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In cases where it would be possible to bring about a circulation of an entire body of water during the warmer months, so that the lower layers would be brought to the surface and stagnation prevented, we would have effective aeration of the water with the prevention of the accumulation of products of decomposition.

In the case of ground water it is now well understood that the more directly from the ground it is supplied to the consumer the more acceptable it is. It needs no aeration. In fact, its storage in open reservoirs results often in the conversion of a cool, clear, palatable water into one which is repulsive to sight, taste and smell.

There is, however, one class of ground waters, not infrequently met with, which are only fit to use after they have been exposed to the air : namely, waters which contain considerable iron in solution in the form of protoxide. These waters deposit iron oxide on standing, owing to their absorption of oxygen from the air. Aeration in connection with settling basins or filter beds might make waters of this class available for general use.

To resume:

1. The oxidation of organic matter in water is not hastened by vigorous agitation with air or by air under pressure.

2. The aeration of water may serve a useful purpose, by preventing stagnation, by preventing the excessive growth of algæ, by removing from water disagreeable gases, and by the oxidation of iron in solution.





In the special report of the Board upon the investigations conducted at the Lawrence Experiment Station on the purification of sewage and water, Professor Sedgwick, Biologist of the Board, has given an historical account of the various methods hitherto employed for the microscopical examination of potable water, and has described an entirely new method introduced by himself for the work of the Board, and afterwards much improved by George W. Rafter, C. E., of Rochester, New York. *

This method, which is now known as the Sedgwick-Rafter method, has been constantly employed in the regular examinations of the drinking waters of Massachusetts for the State Board of Health since Nov. 6, 1890. By its aid and under the supervision of Professor Sedgwick I have made more than 3,000 examinations of potable waters, my whole time having been given to this branch of the biological work.

In the present paper I propose, first, to describe the method in detail — for it is still comparatively unknown — and to suggest certain improvements; and, second, to consider at some length the possible sources of error which its use may involve. I desire to acknowledge my obligations throughout to Professor Sedgwick.

It should be clearly understood at the outset that the microscopical examination of water has nothing to do with the smallest forms of aquatic life (the bacteria) or with the largest forms (such as fishes, frogs, pond-lilies, etc.). For detecting and counting the bacteria special methods, known as “cultures,” are indispensable. For recognizing the larger forms the naked eye is sufficient. Between

* W. T. Sedgwick. Recent Progress in Biological Water Analysis. Journal of the New England Water Works Association, September, 1889.

George W. Rafter, CE. The Biological Examination of Potable Water. Proceedings, Rochester Academy of Sciences, 1890. Rochester, New York.

W. T. Sedgwick. Report of the Biological Work of the Lawrence Experiment Station. Special Report of the Massachusetts State Board of Health on the Purification of Sewage and Water, 1890.

these limits, however, there is a vast host of minute plants and animals, nearly or quite invisible to the naked eye, yet readily studied (as bacteria are not) by the aid of microscopes of ordinary power. These forms abound in lakes and ponds, in open reservoirs and in rivers.

To distinguish them from the bacterial organisms, which cannot so satisfactorily be studied by the aid of the microscope alone, Professor Sedgwick has proposed that they be called the Microscopical Organisms, the whole group of small or micro-organisms being sub-divided and defined as follows:

| Microscopical Organisms

Not requiring special “ cultures."
Easily studied with the microscope.
Microscopic in size or slightly larger.
Plants or animals.

Micro-Organisms. Organisms, either plants or animals,

invisible or barely visible to the

naked eye.

Bacterial Organisms

Requiring special cultures.
Difficultly studied with the microscope.
Microscopic in size.

By this simple and natural classification all of the lowest plants and animals are grouped together in one great class, viz. : the Micro-Organisms. Most of these, though very small, are still easily studied by the aid of the compound microscope. The members of one group only, the bacteria, are so extremely minute that the microscope fails to reveal most of their peculiarities. The microscopical organisms, therefore, include all of the lowest animals and, excepting the bacteria, all of the lowest plants; but until the Sedgwick-Rafter method was devised there was no satisfactory method for the collection, enumeration and comparison of these organisms in different specimens of water. This method, originally devised to meet the exigencies of the work of the Board, though by no means absolutely perfect, has been found to work very well.


THE MICROSCOPICAL EXAMINATION OF DRINKING WATERS. The method consists, first, in the concentration of all the organisms (except the bacteria) in a measured quantity of the water to be examined by its filtration through sand upon which the organisms are detained ; second, in the separation of the organisms from the

* See Report of Prof. Sedgwick cited above, page 797.

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