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Microscopical Examination of Effluent of Filter Tank No. 8.

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The following table contains the averages, by months, of the weekly analyses of the effluent of Filter Tank No. 18 A:

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City water applied until Aug. 18, 1891; afterwards canal water.

Experiments interrupted by high water in the river February 27 to March 1, March 12 to 16 and April 12 to 17; also March 20 to April 9, for repairs.

A summary of twenty-five microscopical examinations of the effluent is given in the following table :

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Microscopical Examination of Effluent of Filter Tank No. 18 A.

[Number of organisms per 100 cubic centimeters.]

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The averages, by months, of the weekly analyses of Filter Tank No. 20 A are given in the following table :

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Experiments interrupted by high water in the river February 27 to March 1, March 12 to 10 and April 12 to 17.

The following table contains a summary of twenty-seven microscopical examinations of the effluent :

Microscopical Examination of Efluent of Filler Tank No. 20 A.

[Number of organisms per 100 cubic centimeters.]

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SPECIAL BIOLOGICAL WORK.

The following account of the special biological work of the station has been prepared by Mr. George W. Fuller, biologist in charge.

The regular biological work has consisted in part, as in former years, in the systematic examination, both microscopical and bacterial, of the applied sewage, city water and canal water, and of the effluents of the different filters. The microscopical examinations have been made after the Sedgwick-Rafter method (see p. 397). In the bacterial analyses unusual care has been taken to make the determinations under conditions as nearly as possible parallel, in order that the results might be directly comparable. Since it has been observed, both in this laboratory and elsewhere, that nutrient gelatine, varying even to a moderate extent in its degree of alkalinity, may yield widely differing numbers of bacteria from duplicate samples of water,

a circumstance probably due to the fact that different species of bacteria require different degrees of alkalinity for their rapid development, — numerous experiments have been made to determine the optimum amount of alkali necessary for the development of those species of bacteria prevailing in the waters and sewage of Lawrence. A special study has also been made of the methods available for securing the same degree of alkalinity in all samples of nutrient gelatine, but it has been found difficult to obtain an indicator sufficiently sensitive to all the compounds present in the gelatine; and it has occasionally been observed that lots of nutrient gelatine, which had been made alkaline to the same degree, so far as could be determined by the ordinary indicators, have also given widely varying numbers of bacteria in duplicate samples of the same water.

As a step toward greater precision in manipulation, all gelatine plates have been grown not at the “ room temperature," as is usual, but in a low-temperature thermostat. This thermostat, which eliminates irregular temperature differentiations, is kept at 20° C. during the summer months by passing a stream of cold water through the water jacket, and, during the rest of the year, by a small gas flame. Devices have also been introduced for protecting the eflluents from contamination by dust and air ; a careful study bas been made of the conditions and methods of taking samples; and these things, together with the use of a nutrient gelatine of a composition as nearly as possible uniform, and under parallel conditions, have served to increase the accuracy of the biological methods of determining the degree of purification of sewage and water. The results of the regular biological analyses have already been tabulated and discussed along with the chemical results on the foregoing pages. (See tables above, showing the chemical and biological purification effected by the different filters.)

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INVESTIGATIONS UPON THE BACILLUS OF TYPHOID FEVER. The importance of determining accurately the life-history of the typhoid fever bacillus, its deportment in sewage and in water, and the possibility of its removal from water by means of filtration, has long been recognized as an important part of the biological work. When the problem was first undertaken, it was found that there was no reliable method for its differentiation from several species of bacteria regularly present in the water of the Merrimack River at Lawrence. The potato test, upon which much dependence was generally placed, has proved to be totally inadequate, and for two reasons: first, because the typhoid fever bacillus does not under all conditions give its characteristic invisible growth on sterilized potato ; and, second, because no fewer than five of the species in the Lawrence waters under some circumstances present the same characteristic.

After it had been discovered that the potato test was not diagnostic, and that it was even contradictory, it became necessary to devise other methods of differentiation. To this end a thorough study was made of the typhoid bacillus, side by side with the species of bacteria present in the water of the Merrimack River at Lawrence. The numerous special methods and media proposed for its differentiation were examined and their diagnostic value determined. This preparatory investigation was continued for several months, and resulted in a method of procedure whereby the typhoid fever bacillus can be differentiated beyond all reasonable doubt from every species of bacteria hitherto met with in the Lawrence waters. The results of this investigation are embodied in a paper which appears beyond (p. 635).

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