The microscopical examination of water consists of the enumeration of
the kinds of microscopic organisms (Plankton), and an estimation of
their quantity.
It may serve any one or more of the following purposes:
(1) To explain the presence of objectionable odors and tastes.
(2) To indicate the progress of the self purification of streams.
(3) To indicate the presence of sewage contamination.
(4) To explain the chemical analysis.
(5) To identify the source of a water.
(6) To aid in the study of the food of fish, shellfish, and other
aquatic organisms.
The term “Microscopic Organisms” shall include all organisms microscopic
or barely visible to the naked eye, with the exception of the bacteria.
It includes the diatomaceae, chlorophyceae, cyanophyceae, fungi,
protozoa, rotifera, crustacea, bryophyta, and spongidae found in water.
Fragments of organic matter, silt, mineral matter, zoöglea, etc., shall
be considered as amorphous matter. The recording of amorphous matter
usually serves no useful purpose and shall not be considered a part of
the standard method.
_Apparatus._—1. A cylindrical funnel about two inches in diameter at the
top, with a straight side for nine inches, narrowed over a distance of
three inches to a bore of one-half inch in diameter, and terminating in
a straight portion of this diameter two and one-half inches in length.
The capacity of this funnel is 500 cc. It shall be provided at the
bottom with a tightly fitting rubber stopper with a single perforation
and a disk of silk bolting cloth over the hole about three eighths of an
inch in diameter.
2. A counting cell consisting of a brass rim closely cemented to a plate
of optical glass. The shape and size of this cell are not essential but
its depth shall be one millimeter. A convenient capacity is about one
cubic centimeter.
3. An ocular micrometer ruled as follows: The ocular micrometer is
commonly of such a size that with a 16 mm. objective and a suitable tube
length, the largest square cuts off one square millimeter on the stage.
_Procedure._—Filter 250 cc. of the water (more or less according to the
clearness of the sample) through a one-half inch layer of quartz sand
(washed and screened between 60 and 120 mesh sieves) supported by the
disk of bolting cloth and rubber stopper at the bottom of the funnel.
Suction may be applied to hasten the filtration.
Remove the stopper and catch the plug of sand and its entrained
organisms in a small beaker or test tube, washing down the inside of the
funnel into the beaker with 5 cc. of clean (preferably distilled) water.
Agitate the mixture of sand, water, and organisms to detach the latter
from the sand grains, and quickly decant the water and the organisms in
suspension to a test tube. If desired the sand may then be again washed
with 5 cc. water and the wash water added to the first portion.
Cover the cell partially with a cover glass, and by means of a pipette
run the concentrate under the cover glass until the cell is completely
filled.
Cover and place on the microscope stage in a horizontal position for
examination.
Count the organisms in twenty fields, i. e., twenty cubic millimeters,
estimating their areas in terms of Standard Units.
_The Standard Unit is the smallest square in the ocular micrometer, and
represents an area 20µ × 20µ, or 400 square microns on the stage._
Results shall be expressed in the number of Standard Units of each kind
of micro-organism per cc. and also the total number of standard units of
all kinds per cc. The general directions as to significant figures given
under Turbidity shall apply also to the microscopical examination.
_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.
[Illustration: THE OCULAR MICROMETER.]