33§ POPULAR SCIENCE MONTHLY 



of the blood, and the plants and animals in the lake. As is the case in 

 the animal, the respiratory gases are absorbed and transmitted to the 

 living structures by means of the unorganized fluid. It is my purpose 

 to trace in outline the history of these processes and their result upon 

 the activity of the lake. 



*J 



The respiration of the lake, like that of the higher animal, may be 

 divided into external and internal respiration. By the former we 

 understand the adsorption of certain gases from the air and the return 

 of other gases to it, as well as the processes by which this exchange is 

 effected. We include in it also the methods by which the gases are 

 distributed in the lake and conveyed to and from the surface of the 

 water, which takes them from the atmosphere and gives them back to 

 it. By internal respiration we mean the gaseous exchanges which take 

 place in the lake itself, between its various organisms and the water 

 surrounding them. With these exchanges come the chemical processes 

 by which the character of the gases is altered or new gases manu- 

 factured, in the course of the vital activities of the inhabitants of the 

 lake. 



The external respiration of the lake closely resembles that of the 

 organism. The lake absorbs oxygen, carbon dioxide and nitrogen from 

 the atmosphere, and returns to it nitrogen, carbon dioxide and some- 

 times other gases. The nitrogen absorbed by the lake, like that taken 

 in by an animal, has very little or nothing to do with the vital processes. 

 In autumn, as the lake cools, larger amounts of nitrogen are absorbed, 

 according to the general law of absorption of gases. As the lake 

 warms during the summer season, the capacity for holding gases in 

 absorption becomes smaller and some of the nitrogen is lost. This 

 process is a purely physical one and has apparently no influence on the 

 life of any of the organisms whose home is in the water. 



The relation of the oxygen to life is, however, far different, and the 

 processes of external respiration are of prime importance to the living 

 beings of the lake. Speaking roughly, and in terms of our comparison, 

 we may say that an inland lake is an organism which takes one full 

 inspiration in the fall, and another, less complete, in the early spring; 

 that during the winter it does not breathe at all and during the summer 

 has only a very shallow and imperfect respiration. As the lake cools in 

 the fall the temperature becomes uniform from top to bottom at a date 

 which will vary from late September to late November or early De- 

 cember, according to the area and the depth of the lake and the con- 

 sequent temperature of the bottom water, the volume of water to be 

 cooled, and the vigor of the cooling processes. When the temperature 

 has thus become uniform, the water of the lake is readily moved 

 throughout its entire depth by the wind. It is turned over and all 

 parts of it are brought into contact with the atmosphere. As a result, 



