CH. XIl] THE CIRCULATION OF NITROGEN 281 



appearance of nitrogen may take place^ and we can explain this 

 fact only by assuming that the gas is dispersed into the atmo- 

 sphere, the compounds containing it being completely broken down 

 by the denitrifying bacteria that are present in the sewage 

 effluents. 



Drainage of nitrogen from land to sea. Thus the substance 

 that constitutes the living materials of the animal and plant 

 organism is sometime or other resolved into simple inorganic 

 compounds, or even into its chemical elements. In the active, 

 rapidly metabolising animal this change is continually taking 

 place, all the more quickly the smaller the organism is. In the 

 sedentary plant the process is much slower but a certain amount 

 of decomposition of organic matter takes place there also. Then 

 when the animal or plant body dies the dispersive process takes 

 place more rapidly. If denitrification occurs then the greater 

 part of the body may be resolved into the intangible gases, 

 nitrogen, carbon dioxide and water vapour, and may vanish into 

 "thin air" just as completely as if the tissues of the organisms 

 had been consumed by fire. All that would be left would be the 

 mineral constituents of the skeleton, the lime or silica, which 

 composed the bones or shells. Even these would ultimately be 

 dispersed through nature. Perhaps a similitude of the skeleton of 

 the animal might be preserved as a fossil ; perhaps as an impression 

 on the strata among which it becomes embedded. But even here, 

 the material of the fossil would be entirely different from that 

 which constituted the dead body. And there is no doubt that 

 only an insignificant fraction of the animals and plants that have 

 existed on the earth has persisted in the fossil form. Of the 

 water and carbon dioxide which result from decomposition processes, 

 some part escapes into the atmosphere, and another part enters 

 the soil and is washed down by rain into streams and water-courses. 

 So also with the nitrogen, and the nitrous and nitric acids : some 

 part escapes into the air as the elementary gas. We cannot trace 

 the further history of this portion which is " imprisoned in the. 

 viewless winds, and blown with restless violence round about the 

 pendant world." But some of these substances enter the rivers 

 and ultimately are carried down into the sea. 



