346 POPULAR SCIENCE MONTHLY 



water of the lake in summer. If the oxygen of this region is studied, 

 it rarely happens that the quantity found is the amount which would 

 be theoretically expected, according to the laws of the absorption of 

 gases by water at different temperatures. It is sometimes largely in 

 excess of the theoretical amount, and sometimes is considerably defi- 

 cient. The fact is that the amount of oxygen in the upper water of 

 the lake is the resultant of very numerous and variable forces. The 

 lake may or may not be absorbing oxygen from the air. If saturated, 

 it will give off oxygen to the air as the water warms, or will take it in 

 as it cools. Both of these processes go on somewhat slowly, and the 

 oxygen is not given off or absorbed as rapidly as the water warms or 

 cools. Into the water the green plants are discharging oxygen during 

 the hours when the light is sufficiently strong; from the water both 

 plants and animals are taking oxygen to assist their vital operations; 

 and the process of decomposition is aiding to exhaust the stock of 

 oxygen. Thus the amount present at any given moment will depend 

 on the relative value of these forces; some of them positive; others 

 negative; and all varying not only from day to day, but from hour to 

 hour. Nor do these factors exhaust the list. The wind has something 

 to do here ; during a calm period the oxygen content of the upper water 

 may differ from that of a stormy period. The vital condition of the 

 successive crops of alga?, as they come and go, may determine for the 

 time the predominance of the manufacture of starch, with accompany- 

 ing liberation of oxygen, or decomposition, with partial exhaustion of 

 oxygen. Thus the ability of the green plant to set free oxygen into 

 the upper water may be of great value in maintaining the supply of 

 the lake. 



This power may be far more important in the lower water. If the 

 transparency of the water and the thickness of the warm layer are such 

 that a good deal of light can penetrate to the colder water, alga? will be 

 able to manufacture starch in the upper part of this stratum. Thus in 

 the region which is practically cut off from access to the atmosphere, 

 large amounts of oxygen maj r be set free. There may be enough not 

 only to serve the ordinary needs of the stratum, but the water may be 

 saturated or even oversaturated with the gas. To illustrate this point 

 I give a diagram (Fig. 1) showing the vertical distribution of oxygen 

 in Elkhart Lake, "Wisconsin. This figure shows clearly the position 

 and amount of the manufactured oxygen, and the addition which it 

 makes to the thickness of that part of the lake that has abundance of 

 oxygen. Lakes whose habitable portion would otherwise be only twelve 

 to twenty feet in thickness may have this depth nearly or quite doubled 

 by the presence of the manufactured oxygen. The plants in this 

 undisturbed cooler water find a peculiarly favorable situation for 

 growth. They obtain for their food the products of decomposition, 



