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In samples of suspected polluted water, spicific organisms were isolated as far. as possible. In pronouncing upon the character of a potable water, the following arbitrary standard was established :-
(1.) Water containing less than 300 colonies of micro-organisms per
cubic centimetre is a good and potable water.
(2.) Water containing from 300 to 1,000 colonies of micro-organisms per cubic centimetre is open to suspicion, and if used for potable purposes, ought to be filtered.
(3.) Water containing over 1,000 colonies of micro-organisms in 1 cubic centimetre is presumably contaminated by sewage or surface drain-
age.
A judicial interpretation of the sanitary quality of water is a matter of extreme difficulty. No absolute standards of purity can be established which shall rigidly separate the good from the bad. The factors involved in sanitary water analysis are so complex and the evidence so indirect that the process of reasoning much resembles au intricate.' question in medical diagnosis Any stan lar which is devised, as to the purity of a water, must be applied with great caution, and I trust that the limits of range set up by me are not stated in ton conservative a fashion. During the past year. I have had ample opportunity of comparing the results of my bacteriological examinations with those furnished by chemical. tests.
On estimating the delicacy and exactness of each method, I have drawn up the following summary of the position which the chemical and the bacteriological methods of examination at present occupy in their hearing upon the detection of pollution in water and water supplies.
The view that the chemical composition of water always has a direct relation to the number of micro-organisms, receives little support, unfortunately, from a large number of experiments which have been made.
Frequently a large number of micro-organisms is associated with an excess of solid matter. But the same chemical conditions may exist when the number of micro-organisms has been reduced to a vanishing point.
Again. water with little or no solid matter may contain a large number of micro-organisms, and the same chemical conditions may be present when these organisms are practically rendered non-existent.
In concluding from the standpoint of our knowledge at the present time, there is little doubt that, for the detection of actual sewage contamination, bacteriological methods are much more delicate and definite than chemical analysis.
It has, in fact, been shown that a recent contamination of a-water supply by sewage can easily be detected by bacteriological measures, even when the pollution is so slight as one part per million microscopic degree of pollution, which it is very doubtful if chemical methods would be able to derect. At any rate chemical analysis would not be able to detect the pollution to such an extent as to allow of an exact diagnosis.
To take a broad view of the value of both methods of examination of water, the following may be said :
Chemical analysis may be of value when an exact quantitative estimation is required. Bacteriological examination should be definitely qualitative.
The two methods ought to go hand in hand.
One must bear in mind that the purification of water by filtration through sand is essentially a biological process. The purifying efficiency of a filtering bed, from a disease point of view, can only be ascertained by resorting to bacterio- logical methods of examination.
morbi.
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Micro-organisms, and not chemical substances, are, after all, the real materies
Though the presence of chemical substances may indicate impurity, it does not follow that it is of a specifically dangerous kind,