Caution.—Many micro-organisms, especially some of those causing odors, are so fragile that they are broken up in filtration, especially if the agitation of the filtrate is too vigorous. A direct examination of a fresh sample is therefore a useful supplementary procedure. For the same reason the concentrate should not stand long before examination. Also some organisms are carried by specific gravity to the top of the cell which should be scrutinized as well as the bottom layer each time.
It is always better to examine the micro-organisms in the field when possible, and for this purpose the sling filter has been devised consisting of a metal funnel slung to a pivoted handle, with a disk of wire gauze in the detachable lower end to support the sand. Filtration is hastened by imparting a whirling motion to the whole and utilizing the centrifugal force thus generated.
THE OCULAR MICROMETER.
MICROSCOPICAL BIBLIOGRAPHY.
a. Kean, A. L. A new method for the microscopical examination of water: Science, Vol. 13, p. 132, 1889; Eng. News, pp. 21, 276, 1889. b. Sedgwick, W. T. Recent progress in biological water analysis: J. N. E. Water Works Assoc., Vol. 4, pp. 50–64, 1889. c. ——. A report of the biological work of the Lawrence Experiment Station: Examinations by the State Board of Health of water supplies of Mass., 1887–90, pt. 2, Purification of sewage and water, pp. 793–862, 1890. d. Parker, G. H. Report upon the organisms, excepting the bacteria found in the waters of the State: Examinations by the State Board of Health of water supplies of Mass., 1887–90, pt. 1, Examination of water supplies, pp. 579–620, 1890. e. Rafter, G. W. The microscopical examination of potable water, D. Van Nostrand Co., New York, 1910. (Contains bibliography.)
BACTERIOLOGICAL EXAMINATION.
I. APPARATUS.
1. Sample Bottles.—Any size, shape or quality of bottle may be used for a bacterial sample, provided it holds a sufficient amount to carry out all the tests required and is such that it may be properly washed and sterilized and will keep the sample uncontaminated until the analysis is made. Four- or eight-ounce, ground-glass-stoppered bottles are recommended. These should be protected by being wrapped in paper, or their necks covered with tin foil, and should be placed in proper boxes for transportation.
2. Pipettes.—Pipettes may be of any convenient size or shape provided it is found by actual test that they deliver accurately the required amount in the manner in which they are used. The error of calibration shall in no case exceed 2 per cent. Protecting the pipettes with a cotton stopper is recommended.