50 The Water of the Mississippi river — Dodge. 



thougli chemistry cannnot prove any existing infectious property^ 

 it can prove, if existing, certain degrees of sewage contamination. 

 And every sewage contamination which chemistry can trace ought, 

 jn-inuf facie, to beheld to include the possibility of infectious prop- 

 erties. * * * There is always a risk lest some portion (not 

 detectable by chemical or microscopical analysis) of the noxious 

 constituents of the original animal matters should have escaped 

 that decomposition which lias resolved the remainder into innocu- 

 ous compounds. * * * In the case of river Avater there is 

 great probability that the morbific matter sometimes present in 

 animal excreta will be carried rapidly down the stream, escape 

 decomposition and produce disease in those persons who drink the 

 water, as the organic matter of sewage undergoes decomposition 

 very slowly when it is present in running water. The researches 

 of Chauvean, Burdon, Sanderson, Klein and others leave no room 

 for doubt that the specific poisons of the so-called zymotic diseases 

 consist of organized and living organic matter; and it is now certain 

 that water is the medium through which some at least of these 

 diseases are propagated. It is evident, therefore that an amount of 

 exposure to oxidizing influences which may resolve the dead or- 

 ganic mitter present in water into innocuous mineral compounds, 

 may, and probably will, fail to affect those constituents which are 

 endowed with life." Again, in speaking of the possibility of ren- 

 dering polluted waters again wholesome, the same authority says: 

 '•When the sewage of towns or other polluting organic matter 

 is discharged into running water, the suspended matters may })e 

 more or less perfectly removed by subsidence and by tiltratiou, 

 but the foul organic matters in solution are very persistent. They 

 oxidize very slowly, and they are removed only to a sliglit extent 

 by sand filtration. * * The most efficient artificial filtration 

 leaves in water much invisible matter in suspension (as well avS in 

 solution) and constitutes no efPective safe-guard against the pro- 

 pagation of epidemic diseases })y polluted watei-. Boiling the 

 water for half an hour is a probable means of destroying its power 

 of communicating these diseases." Since this last paragraph was 

 published, more reliance, I Avill say, has come to be placed on fil- 

 tration and especially filtration through spongy iron, that is 

 metallic iron reduced from a porous oxide of iron, as a means of 

 purifying water. An application of the filtering process through 

 layers of spongy iron and sand has recently been made on a large 



