of nitrogen, which is 



carried down to the sea, as nitrous and nitric acids, in the rainfall. 



Unfortunately it is impossible to estimate the amount of nitrogen 



fixation that takes place in this manner^. Then we have seen 



that bacteria are present in the sea which possess the power of 



1 In some parts of England £1 per ton is paid for this produce. 



- See Appendix, Palaeochemistry of the primitive ocean. 



'^ The amount of nitrogen in the form of nitrates, nitrites and ammonia that is 

 annually added to the land from the atmosphere has been variously estimated. 

 According to the results of the Rothamsted experiments about 4-4: lbs. are annually 

 deposited by rain on each acre of land. "Rain" includes snow, hail and dew. 

 Other and different estimates have, however, been made. 



