' 2G 



the quantities of organic matters in the water of the Illinois 

 Kiver and in the lakes adjacent thereto are ordinarily very high 

 even for surface waters. During the last twelve mouths 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 distril)uted rainfall during the year of '97-'98 

 than Avas 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 Eiver 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 someMhat rapid decomposition. 

 The determinations of dissolved oxygen, consequently, commonly 

 give us figures someAvhat 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, lioAvever, have been obtained under 

 practically similar conditions throughout the season and they 

 show considerable variation, at times the quantity of dissolved 

 oxygen l)eing 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 Avhicli exists not in combination with the 

 bases in the water, but as gas merely dissolved in the water — 

 varies yet more greatly tlian 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 iuflnenced by i^ermitting the samples to 



