188 
determined for each. Since the samples were collected at in- 
tervals throughout the year, the averages may be regarded as 
presenting in succinct form the chemical characteristics of the 
stations examined, and they may therefore serve as a basis for 
a comparison of the relative fertility of the localities. 
The residue upon evaporation, which comprises the solid 
matters left upon evaporating the water and drying the residue, 
includes both organic and inorganic substances. The inorganic 
constituents are salts, and comprise mainly compounds of lime, 
magnesia, soda, potash, iron and alumina with chlorine and with 
carbonic, sulphuric, nitric, and silicic acids. In this residue lie 
both the mineral constituents of the food of the phytoplank- 
ton and the undecayed organic matter found in the water. Not 
all of the constituents of the residue are equally utilized as 
food by the phytoplankton, so that the quantity of the residue 
gives a basis only fora very rough estimate of the fertility of 
the different waters. Some significance, however, attaches to 
the marked differences shown in the table. 
The differences in total residue in Illinois and Spoon rivers 
(367.5 and 522.3) and Quiver and Thompson’s lakes (268.9 and 
326.4) show no particular correlation with those of the average 
plankton production of these waters for corresponding periods 
(1.91, 0.884, 1.62, and 6.68 cm.> per m.3, as shown in Tables X.- 
XIII.). The amounts and relative proportions of the dissolved 
and suspended residue in these localities show some relation to 
the plankton production. The residue in suspension is not, in 
its present form at least, available for plant food. Its occurrence 
in the four localities is almost directly correlated with the rela- 
tive turbidity of the water Spoon River has from four to eleven 
times as much suspended matter (274.3) as the other localities, 
and this consists largely of clayey material with considerable 
fine quartz, neither of which contributes any considerable 
source of nutrition to the phytoplankton. The suspended ma- 
terial in the other locations at times of flood partakes of the 
character of that in Spoon River. At other times it contains 
a considerable proportion of debris of plant or animal origin 
