342 POPULAR SCIENCE MONTHLY 



with reference to that of the cells which take their oxygen from it. 

 Yet is it none the less true that the supply of oxygen in most lakes is 

 very small as compared with that of an animal, and the mechanism for 

 renewing it is always very inefficient as compared with the demand for 

 the gas. 



The disadvantage of the lake in the matter of respiration appears 

 still more clearly when we consider the means of transporting the 

 oxygen from the region where it is absorbed — the surface — to the deeper 

 parts of the lake, where much of it is to be used. The animal shows 

 a complex and very efficient mechanism for the circulation of the 

 blood; an apparatus whose complexity and efficiency are in large meas- 

 ure determined by the necessity for a rapid distribution of the oxygen 

 and a rapid disposal of the gaseous wastes of the body. In the lake 

 the means of transport are three : diffusion, by which the gas is slowly 

 passed from point to point in the water independently of currents; 

 currents produced by the wind; and convection currents, produced by 

 the cooling of the surface water to a temperature below that of the 

 water beneath. 



Diffusion is a process which operates rapidly when the distances are 

 minute, but whose efficiency decreases greatly as the distances increase. 

 In our lungs, or the gills of a fish, for instance, where the distance 

 between blood and air is measured in thousandths of an inch, the process 

 of diffusion goes on with great rapidity. But where, as in the lake, the 

 distances are measured by inches or by feet, or even by scores of feet, 

 the process is practically worthless for the processes of distribution. By 

 diffusion alone oxygen would penetrate the lake only to the depth of 

 a very few feet in a whole season. While diffusion, therefore, plays 

 an active and important part in the exchange of gases between the 

 individual plant and animal and the water immediately surrounding it, 

 it has little or nothing to do with the general circulation of gases within 

 the lake. 



During the fall, when the lake is cooling, convection currents aid 

 materially in carrying oxygen down to considerable depths. The sur- 

 face water, saturated with oxygen, cools, becomes heavier, and sinks, 

 carrying the gas with it. The same process takes place at night in 

 summer, but ordinarily to very small depths. In general, we may say 

 that during early and mid-summer, before the period of general cooling 

 begins, these processes do not extend to greater depths than ten or 

 fifteen feet. At the season, therefore, when vital processes are most 

 active and the need for oxygen is greatest, convection currents afford a 

 minimum of assistance in distributing it. The main reliance, there- 

 fore, for the distribution of oxygen is in the third factor, the wind. 

 This, as already said, is very efficient when the lake is uniform in 

 temperature; but during the spring, as the lake warms, it becomes in- 



