26 
the quantities of organic matters in the water of the Tllinois 
River and in the lakes adjacent thereto are ordinarily very high 
even for surface waters. During the last twelve months the 
average in the river and also in the lakes shows that the 
quantity of sewage contained has been considerably less than it 
was during the preceding twelve months, due of course to the 
fact that there have been repeated periods of high water and a 
more generally distributed rainfall during the year of ’97—98 
than was the case during ’96-’97. 
Attempts have also been made during the last year to deter- 
mine the quantity of dissolved oxygen and of dissolved or free 
carbonic acid gas in the waters of the Illinois River and the 
lakes at Havana. We have met with considerable difficulty in 
making these determinations because of the necessity of doing 
most of the work here at Champaign. The conditions are 
particularly unfavorable for work at a distance because of the 
large quantities of organic matters contained in the water, and, 
further, because these matters are in a state of putrefactive 
change, that is, are undergoing somewhat rapid decomposition. 
The determinations of dissolved oxygen, consequently, commonly 
give us figures somewhat too low. Tests made on the spot at 
the time of collection show that the quantity of dissolved oxygen 
is ordinarily diminished very quickly on standing a few hours, 
particularly when the vessels containing the water are exposed 
to the light. Our results, however, have been obtained under 
practically similar conditions throughout thé season and they 
show considerable variation, at times the quantity of dissolved 
oxygen being exceedingly low, while at other times the quantity 
reached the approximate maximum figure. This is true not 
only of water contained in the river, but also of the water of the 
lakes. The data which we have in hand has not yet been 
digested and cannot be until the work has been carried on 
“somewhat more extensively. The free carbonic acid,—that is 
the carbonic acid which exists not in combination with the 
bases in the water, but as gas merely dissolved in the water— 
varies yet more greatly than does the dissolved oxygen. At 
times there seems to be none present; at other times the 
quantity present is quite considerable; but these determinations 
are even more greatly influenced by permitting the samples to 
