48 The Water of the Mississippi river — Dodge. 



manganate) shows at once to the eye that there has been oxida- 

 tion of organic matter in the water while on the way from St. Paul to 

 Hastings. I will say at this point that, in fact, the difference between, 

 the two samples of water here referred to, as shown by these tests, 

 proved itself greater than I had anticipated. For, seeing that the 

 river was at the time, and for months previously, clothed with a 

 comparatively close covering of ice, it seemed that the mount of 

 air finding access to the water and consequently the degree of ox- 

 idation of organic matter therein would be small. Still there have 

 been openings in the ice, made in various ways, through which 

 the air has found entrance 



The last column in the table shows the results for the water 

 taken from the river just above Winona. In tbe case of this 

 water, there are circumstances making conclusions rather less easy. 

 This case is complicated on the one hand by the entrance of the 

 St. Croix river and some other streams into the Mississippi between 

 Hastings and Winona, and on the other hand by the fact that there 

 are several towns along the way which contribute more or less 

 sewage to the river. The inflowing of those streams would un- 

 doubtedly make the results of our tests lower, because they would 

 bring a water somewhat purer than that of the Mississippi. The 

 presence of the settlements along the bank will of course raise 

 the results. 



Hence, as said, our conclusions must be somewhat uncertain, 

 in regard to the extent of the self-purification of the water in thi 

 case. In fact, on considering the results of the determinations of 

 chlorine and of solid residue, we see that the lower figures all 

 through the column of results for the Winona water are proba- 

 bly due partly to dilution by purer water, namely that of the St. 

 Croix, Chippewa and other streams; still the results under "free" 

 and- '^albuminoid" ammonia are so: considerably different from 

 those of the waters higher up the river that we are disposed to 

 take them as good evidence of the oxidizing process. 



It will be noticed that I have a column of figures for the 

 water taken from the river just above Minneapolis also. These 

 are the results which have been already communicated to the Acad- 

 emy in a recent repoit of a committee. They have very little 

 bearing on the present investigation, but are placed beside the 

 others merely for incidental reference. The real purpose of the 

 analyses outside of Minneapolis was to ascertain if the tests which 



