ORGANIC CONTENT OE LAKE WATER 
195 
We found that the De Laval centrifuge, employed in the extraction of the 
plankton for the plankton report, removed 35 to 50 per cent of the bacteria. The 
Sharpies supercentrifuge, used by Domogalla, Juday, and Peterson, removed about 
70 per cent of the bacteria (p. 270). It is a fair conclusion that about the same 
percentage of the particles of debris also was removed, at least down to the size 
of the bacteria. 
It is a fair conclusion also that the weight of the very numerous particles left 
in the water by the centrifuge is only a small fraction of the amount extracted by the 
centrifuge. In the plankton report (p. 89) there was computed the weight of the 
bacteria of Lake Mendota. At the summer maximum of 30,000 per cubic centimeter 
there were about 20,000 rods averaging 2.5m long and 0.9m in diameter, and about 
10,000 cocci averaging 0.44m in diameter. Such a crop would yield 0.0048 milligram 
of dry organic matter per liter, which would represent about one-half that weight 
of organic carbon. Such an amount is almost infinitesimal when compared with 
the average of nearly 6.0 milligrams per liter of dissolved organic carbon found 
in the water of Lake Mendota. 
We may conclude, therefore, that only a small part of the organic material found 
in the evaporated residues is in the particles left behind by the centrifuge. The 
nitrogen and carbon of Table 2 may be called “dissolved” in any sense of that word 
in which it is properly applicable to colloids. 
The same conclusion was reached directly by filtration experiments. In two 
cases 100 liters of water from Lake Mendota were divided into two parts after centri- 
fuging. One-half was evaporated at once and the other was passed through 
a Berkefeld filter before evaporation. In both cases the nitrogen was determined 
both as to amount and kinds. The filtered water yielded slightly less organic nitrogen 
than did the unfiltered. 
Table 7. — Organic nitrogen in filtered and unfiltered water, milligrams per liter 
Date 
Filtered 
Unfiltered 
Dee. 16, 1924 
0.306 
.341 
0.319 
.351 
Feb. 19, 1925 
These specimens were used in the study of the nitrogenous compounds, which 
agreed in detail as closely as in the total. 
As some part of any colloids in the water would adhere to the filter and thus 
reduce the total nitrogen, it does not seem probable that any considerable part of 
the weight of the organic substances left in the water by the centrifuge exists in the 
form of particles. It is certain that very little is left that is removed by a Berkefeld 
filter. 
Those who have studied the organic content of sea water (Henze, 1908; Raben, 
1910; Moore et al., 1912) have left the subject with the statement of the amount 
of organic nitrogen and carbon. Table 2, therefore, gives a basis of direct compari- 
son between the conditions existing in fresh and in salt water. If comparisons 
regarding nonorganic nitrogen are desired also, the data may be found in the paper 
on the forms of nitrogen (p. 273). 
