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16 ILLINOIS ACADEMY OF SCIENCE 
course, to the greater volume and more rapid movement of the 
stream since Lake Michigan water was introduced into it 
through the Sanitary Canal. 
We also find that the river in its middle course, as at Havana, 
now carries a larger store of nitrogen in combinations which 
make it available as food for the river plankton than before 
1900, but that a very much larger volume of these food materials 
now passes on into the Mississippi unused. In other words, the 
Illinois is now receiving a much larger food supply than it can 
digest and assimilate within its own course, and these pro- 
cesses are consequently carried forward in the Mississippi to a 
much greater extent than formerly. 
Quite consistently with the foregoing, the successive steps of 
the self-purification process, evidenced by the appearance of a 
green plankton and an increasing saturation of the water with 
dissolved oxygen, occur now much farther down the stream 
than before 1900. It is important to notice, however, that the 
biological conditions of the river become normal much farther 
up-stream than the chemical conditions. In respect to the 
plankton forms, Averyville (just above Peoria) is now about as 
La Salle was before 1900; but the former chemical conditions 
at La Salle are now scarcely reached at Grafton, at the mouth 
of the river. We also find that the return to normal conditions, 
both chemical and biological, progresses more slowly down- f 
stream in the bottom sediments of the river than it does in the 
water over them—that the bottom mud of a given area in the 
upper river is relatively fouler than the water which flows 
over it. 
This whole subject is very complex and difficult, but Mr. R. 
E. Richardson, one of the biologists of the Survey, has given his 
entire time to its study for a little more than a year, and we 
hope to have a detailed report in print before the summer is 
over. 
Since the last report of the committee, Dr. V. E. Shelford, 
serving as one of the biologists of the Natural History Survey, 
has completed and published an investigation of the effects of 
coal gas on fishes. A preliminary summary of the results of 
this work was, indeed, printed in the Academy Transactions 
