ORGANIC CONTENT OF LAKE WATER 
187 
dues, and at present there is no way of distinguishing them. Probably by far the 
greater portion of the “soluble nitrogen” or the “dissolved carbon” is dissolved 
only as egg albumen or starch may be said to be dissolved in water. 
The organic carbon was determined by Doctor Wise, who had for his exami- 
nation 31 specimens of dry residues; 14 of these were from Lake Mendota, 14 from 
12 other lakes, and 3 from 2 rivers. They represented the larger samples prepared 
by Doctor Domogalla, the material of which had not been used up in the study of 
the nitrogen. 
The specimens for determination of organic carbon were dried carefully to 
constant weight, over concentrated H 2 S0 4 , in a vacuum. The temperature was 
kept at 46 to 47° by an electrically heated nichrome wire, and a pressure of 60 to 
70 millimeters of mercury was maintained. Under these conditions the organic 
matter should not be decomposed and the water mechanically held should be 
dissipated. Most of the air-dry specimens lost 4 to 5 per cent in this process. 
The total carbon dioxide was determined by the ordinary methods of organic 
combustion. The evolved (carbonate) carbon dioxide was determined by a method 
almost identical with that of Hillebrand (1919). Two soda-lime tubes, however, 
were found to be more satisfactory than one. 
All determinations were made in duplicate and checked closely. Two examples 
are given; these are Nos. 21 and 42, each the residue from the evaporation of 100 
liters of water from Lake Mendota. No. 21 came from surface water; No. 42 was a 
bottom sample; both were collected on July 24, 1923. 
Table 1. — Determination of carbon dioxide in dry residue from water of Lake Mendota 
No. 21 
No. 42 
A 
B 
A 
B 
Total COj 
Per cent 
47. 75 
27. 65 
Per cent 
47. 77 
27.83 
Per cent 
44. 45 
27.99 
Per cent 
44.89 
28. 21 
Evolved COi 
Difference 
20. 10 
19.94 
16.46 
16.68 
The results show 20.02 per cent of organic C0 2 in the surface sample and 16.57 
per cent in the bottom sample, corresponding, respectively, to 5.45 and 4.53 per 
cent of organic carbon. The surface yielded 135 milligrams of dry residue per 
liter of lake water, and the bottom 129.2 milligrams. The organic carbon, there- 
fore, was 7.36 milligrams per liter in surface water and 5.85 milligrams in water at 
20 meters. Already there had been found 0.407 milligram per liter of soluble 
organic nitrogen in the surface sample and 0.404 milligram in the water from the 
bottom. These figures are reached by adding the amounts of nitrogen given 
(nitrogen report, p. 274) under the heads of free-amino, peptide, and nonamino 
nitrogen. 
