| 0- | 100 bacteria per cc. = | a good potable water, |
| 100- | 500bact"erieri"teria= | a suspicious water. |
| 500- | 1000 or moreeri"teria= | a water which should have further filtration before being used for drinking purposes. |
The personal view of the writer after some experience of water examination would favour a standard of "under 500" being a potable water, if the 500 were of a nature indicating neither sewage pollution nor disease. Miquel holds that not more than ten different species of bacteria should be present in a drinking water, and such is a useful standard. The presence of rapidly liquefying bacteria associated with sewage or surface pollution would, even though present in fewer numbers than a standard, condemn a water. Thus it will be seen that it is impossible to judge alone by the numbers unless they are obviously enormously high.
Wolfhügel's Counter
When we are counting colonies upon a Koch's plate, Wolfhügel's counter may be used. This is a thin plate of glass a size larger than Koch's plates, and upon it are scratched squares, each square being divided into nine smaller squares. The Wolfhügel plate is superimposed upon the Koch's plate, and the colonies counted in one little square or set of squares and multiplied.
Petri's Dish
By using flat, shallow, circular glass dishes, generally known as Petri's dishes, instead of Koch's plates, much manipulation and time is saved, and, on the whole, less risk of pollution occurs. Moreover, these are easily carried about and transferred from place to place. When counting colonies in a Petri's dish it is sufficient to divide the circle into eight equal divisions, and counting the colonies in the average divisions, multiply and reduce to the common denominator of one cc. For example, if the colonies of the plate appear to be distributed fairly uniformly we count those in one of the divisions. They reach, we will suppose, the figure of 60; 60 × 8=480 micro-organisms in the amount taken from the suspected water and added to the melted gelatine from which the plate was made. This amount was .25 cc. Therefore we estimate the number of micro-organisms in the suspected water as 60 × 8=480 × 4= 1920 m.-o. per cc., which is over standard by about 1500. A water might then be condemned upon its quantitative examination alone or qualitative alone, or both. If the quantity were even that of an artesian well, say 4–10 m.-o. per cc., but those four or ten were all Bacillus typhosus, it would clearly be condemned on its quality, though quantitatively it was an almost pure water. If, on the contrary, the water contained 10,000 m.-o. per cc., and none of them disease-producing, it would still be condemned on the ground that so large a number of organisms indicated some kind of organic pollution to supply pabulum for so many organisms to live in one cc. of the water. It is not the number per se which condemns. The large number condemns because it indicates probable pollution with surface water or sewage in order to supply pabulum for so many bacteria per cc.
It should always be remembered that a chemical report and a bacteriological report should assist each other. The former is able to tell us the quantity of salts and condition of the organic matter present; the latter the number and quality of micro-organisms. Neither can take the place of the other and, generally speaking, both are more or less useless until we can learn, by inspection and investigation of the source of the water, the origin of the organic matter or contamination. Hence a water report should contain not only a record of physical characters, of chemical constituents, and of the presence or absence of micro-organisms, injurious and otherwise, but it should also contain information obtained by personal investigation of the source. Only thus can a reasonable opinion be expected. Moreover, it is generally only possible to form an accurate judgment of a water from watching its history, that is, not from one examination only, but from a series of observations. A water yielding a steady standard of bacterial contents is a much more satisfactory water, from every point of view, than one which is unstable, one month possessing 500 bacteria per cc. and another month 5000. It is obvious that rainfall and drought, soil and trade effluents, will have their influence in materially affecting the bacterial condition of a water.
It is perhaps scarcely necessary to add that we have not in the above account of the examination of water included all, or nearly all, the various methods adopted for acquiring a knowledge of the bacterial contents of the water. Many of these are of too detailed and technical a nature to enter into here. Three points, however, we may touch upon. In the first place, as we have said, the particulate matter out of a large body of water should be concentrated in a small quantity. Accordingly it has become the custom to pass 2000 or 3000 cc. of the suspected water through a Berkefeld filter. When this has been accomplished, by means of a