The ]J\ifer of the Mississipj)i river — Dodge. 47 



plicating the question ; there are on the other hand no towns or 

 settlements of importance along that distance and hence no influx 

 of impurity on the way. The distance is one of about thirty 

 miles, a sufficiently long one to offer a fair test of the point in 

 question. If the change in the character of the water by oxida- 

 tion or other natural causes within the distance of thirty miles be 

 not enough to shew itself through the ordinary methods of water 

 analysis, then it would be rather too unim])ortant to be referred to 

 in connection with sanitary matters. Now, on referring to the table,, 

 we observe in the case of each test that the waier from the river above 

 Hastings (it was in fact taken just above that town) shows itself 

 better than the water from just below St. Paul. Setting aside the 

 determinations of chlorine and of the total residue from evapora- 

 tion, as from the nature of the case of little concern here, since 

 there are evidently no special causes for their diminution, we ob- 

 serve that the water from above Hastings gives us lower figures- 

 than the water below St. Paul. The free ammonia has suffered a 

 very considerable diminution. The albuminoid mattei's from which 

 the "albuminoid ammonia" is derived by the method of Wanklyn,^ 

 are also much less in amount. The explanation of their diminu- 

 tion is to be found mainly in a process of actual oxidation. The 

 amount of nitrogen present in nitrates and nitrites is also less in 

 the Hastings water. I had hardly expected much difference be- 

 tween the two in this respect, as T supposed the oxidation of nitro- 

 genous matter represented by the ammonia would keep up the 

 amount of nitrates and nitrites, so that the nitrogen would re- 

 main in the water in another combination. The explanation of 

 the lower figures for nitrogen is, probably, that the nitrates and 

 nitrites which were in the water at St. Paul, mixing with other 

 organic matter also introduced at St. Paul, were reduced by this 

 other organic matter and the nitrogen was set free in the elemen- 

 tary state. The same change must have happened to such nitrates 

 and nitrites as were produced by the above mentioned oxidation 

 of ammonia. Looking at the figures giving the results of the 

 permanganate test (Forschammer's and Tidy's process,) we again 

 find a considerable difference to the advantage of the Hastings water. 

 That is, the amount of oxygen required for performing a work of 

 destruction on organic matter, carbonaceous and nitrogenous, in 

 the Hastings water was much less than in the water at St. Paul. 

 This very convenient and, in the main, satisfactory test (by per- 



