48 



AERATION AND AIR-CONTENT. 



July 20, when the water-bloom appeared. He found 4.6 c.c. of oxy- 

 gen and 7.7 c.c. of CO 2 in tap-water per liter. The lowest amount 

 of oxygen was 2.32 c.c., when the water was muddy after heavy 

 rains, and the highest amount of CO 2 was 10 c.c., when the lagoon was 

 covered with ice. Richards (1917 : 331) concluded that rain-water 

 is very nearly saturated with oxygen when its temperature as col- 

 lected is below 15 C., but is less than saturated to as much as 25 per 

 cent when the temperature is above this. The amount of oxygen in 

 summer rain has been shown by Russell and Richards (1919 : 328) 

 to be 95 per cent of saturation and in winter rain 99 per cent. The 

 number of pounds per acre is 26 for the winter from November to 

 February inclusive, and 20.8 for the summer, May to August. It is 

 also estimated that 66.4 pounds of dissolved oxygen per acre is 

 brought down by rain in a year. 



Bergmann (1920 : 23) has shown that the oxygen-content of lake 

 and bog water decreases with the amount of vegetation, while that 

 of carbon dioxid correspondingly increases, as seen in table 13. 



TABLE 13. 



Pearsall (1920 : 171) has determined the changes in the dissolved 

 gases of English lakes during the growing-season in relation to the 

 plant communities. The avergages were expressed in per cent as in 

 table 14. 



TABLE 14. 



Samples taken above different communities showed no important 

 difference in the carbon-dioxid content (table 15). 



