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of the bacteria in the applied water, constitute less than forty per cent. of those in the effluent. These two species are very prominent ones in the underdrains (No. 20) and in the air (No. 11.0). A comparison of the remaining species indicates, also, that it is far more reasonable to conclude that they have had their origin in either the underdrains or the air, than that they have passed with the applied water (containing very small numbers) through the filtering material. The second portion of the table shows a very striking resemblance between not only the species of bacteria but their relative numbers, in the effluent and those in the underdrains and outlet pipe. The relation of the species and their numbers, in the case of the bacteria of the applied water and the effluent, is far less marked ; while two species present in the applied water, No. 11.2 = B. coli communis, and No. 55 = B. aurantiacus, were not found in the effluent.

The investigation up to this point confirmed the results of the work of previous years ; namely, that the few bacteria in the effluent have their origin in the underdrains and outlet pipe, and do not pass through the filter. In order to obtain still more definite data, it was determined to increase as much as possible the number of applied bacteria, and to guard, with the utmost care, against contamination of the efluent. To this end " canal” water taken from the Merrimack River above Lawrence, and containing from one thousand to fifteen thousand bacteria in a cubic centimeter, was applied to the filter, commencing Nov. 13, 1891. The etiluent was protected from contamination by dust and air by covering the outlet pipe with a large zinc-lined box, which was frequently washed with corrosive sublimate, and so arranged that the stopper of a sample bottle could be removed beneath it and all manipulations could be done without disturbing it.

From the date of application of canal water to the filter several determinations of the number of bacteria in the applied water and in the effluent were made daily from samples collected at different hours. The results are as follows:

Summary of Bacterial Counts. (Nov. 13– Dec. 31, 1891.)

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Eduent from Filter Tank No. 8,

102

0.83

99.97

Of the one hundred and two determinations made of the effluent, fifty-eight showed no colonies on the gelatine plates. The determinations of the number of bacteria in canal water and in the effluent from Filter Tank No. 8 were made on the same gelatine, grown for the same length of time and at the same temperature. Many duplicate determinations were made of the effluent, and the plates were allowed to stand eight and even ten days before counting. They agreed very well with those counted after three days; rarely a colony was found which proved to be a spore-former, and had no relation to the leading species in the applied water.

A quantitative species determination was made of all samples of Filter Tank No. 8 effluent, as well as of canal water, at frequent intervals. The results are given below. It will be seen that these determinations indicated the presence of twelve species in the canal water. This does not mean that they were the only species of bacteria present, but that these species were the leading ones, occurring in numbers sufficiently great to be found on plates of samples diluted one to one hundred.

Number of Bacteria of Each Species found in a Cubic Centimeter of Canal Water.

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Number of Bacteria of Each Species Found in a Cubic Centimeter of Effluent

from Filter Tank No. 8.

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Number of Bacteria of Each Species Found in a Cubic Centimeter of Effluent

from Filter Tank No. 8- Concluded.

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Table showing the Average Number of Each Species of Bacteria in a Cubic Centi

meter in Canal Water and in the Efluent from Filter Tank No. 8.

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The results of this investigation may be stated as follows:

1. The application of water taken from the canal (Merrimack River water), instead of city water which contains a much smaller number of bacteria, commencing Nov. 13, 1891, was followed by a decrease rather than an increase in the total number of bacteria found in the effluent from Filter Tank No. 8. The decrease was due to improved manipulation.

2. No relation could be distinguished between the rates of flow and the number of bacteria in the effluent.

3. The quantitative determination of the species of bacteria in the applied canal water and in the effluent indicated that three prominent species, Nos. 20.1, 11.2 and 81, were not present in the water after its passage through the filter. Of these species, No. 11.2 is B. coli communis, the leading species in human fæces, and also, during November and December, 1891, in the water of the Merrimack River at Lawrence, in which it formed thirty-nine per cent. of the bacteria. It is also to be remembered that this species resembles very closely the bacillus of typhoid fever.

4. The presence of the very small numbers of bacteria in the effluent was satisfactorily explained by their specific similarity to those present in the air, the outlet pipe and the underdrains.

5. Of the one hundred and two bacterial examinations made of the effluent from November 14 to December 31, inclusive, fiftyeight proved to be sterile.

From this evidence it is fair to conclude that with the filtering materials here used all of the bacteria of the Merrimack River may ·be removed by intermittent filtration. This important result has not been reached hitherto.

DETERMINATION OF THE SPECIES OF BACTERIA FROM DIFFERENT

POINTS IN THE LAWRENCE WATER SUPPLY.

Beginning on Jan. 1, 1891, a series of determinations was made, once a month or oftener, of the numbers of bacteria found in the water of the Merrimack River from samples taken, first, from the force main at the pumping station of the Lawrence water works, second, in the water after it has passed through the reservoir, and again at certain points along its passage through the service pipes. In connection with the results, which are given in the following table, it is to be stated that the reservoir, where the greatest bacterial purification takes place, has a capacity equal to about two weeks' consumption, and that the city hall is distant about one and one-half miles, and the Experiment Station about two and one-half miles, from the reservoir.

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