The microscopy of drinking-water

Front matter

To

ilostucll

CORNELL UNIVERSITY.

Preface

This book has a twofold purpose. It is intended primarily to serve as a guide to the water-analyst and the water-works engineer, describing the methods of microscopical examination, assisting in the identification of the common microscopic organisms found in drinking-water and interpreting the results in the light of environmental studies. Its second purpose is to stimulate a greater interest in the study of microscopic aquatic life and general limnology from the practical and economic standpoint.

The work is elementary in character. Principles are stated and briefly illustrated, but no attempt is made to present even a summary of the great mass of data that has accumulated upon the subject during the last decade. The illustrations have been drawn largely from biological researches made at the laboratory of the Boston Water Works and from the reports of the Massachusetts State Board of Health. In considering them one should remember that the environmental conditions of the Massachusetts water-supplies are not universal, and that every water-supply must be studied from the standpoint of its own surroundings. So far as the microscopic organisms are concerned, however, the troubles that vi PREFACE.

they have caused in Massachusetts may be considered as typical of those experienced elsewhere.

The descriptions of the organisms in Part II are necessarily brief and limited in number. The organisms chosen for description are those that are most common in the water-supplies of New England, and those that best illustrate the most important groups of microscopic animals and plants. In many cases whole families and even orders have been omitted, and some readers will doubtless look in vain for organisms that to them seem important. The omissions have been made advisedly and with the purpose of bringing the field of microscopic aquatic life within the scope of a practical and elementary survey. For the same reason the descriptions, stop at the genus and no attempt has been made to describe species and varieties. Notwithstanding this it is believed that the illustrations and descriptions are complete enough to enable the general reader to obtain a true conception of the nature of the microscopic life in drinking-water and to appreciate its practical importance. To the student they must serve as a skeleton outline upon which to base more detailed Study.

The illustrations, for the greater part, have been drawn from living specimens or from photo-micrographs of living specimens, but some of them have been reproduced from published works of standard authority. Among these may be mentioned: Pelletan and Wolle on the Diatomaceas; Wolle, Rabenhorst, and Cooke on the Chlorophyceae and Cyanophyceae ; Zopf on the Fungi; Leidy, Biitschli, and Kent on the Protozoa; Hudson and Goss on the Rotifera; Baird and Herrick on the Crustacea; Lankester on the Bryozoa; Potts on the Spongidae; and Griffith and Henry on miscellaneous organisms.

PREFACE. vil

This book has been prepared during the leisure moments of a busy year. Its completion has been made possible by the kind assistance of my present and former associates in the laboratories of the Boston and Brooklyn water-supply departments and of other esteemed friends, to all of whom I tender my sincere thanks. I desire also to acknowledge the valuable assistance of my wife, Mary R. Whipple, in revising the manuscript and correcting the proof. To many others I am indebted indirectly, and among them I cannot refrain from mentioning the names of Prof. Wm. T. Sedgwick of the Massachusetts Institute of Technology; Mr. Geo. W. Rafter, C.E., of Rochester, N. Y. ; and Mr. Desmond FitzGerald, C.E., formerly Superintendent of the Boston Water Works and now Engineer of the Sudbury Department of the Metropolitan Water Works. To Prof. Sedgwick and Mr. Rafter water-analysts are indebted for the most satisfactory practical method of microscopical examination of drinking-water yet devised, and Mr. FitzGerald will be remembered not only as an eminent engineer but as the founder and patron of the first municipal laboratory for biological water-analysis in this country.

George Chandler Whipple.

New York, January, 1899.

HISTORICAL.

PAGE

Karly Investigators in Europe and the United States. — Cloth Method. — Kean's Sand Method. — Sedgwick's Improvements. — Sedgwick-Rafter Method. — Recent Improvements. — Plankton Studies in Europe and America I

Chapter II. The Object Of The Microscopical Examination

Sanitary Analyses. — Interpretation of Analyses. — Use of Microscopical Examination in Indicating Sewage Contamination — in Explaining the Chemical Analyses — in Explaining the Turbidity and Odor of Waters — in Studying the Food of Fishes 8

Chapter III. Methods Of Microscopical Examination

Sedgwick-Rafter Method.— The Filter.— Concentration.— The Cell— The Microscope. — Enumeration. — Sources of Error. — Precision of the Method. — Results of Examination. — The Standard Unit. — Records. — The Plankton Net Method. — The Plankton Pump. — The Planktonokrit 15

Chapter IV. Microscopic Organisms In Water From Different Sources

Rain-water. — Ground- water. — River- water. — Canals. — Raphidomonas

in the Lynn Water. — Pond-water 41

ix x CONTENTS.

Chapter V. Limnology

PAGE

Physical Properties of Water.— Compressibility. — Density.— Mobility. — Thermal Stratification.— Diathermancy.— Temperature of Lakes. — Methods of Observation. — Thermophone. — Seasonal Variation of Temperature. — Periods of Circulation. — Periods of Stagnation. — Thermocline. — Classification of Lakes according to Temperature. — Transmission of Light by Water. — Color of Water. — Seasonal Change of Color. — Bleaching of Color by Sunlight. — Turbidity of Water. — Transparency of Water. — Absorption of Light by Water 50

Chapter VI. Geographical Distribution Of Microscopic Organisms

Common Organisms Classified according to the Frequency of Their Occurrence. — Statistics of Their Occurrence in Massachusetts Surface-water Supplies, — Relation of Each Class of Organisms to the Sanitary Chemical Analyses 77

Chapter VII

SEASONAL DISTRIBUTION OF MICROSCOPIC ORGANISMS. Seasonal Succession of Organisms. — Spring and Autumnal Growths of Diatoms. — Explanation of this Seasonal Distribution. — Effect of Temperature. — Effect of Light. — Heliotropism. — Food-material. — Stagnation. — Seasonal Distribution of Chlorophyceae, Cyanophycea:, Schizophycea?, Fungi, Protozoa, Rotifera, Crustacea 93

Chapter VIII. Horizontal And Vertical Distribution Of Microscopic Organisms

Littoral Organisms.— Limnetic Organisms.— Effect of Winds and Currents on Horizontal Distribution.— Conditions Affecting Vertical Distribution.— Growth above the Thermocline.— Effect of the Specific Gravity of Organisms.— Peculiar Vertical Distribution of Mallomonas.— Protozoa.— Statistics of Vertical Distribution 105

Chapter IX. Odors In Wa Ter-supplies

The Senses of Taste and Odor.— Odors Caused by Organic Matter.— Odors of Decomposition.— Odors Caused by Living Organisms.— Character of Odoriferous Substances.— Intensity. . of Odors.— Characteristic Odors of Different Organisms.— Extent to which

CONTENTS. xi

PAGE

Water-supplies are Afflicted with Odors. — Cucumber Odor Not Caused by Fresh-water Sponge. — Cucumber Odor in Boston Water 113

Chapter X. Storage Of Surface-water

Clean Watersheds. — Effect of Swamps. — Anabaena in Cedar Swamp. — Drainage of Swamps. — Self-draining Watersheds. — Self-draining Reservoirs. — Stagnation of Water in Deep Reservoirs. — Lake Cochituate. — Removal of Organic Matter from the Sides and Bottom of Reservoirs. — Blow-off at the Bottom of Deep Reservoirs 150

Chapter XI. Storage Of Ground-water

Ground-water to be Stored in the Dark. — Growth of Organisms in Open Reservoirs. — Storage of Surface-water and Ground-water Together. — Asterionella in Brooklyn Water-supply. — Storage of Impure Ground-water. — Crenothrix. — Storage of Filtered Water.. 141

Chapter XII. Growth Of Organisms In Water-pipes

Effect of Pipes upon the Biology of Water. — Temperature. — Microscopic Organisms. — Amorphous Matter. — Bacteria. — Effect of Water upon the Biology of Water-pipes. — Hamburg. — Rotterdam. — Boston. — Food of Organism* Dwelling in Pipes. — Polyzoa. — Fresh-water Sponge. — Effect of Pipe-moss. — Friction. — Odor of Water. — Paludicella in Brooklyn 146

Chapter I. Historical

THE study of the microscopic organisms in water dates back to the seventeenth century. With the invention of the compound microscope enthusiastic observers began to search ponds and streams and ditches for new and varied kinds of microscopic life. Among the pioneers in this field of Natural History were Hooke(i665), Leeuwenhoek (1675), Ray (1724), Hudson (1762), Muller (1773), Dillwyn (1809), Kiitzing (1834), Ehrenberg (1836), Dujardin (1841), and Stein (1849).

It was not until 1850 that the study of the organisms in drinking-water was recognized as having a practical sanitary value. Dr. Hassall of London was the first to call attention to it. His method of procedure is unknown, but in all probability it consisted of the examination of a few drops of the sediment collected in a deep vessel after allowing the water to stand for a longer or shorter interval. Radlkofer (1865) of Munich, and Cohn (1870), Hirt (1879) and Hulwa of Breslau, pursued the study and emphasized its importance, but they made no radical improvement in the method.

In 1875 Dr. J. D. Macdonald of London suggested improvements in the sedimentation method, and made a rude attempt to obtain quantitative results by allowing the water to settle for a definite length of time, collecting the sediment on a removable glass disk or watch-glass at the bottom of a tall jar, and afterwards transferring this glass disk with its accumulated sediment to the stage of the microscope for direct examination.

In 1884 Dr. H. C. Sorby of England attempted to obtain a more exact enumeration by passing a gallon of the sample through a fine sieve (200 meshes to an inch) and then washing the collected organisms into a dish and in some way counting then.

In America important researches were made by Torrey, Vorce, Mills, Leeds, Potts, Nichols, Farlow, and others, but previous to 1888 the work was chiefly of a qualitative character.

In 1887 the Massachusetts State Board of Health began a systematic examination of all the water-supplies of the State, and two years later the State Board of Health of Connecticut began a similar but less extensive series of examinations. In 1889 the Water Board of the City of Boston established a biological laboratory* at the Chestnut Hill Reservoir for the purpose of studying systematically the biological character of the various sources of supply. In 1893 a small laboratory was established by the Public Water Board of the City of Lynn, Mass. In 1897 Mt. Prospect Laboratory, connected with the Department of Water Supply of Brooklyn, N. Y. was equipped and put in operation. It is devoted to general water analysis, and the microscopical examination of water

*For the first eight years of its existence it was conducted by the author under the general direction of Mr. Desmond FitzGerald, Superintendent of the Western Division of the Water Works.

HISTORICAL. 3

from the different sources of supply forms an important part of the routine work.

Similar biological work has been lately undertaken by health departments and by water experts in other parts of the country.

The reports of the various laboratories show that during the last ten years more than fifty thousand samples of water have been submitted to microscopical examination in New England and New York, and that this number is increasing at the rate of about eight thousand a year.

The method of microscopical examination first used by the Massachusetts State Board of Health was that suggested by Mr. G. H. Parker. A piece of cotton cloth was tied firmly over the end of a glass funnel and 200 c.c. of the sample were made to pass through it. The organisms were left as a deposit on the cloth. After this straining the cloth was removed and inverted over an ordinary microscopical slip. The organisms, together with a small quantity of water, were dislodged upon the slip by blowing downwards upon the cloth through a piece of glass tube. This method was useful, but it did not give accurate quantitative results. Mr. F. F. Forbes of Brookline, Mass., used a modification of the cloth method. The water was filtered as in Parker's method, but the neck of the funnel passed into a tank from which the air was exhausted by an aspirator. This hastened the filtration and allowed a larger amount of water to be filtered.

The present method of examination was foreshadowed in the work of Mr. A. L. Kean. He filtered ioo c.c. of his samples through a small quantity of coarse sand placed at the bottom of a glass funnel and supported by a plug of wire gauze. After filtration the plug was removed and the sand with its contained organisms was washed into a watch-glass with i c.c. of water. This was stirred up to separate the organisms from the sand and a portion was transferred to a cell holding one cubic millimeter. From the number of organisms found in this cell the approximate number originally present in the water could be obtained. This method has become known as the " sand method."

In 1889 Prof. Wm. T. Sedgwick and Mr. Geo. W. Rafter made valuable improvements upon Kean's original idea. Prof. Sedgwick suggested the use of a cell much larger than that used by Kean, bounded by a brass rim and having an area of 1000 square millimeters ruled by a dividing engine into 1000 squares. The filtration was made as before, and the sand was washed into the cell with one or two cubic centimeters of water and distributed over the bottom. The cell was then placed under the microscope and the organisms counted in a certain number of the small squares. From this count the number of organisms present in the sample was estimated. A modification of this method was the one first used by the Connecticut State Board of Health. In the Connecticut method precipitated silica was used instead of sand for the filtering medium, and this was supported upon a pledget of absorbent cotton.

Mr. Rafter's improvements consisted in the substitution of a ruled square in the ocular of the microscope for the ruling upon the plate, in the separation of the sand from the organisms by decantation, in the use of a cell covered by a cover-glass and containing just one cubic centimeter, and in the use of a specially constructed mechanical stage. The Sedgvvick- Rafter method has been modified somewhat by recent experimenters,* but its essential character has not been changed.

* Dr.. Gary N. Calkins substituted a. perforated rubber stopper capped

HISTORICAL. 5

While sanitarians have been pursuing the study of the microscopic organisms because of their effect on the quality of water-supplies, other scientists have approached the subject from an entirely different standpoint. In the same year that the Massachusetts State Board of Health began its examination of the water-supplies of the State, Victor Hensen of the University of Kiel, Germany, published a description of a new method of studying the minute floating organisms found in lakes. To these organisms he gave the name " plankton," * a collective word applied to all minute animals and plants that float free in the water and that are drifted about by waves and currents. Plants attached to the shore, and animals that possess strong powers of locomotion, are not included in the plankton, but fragments of shore plants, fish-eggs, young fishfry, etc., are included. The term "plankton," however, may be said to be synonymous with the term " microscopic organisms " of the sanitary biologist.

Hensen's method is radically different from the Sedgwick- Rafter method. The latter is strictly a laboratory process. The samples of water operated on are small ; the concentration of the organisms is made in the laboratory. Hensen devised a net by which the organisms could be concentrated in the field, so that only the collected material need be taken to the laboratory.

Even before the publication of Hensen's paper, scientists

■by a circle of bolting- cloth in place of the plug of wire gauze. Mr. D. D. Jackson suggested a cylindrical funnel in place of the ordinary flaring chemical funnel, and added an attachment at the lower end to control the concentration and prevent the sand from becoming dry. The author has .graduated the funnels, designed a simple automatic concentrating device, and applied an aspirator to hasten the filtration. He also designed the ocular micrometer and the record blank now used, and suggested the idea of a standard unit of size for estimating the organisms and amorphous matter. * From the Greek planktos, wandering.

O THE MICROSCOPY OF DRINKING-WATER.

on the Continent had become interested in the study of lakes. The early observations of Prof. F. A. Forel, of Morges, Switzerland, on Lake Geneva were followed by the establishment of a Limnological Commission in Switzerland. Under its direction many valuable lines of physical and biological research were undertaken. This was followed in 1890 by an International Commission. From this time increased attention has been given to the biology of ponds and lakes. A biological station was established by Zacharias at Lake Plon in 1 891 , and a group of scientists have contributed important articles to its annual reports. Apstein at Kiel, Schroeter at Zurich, and many others have made extensive and valuable observations. Biological stations are multiplying and the work is being extended to France, Italy, Austria, Denmark,, and Norway.

Similar investigations have been carried on in the United States. In 1893 Prof. J. E. Reighard, acting under the direction of the Michigan Fish Commission, made a biological study of Lake St. Clair. This was followed by an examination of Lake Michigan by Prof. Henry B. Ward, and by studies of the Crustacea in Lake Mendota by Prof. E. A. Birge, and in Green Lake by Prof. C. Dwight Marsh.

Biological stations have been recently established by a number of Western universities on and in the vicinity of the Great Lakes.

For several years Prof. James L Peck, acting under the direction of the U. S. Fish Commission, has been making important studies of the food of certain fishes, notably the menhaden. He uses the Sedgwick-Rafter method instead of the plankton net for concentrating the microscopic organisms.

In 1896 Dr. C. S. Dolley of Philadelphia suggested the use of the centrifugal machine for the purpose of concentrat-

HISTORICAL. 7

ing the microscopic organisms. This " planktonokrit," as it is called, has not been developed to completeness, but recent experiments by Field, Kofoid, and others show that it is likely to prove a valuable adjunct to the Sedgwick- Rafter method, but it is not likely to supplant it.

Prof. H. B. Ward and Mr. Chas. Fordyce have recently devised a "plankton pump" for collecting Crustacea and other plankton organisms at particular depths below the surface of a lake. It seems to be a decided improvement over the plankton net.

Chapter II. The Object Of The Microscopical Examination

A COMPLETE sanitary examination of water, as conducted in modern laboratories, consists of three parts, — the physical, the biological, and the chemical. The biological examination may be divided into the microscopical and the bacterial, because of different methods employed. The data obtained are as follows:

Physical Examination.

Temperature — Turbidity — Sediment — Color — Odor, both cold and hot.

Biological Examination.

Micros cop tea I.

Number of microscopic organisms per c.c. — Amount of inorganic, amorphous matter, etc.

Bacteriological.

Number of bacteria per c.c. — Presence of intestinal or pathogenic bacteria.

Chemical Examination.

Total Residue on Evaporation — Loss on Ignition

Fixed Solids — Alkalinity — Hardness — Chlorine — Iron — Nitrogen as Albuminoid Ammonia— Nitrogen as Free Ammonia — Nitrogen as Nitrites — Nitrogen as Nitrates — Total Organic Nitrogen (Kjeldahl Method)— Oxygen consumed — Dissolved Oxygen. (The last three are of use only in special cases.)

THE OBJECT OF THE MICROSCOPICAL EXAMINA TION. 9

Such an analysis is intended to show whether or not the water is of such a character that it would cause sickness if used for drinking; whether or not it contains anything that would render it distasteful or unpalatable; and whether or not it contains any ingredient that would make it unfit for laundry use or for general domestic or industrial purposes. Sanitary examinations are necessary also in studying the effect of processes of purification.

Opinions regarding the function and value of sanitary water analyses have undergone a change in recent years. The numerical results of a single analysis of a sample of water, when considered by themselves, are now believed to have little intrinsic value. It has been found that the value of the analysis lies in its interpretation, and that each part of the analysis must be interpreted by comparison with all the other parts and in the light of exact knowledge of the environment of the water. The interpretation of an analysis is as much a matter of expert skill as is the making of the analysis itself. The physical, biological, and chemical examinations should be interlocking in their testimony, yet these different parts are to be given different weight in the study of different problems. For example, in the detection of pollution the chemical and bacterial examinations furnish the most information, in the study of the aesthetic qualities of a water the physical and microscopical examinations are most important, while in investigations concerning the value of a water for industrial purposes the chemical and physical examinations sometimes suffice.

The biological examination is concerned with the micro-organisms found in water. The term " micro-organisms," when used in its broadest and most literal significance, includes all organisms which are invisible or barely visible to the

IO

THE MICROSCOPY OF DRINKING-WATER.

naked eye. It is frequently used in a narrower sense, however, as a synonym for bacteria. Using the word in its broad sense we may divide the micro-organisms found in water into two classes, as suggested by Prof. Sedgwick.

Micro-organisms.

Organisms, either plants or animals, invisible or barely visible to the naked eye.

Microscopic Organisms.

Not requiring special culture.

Easily studied with the microscope.

Microscopic in size, or slightly larger. Plants or animals.

Bacterial Organisms.*

Requiring special cultures.

Difficultly studied with the microscope. Microscopic or sub-microscopic in size.

I Plants.

This subdivision is convenient for the sanitarian as well as for the biologist, because the two classes of organisms affect water in different ways. With certain reservations it may be said that the bacteria make a water unsafe, the microscopic organisms make it unsavory.

Microscopical Examination. — The microscopical examination of water may be considered in four aspects: i. As indicating sewage contamination. 2. As explaining the chemical analysis. 3. As explaining the cause of turbidity, odors, etc., in water. 4. As a method of studying the food of fishes and other aquatic animals.

1. The microscopical examination cannot be depended upon to determine the pathogenic qualities of a drinking-

* The bacteria are not considered in this volume. The reader is referred to the numerous works on Bacteriology, and in particular to. "Micro-organisms in Water." by Percy and G. C. Frankland.

THE OBJECT OF THE MICROSCOPICAL EXAMINATION. II

water. To be sure, the germs of disease are microscopic bodies, and when artificially cultivated or when found in the tissues of the body they can be studied with microscopes of high power. But when scattered through a mass of water they cannot be detected by ordinary microscopical methods, because of their small size and because they are greatly outnumbered by the ordinary water bacteria. It is questionable whether they can be discovered even by methods of culture. Not only may water contain pathogenic bacteria without discovery, it may contain the ova or larvae of some of the endoparasites of man. It is probable that endoparasitic diseases are more common than has been generally supposed ; and while diseased pork, beef, etc., are the chief agencies of infection, it is known that water polluted by animal excrement may contain the ova or larvae of such endoparasites as Tmnia solium, Tania saginata, Bothriocephalus latus, Ascaris lumbricoides, Trichocephalus dispar, and Anchylostomum duodenale. Infection of animals by the drinking of water contaminated by barnyard wastes has been several times recorded, while a microscopical examination of the water has seldom revealed the presence of the suspected ova or larvae. This is not because they are too minute to be detected, but because the quantity of water examined is necessarily too small.

The microscopical examination cannot show definitely whether a water is polluted by sewage unless the pollution is excessive. It can, however, give evidence which, taken with the chemical and bacterial examinations, may establish the proof. A microscopical examination of sewage reveals few of the living organisms that are found ordinarily in water. Ciliated infusoria, such as Paramsecium and Trachelocerca; fungus forms, such as mold hyphae, Saprolegnia, Leptomitus, Leptothrix, and Beggiatoa; and miscellaneous objects, such as yeast-cells, starch-grains, fibres of wood and paper, fibres of muscle, epithelial cells, threads of silk, woolen, cotton and linen, insect scales, feather barbs, etc., may be observed. Most of these objects are foreign to unpolluted water, and their presence in a sample of water leads one to suspect its purity.

Furthermore, there are other organisms, such as Euglena viridis, which live on decaying vegetable matter and which, though not found in sewage, are often associated with it in polluted water. Their presence in a sample is a cause of suspicion. These evidences, however, should be weighed only in connection with an environmental study and with the entire sanitary analysis. The common microscopic organisms found in water are not themselves the cause of disease, nor does their presence indicate sewage pollution.

2. The chemical examination determines the amount of organic matter that a sample of water contains, but it does not determine the nature of it. As the character and condition of the organic matter is very important from the sanitary point of view, the microscopical examination gives valuable information by showing not only whether the organic matter in suspension is vegetable or animal, but by determining whether it is made up of living organisms or of decomposing fragments. For example, the amount of albuminoid ammonia in suspension is sometimes so great that one might suspect that the water was polluted did the microscope not show that the high figure was due to a growth of some organism. Or in a series of samples from a reservoir it might be difficult to account for a sudden decrease in the nitrates or free ammonia were it not for the appearance of some microscopic organism that had appropriated the nitrogen as a part of its food.

THE OBJECT OF THE MICROSCOPICAL EXAMINATION. 1 3

3. By far the most important service that the microscopical examination renders is that of explaining the cause of the taste and odor of a water and of its color, turbidity, and sediment. Several of the microscopic organisms give rise to objectionable odors in water and, when sufficiently abundant, have a marked influence on its color. They also make the water turbid and cause unsightly scums and sediments to form. Upon all such matters related to the aesthetic qualities of a water the microscopical examination is almost the only means of obtaining reliable information.

4. The microscopic organisms form the basis of the food-supply of fishes and other aquatic animals. Sometimes the relation is a direct one; that is, the microscopic organisms are themselves eaten by fish. This has been well illustrated by Prof. Peck. The menhaden swims with its mouth open, and is provided with a peculiar filtering apparatus by which the minute organisms are caught. It has been shown that the presence or absence of these fish from certain sections of the Massachusetts coast depends upon the abundance of microscopic life in the water, and also that the weight of fish of any particular length depends upon the quantity of this food material at hand.

The relationship between the plankton and fish life is not always so direct. In many cases the fish feed upon Crustacea and insect larvae; the Crustacea feed upon the rotifera and protozoa; the rotifera and protozoa feed upon algae; while 'the algae nourish themselves by the absorption of soluble inorganic substances.

The interrelations between different organisms of the lower world, and between the organisms and their environment are matters of intense scientific interest, and limnology is fast assuming an important place in scientific literature.

The physical condition of lakes, the currents, waves, temperature, and transparency of water, the chemistry of water, the life-history of organisms, and various bio-chemical and biophysical problems are more and more attracting the attention of scientists and of water-works engineers.

Chapter III. Methods Of Microscopical Examination

The most important methods of microscopical examination of water now in use are: I. The Sedgwick- Rafter Method; 2. The Plankton Net Method; 3. The Plankton Pump Method; 4. The Planktonokrit. They differ chiefly in the manner of concentrating the organisms.

I. The Sedgwick- Rafter Method.

The Sedgwick-Rafter Method consists of the following processes: the filtration of a measured quantity of the sample through a layer of sand upon which the organisms are detained; the separation of the organisms from the sand by washing with a small measured quantity of filtered or distilled water and by decanting ; the microscopical examination of a portion of the decanted fluid; the enumeration of the organisms found therein; and the calculation from this of the number of organisms in the sample of water examined. The essential parts of the apparatus are the filter, the decantation-tubes, the cell, and the microscope with an ocular micrometer.

The Filter. — The sand may be supported upon a plug of rolled wire gauze at the bottom of an ordinary glass funnel 7 or 8 inches in diameter, but the cylindrical funnel shown in Fig. I is preferable. The inside diameter of this funnel at

THE MICROSCOPY OF DRINKING-WATER.

the top is 2 inches; the distance from the top to the beginning of the slope is g inches; the length of the slope is about 3 inches; the length of the tube of small bore is 2£ inches, and its inside diameter is \ inch. The capacity of the funnel is 500 c.c. The support for the sand consists of a perforated rubber stopper pressed tightly into the stem of the funnel and capped with a circle of fine silk bolting-cloth. The circles of bolting-cloth may be cut out with a wad-cutter. Their diameter should be a little less than that of the small end of the rubber stopper. When moist the cloth readily adheres to the stopper. The sand resting upon the platform thus prepared should have a depth of at least three fourths of an inch. The quality of the sand is important. Ordinary sand is unsatisfactory unless very thoroughly washed. Pure ground quartz is preferable. Its whiteness is a decided advantage. The necessary degree of fineness of the sand depends somewhat upon the character of the water to be filtered. A sand which will pass through a sieve having 60 meshes to an inch, but which will be retained by a sieve having 120 meshes, will be found satisfactory for most samples. Such a sand is described as a 60-120 sand. When very mi-