214 



The Sanitary Value of the 



reservoir, pond or river is defiled by sewage matter. Now in the first 

 case the pollution of the well may be harmless, but in the event of 

 specific polluting material entering at any time, as it is quite likely to 

 do at some time, such a well may become a center for spreading dis- 

 ease, and if so its water should not be used at all, and in the other 

 case, if a larger body of water is markedly polluted by sewage, while 

 the certainty of its proving unwholesome or producing specific diseases 

 may not be provable, we shall err on the safe side if we refuse to use 

 such water, the chemical evidence beiug considered, at least, until we 

 can cut off the access of contaminating matter. 



And lastly an analysis may assure us that a given water is of such a 

 degree of purity that the probabilities are that it carries with it no 

 harmful matter, but this kind of evidence is always to be received with 

 caution unless the results of the examination are borne out by other 

 evidence which proves that pollution is not probable. Thus while a 

 chemical analysis alone cannot tell us that a water is absolutely safe 

 or necessarily harmful, it can tell us that a water contains those con- 

 stituents which may reasonably be believed to accompany harmful 

 matter, and if the question be as to the employment of a given water 

 for a city supply, an exhaustive series of analyses made at different 

 points, stages, times and seasons may reveal to us the degree of pollu- 

 tion, points at which the polluting matter enters, rapidity of its de- 

 struction by natural processes, and the conditions of the water un- 

 der varying circumstances, and also indicate to us the methods to 

 be employed for improving its quality and obtaining it at its best es- 

 tate. Such series of analyses have been made during late years in our 

 own country by various chemists, and with very satisfactory results. 

 But a single analysis, or a few analyses, may give us little information 

 or be entirely misleading. 



Previous reference has been made to the investigations of Professor 

 Mallet, and as no more thorough work has probably ever been done in 

 this direction, at home or abroad, and as the conclusions arrived at 

 as the result of an exhaustive study of a multitude of facts confirm 

 the opinions just expressed, some of these conclusions from his re- 

 port are quoted: 



" 1. It is not possible to decide absolutely upon the wholesomeness 

 or unwholesomeness of a drinking water by the mere use of any of 

 the processes examined for the estimation of organic matter or its con- 

 stituents." 



" 2. I would even go further and say that in judging the sanitary 

 character of a water, not only must such processes be used in con- 

 junction with the investigation of other evidence of a more general 

 sort as to the source and history of the water, but should even be 



