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DIAGRAM SHOWING THE SEMIWEEKLY VARIATION IN THE NUMBER OF BACTERIA IN ONE CUBIC CENTIMETER

OF POTOMAC WATER. SHADED, AËROBES. BLACK, ANAEROBES.

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CHART B.
CHART SHOWING COMPARATIVE NUMBER OF BACTERIA FOUND IN ONE CUBIC CENTIMETER OF POTOMAC WATER IN 1886. ALSO DEATH RATE

OF TYPHOID FEVER IN 1886.
By Dr. Theobald Smith.

and the relation it bears to the monthly death rate of typhoid fever. It is an established fact that during the summer time the number of bacteria is diminished by certain agencies which will be spoken of later. It has been found that there is a relative increase in the number of intestinal and sewage bacteria during this time. This fact alone, it is believed, will go far in establishing the connection between typhoid fever and this group of bacteria. There is a wide difference of opinion concerning the viability of bacteria in water. Some claim that they will not develop in water unless it is very rich in nutrient material, and then only under the most favorable conditions, while there are others who cite instances where the bacteria material increases under the ordinary conditions, especially those of the intestinal group. It is also a well-recognized fact that temperature plays an important rôle in the development of bacteria, and we would expect a large increase in their number during the summer and fall. Practically this does not occur. Water containing sewage will preserve bacteria for a considerable length of time, especially if the temperature is low. The bacteria in water may be diminished by the germicidal action of strong sunlight, especially if the water is clear and still; yet under the most favorable conditions the sunlight affects only the under stratum, and has scarcely any effect upon those a foot or so below the surface. There is little or no effect if the water is turbid or moving rapidly. It requires a long time for bacteria to be affected by aeration. They will diminish under its influence if it is persistent, but not otherwise. The inference drawn from the oxidation of the bacterial and chemical products does not apply to the bacteria. The number of bacteria in the Niagara River are but slightly diminished in their plunge over the falls and subsequent aeration in the stream below. Perhaps the stream is not long enough to test this thoroughly, but I am of the opinion that there are no rivers in this country of sufficient length to accomplish this purpose. There is no doubt but that sedimentation accomplishes considerable in diminishing the number of bacteria. Sedimentation of bacteria is quite another thing from the deposition of suspended matter. A stream may free itself from turbidity and yet contain a large number of bacteria. Sedimentation of bacteria may, and often does, occur as the result of time and the distance the water is carried. The slower the volume of water is moved the more rapid will be the deposition. It remains to be demonstrated whether any sedimentation occurs in the Potomac River above Washington. If we take the turbidity as an index the sedimentation must be slight. So far the number of observations have not been sufficient to settle this question. A sesses of observations were recently made of the water in the receiving reservoir, which is located a short distance above the city. These observations were to determine the number of bacteria at different depths in the reservoir. The following are the results:

- Bacteria to

Station. Location. Sample. | 1 cubic centimeter. Station 1------------- Southwest corner of Res- | 1. Water from surface---- ------- 2,800 ervoir I. 2. At 14 meters -------------------- 1,670 3. Within 1 foot of bottom, at 34 330

meters.

Station 2------------- At influent gate, Reser- | 1. Water from surface------------ 330 voir No. 1 2. At 2 meters depth ----- 267 3. At bottom, 34 meters 193 Station 3--- At central gatehouse------ 1. Water from surface--- 1,067 2. From bottom, 34 meters 633 Station 4------------- Southwest corner of Res- | 1. One foot below surface -- 1,200 ervoir No. 2. 2. At bottom, 34 meters----- ---- 567 Station 5------------- Effluent gate, Reservoir 1. One foot below surface--------- 600 No. 2. 2. At bottom, 34 meters.---------- 8.33

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