190 
seem that the river water might under more favorable condi- 
tions develop a more abundant plankton. These favorable 
conditions are to be found in the quiet backwaters of river-fed 
lakes, where time for breeding is afforded. 
The loss which the residues of total solids suffer upon igni- 
tion (heating to redness) includes the organic matters which 
are burned away and such constituents of the mineral matters 
as are volatile or are decomposed by heat into volatile sub- 
stances. In stream waters the suspended portion of this mate- 
rial may be a rough index of the quantity of plankton and silt 
of organic origin, all of which on decay add to the water sub- 
stances available for plant food. From the data in the table it 
may be ascertained that the four localities yield respectively, 
in the order of the the table, 7.7, 17.5, 1.9, and 8.2 parts per 
million of such material. The excess in Spoon River (17.7) is 
doubtless due to silt of organic origin, while the plankton pre- 
sumably forms a larger proportion in Thompson’s Lake and in 
the Illinois River. The poverty alike of plankton and of silt in 
Quiver Lake is reflected in the small amount (1.9) lost on igni- 
tion in its waters. The loss, on ignition, of substances held in 
solution shows no differences at all commensurate with the 
relative production of plankton, though the trend of the differ- 
ences is similar in three instances of the four. 
The chlorine is contained in surface waters in combination 
with various basic elements, but chiefly in the form of common 
salt. Its principal source is animal matter, sewage, or drain- 
age from refuse animal matter. In our river and lake waters 
it is largely an index of their relative contamination with sew- 
age from cities within the drainage basin. Since its combina- 
tions are not utilized by plantsas foodinany considerable quan- 
tity, at least as compared with other constituents of the sew- 
age, such as the nitrates, the chlorine becomes the best crite- 
rion of the amount of sewage andthus of the principal adventi- 
tious fertilizer which the waters examined by us contain. The 
differences in the four localities are striking and significant. 
The average chlorine in the Illinois River (21.6) is more than 
