Henry R. (Henry Richard) Kenwood.

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that, with the plummet immersed in distilled water, and the
largest rider placed on the hook (which represents the tenth
notch, or i,ooo), this pointer rests vertically opposite a small
projection on the frame. The adjustment is made by means
of the small screw shown on the vertical support to the frame.
The liquid is placed in a glass cylinder and the plummet just
completely immersed in the hquid, and by placing the riders on
various notches the two pointers are again brought opposite to
each other. If, for instance, in order to obtain this result, the
largest rider is on the ninth notch, the next largest on the
seventh, and the smallest on the fifth, the specific gravity would
be 975.

A correction for temperature is necessary for an exact observa-
tion by float hydrometers, since all such instruments are originally
graduated by water at the temperature of i5'5° C, and the
specific gravity varies with the temperature. Within the ordinary
ranges of temperature in a laboratory it is sufficient to add 1° of
specific gravity for every 3° of temperature above i5'5° C, and
to subtract 1° for every 3° below I5"5° C.

The Extraction of Fat by Soxhlet's Apparatus.

Soxhlet's apparatus is shown in Fig. 6. A is the small flask
which has been thoroughly dried and weighed and then about
half filled with ether ; the extraction apparatus is shown attached
to the flask between it and the condenser (K), the latter being
fixed in a very slanting position. F represents a piece of fat-
freed paper containing the substance to be extracted; this is
placed in D, care being taken that it is entirely below the level
of the smiall siphon E, so that it may be completely immersed
in the solvent, and also that it does not close the opening to the

The weighed flask of the Soxhlet should have a capacity of
about 150 c.c, and contain about 75 c.c. of ether.

The dish on which the flask stands is partially filled with water,
and this is cautiously heated; the ether vapour then ascends G,
passes into the condenser, and is at once condensed and drops
on to F; the ether goes on accumulating, rising the while in the
ascending arm of E, until it reaches the level of the upper bend.



and overflows, when siphonage takes place, and the ether passes
out of D back to the flask. Thus the circulation of the ether is
completed every few minutes. Immediately after a siphon dis-
charge has returned all the ether to the flask, the latter is
removed, the ether driven off over the water-bath at a temper: -
ture sufficient to make the ether boil, after which the flask and its
contents are dried at ioo° C. until a constant weight is obtained.
Of course, there must be no doubt as to whether the extrac-
tion has been complete; this may be tested by fixing a second

FIG. 6. — soxhlet's fat-extraction apparatus.

small flask containing more ether, and after about half an hour
evaporating off the ether and drying at ioo° C. ; it can then be
noted whether there is any material increase over the original
weight of the flask.

It is well to place a small plug of blotting-paper in the mouth
of the open tube at the top of the condenser so as to limit the
access of air, the moisture of which would otherwise condense
and slightly wet the ether.



The vSpectroscope.

A knowledge of the spectroscope is useful to the public health
worker, and for those unacquainted with the use of this instru-
ment a brief description is given:

If a compound light, such as sunlight, is made to pass through
a glass prism, the different coloured rays of which it consists are
unequally refracted (or bent out of their original course), so that
beyond the prism they form, upon a white surface, a continuous
line of colours called the " spectrum "; and the spectrum of the
compound white light will be seen to consist, in order from right
to left, of red, orange, yellow, green, blue, indigo, and violet. A
number of dark hues — called " absorption bands " or " Fraun-
hofer's hues " — are also seen to cross the image of the solar


spectrum. These lines indicate the absence of raj^s of certain
refrangibilities from the beam of solar hght; each occupies a
definite position, and therefore affords a means of accurately
localizing the parts of the spectrum.

In other lights the spectrum will only show a few bright bands
(that of the sodium flame only one), and the remainder of the
spectral image is almost — or quite — invisible, by comparison.

If we transmit solar light through different coloured solutions,
we then get different absorption bands. If a solution of fresh
blood, for instance, be taken, and a small colourless cell containing
it is placed before the slit in the instrument which admits the
hght, two distinct and characteristic dark stripes or absorption
bands appear in the yellow and green parts of the solar spectrum.


Fig. 7 will serve to show the manner in which a spectroscope
is constructed.

A firm iron stand is seen to support at its upper end a brass
plate carrjing the glass prism; laterally, a cjdinder is also
fastened to the brass plate, and in the end of this cylinder which
is nearest the prism a lens is placed, the other end being closed
by a plate with a vertical slit in it (the width of which can be
regulated by a screw to meet requirements) ; through this slit the
light is admitted to the prism, the rays first passing through the
lens and thereby being rendered parallel and condensed. The
spectroscopic appearance is then viewed through a small tele-
scope (with a low magnifying power), and this (the tube on the
right as seen in the figure) is fitted on to the cast-iron foot so as
to be movable in a horizontal plane about the axis of the foot.
The telescope is made to move over a scale which can be read
^^^th a vernier.

All foreign light must, of course, be cut off; and this may be
done by a black cloth, which is thrown over the prism and the

The sht may be furnished with a reflecting prism, by means of
which two spectra can be compared at the same time.

Thus, by a spectroscopic examination, the colour, number,
and position of the bright lines on the spectroscopic scale may
be carefully observed and noted. If it is desired to distinguish
metals by means of their spectral lines, the substance is dissolved
in a drop of the purest hydrochloric acid; a piece of recentty
ignited platinum wire is then dipped in the solution and held in
a Bunsen flame.

A convenient method of performing spectroscopic observations
is by means of the Sorby- Browning micro-spectroscope, which
consists of a small spectroscope placed in connection with a
microscope in such a way that the former fits into the tube of
the latter, similar to an eyepiece.

The Polariscope.

A simple form of half-shadow polariscope consists of a hori-
zontal brass tube mounted on a vertical stand, and having a
Nicol prism at each end, one being the " polarizer," and the
other the " analyzer." A monochromatic hght, such as the
yellow sodium flame (which may be obtained by placing a


platinum cup containing sodium chloride in the flame of a Bunsen
burner) is admitted to the polarizer. At the opposite end of the
brass tube an eyepiece is fitted just in front of the analyzer.
In the brass tube can be placed a clean and dry glass tube con-
taining the solution under examination.

Light consists of vibrations of ether in all planes, and its trans-
mission occurs in waves; but the monochromatic light consists
of light of a single wave-length. The polarizer allows only the
vibrations taking place in one plane to pass, others being inter-
cepted. Now, when the analyzer is placed parallel to the polar-
izer, all the vibrations pass through the analyzer also, and equal
illumination is seen on both sides of a sharply defined vertical
middle hne when looking through the eyepiece, this point of


equal illumination being called the " zero-point." The slightest
rotation of the analyzer will then produce a difference in the
illumination of the two sides.

In using the instrument, the zero-point is first obtained, with
the glass observing-tube filled with distilled water and placed in
position; then if some sugar solution (or other optically active
liquid, which has the property of rotating the plane of polarized
light) be placed in the glass tube, the rays will no longer pass
through the analyzer, and the equal illumination is disturbed.
If the analyzer be turned round, it is possible to obtain an equal
illumination, or, in other words, to compensate for the optical
disturbance of the rotating substance; but the direction and the
angle through which it has been turned (as indicated on a dial
fitted with a vernier) vary with the amount and nature of the
rotating substance examined, the number of degrees being termed


the " index of refraction," from which the so-called " specific
rotary power " of the substance may be calculated.

The polariscope is used to find the percentage adulteration of
butter with other fats (refractometer), and also the strength
of saccharine solutions (saccharimeter). With pure butter an
equally distributed light can be obtained, but with butter con-
taining fat which has been melted (margarine) this is impossible,
since such fats rotate the plane of polarization. Glucose in
honey and added sugar to milk may also be detected by this
instrument, for while most sugars have the property of deflecting
the ray of polarized light to the right (dextro-rotary, indicated
by the sign +), others deflect to the left (levo-rotary, indicated
by the sign — ), and this affords a means of distinguishing
between them. If the nature of the substance is known, one
can, moreover, estimate its quantity, since i gramme of a
particular optically active substance has its own specific rotary

But for the work demanded of the public health worker it is
not necessary to determine specific rotary powers, useful as these
may be in some of the work which a public analyst may be
called upon to perform. Indeed, from the standpoint of the
public health worker the micro-polariscope (in which the polari-
scope is adjusted to an ordinary microscope) will generally
suffice. In this instrument one of the Nicol prisms (the
analyzer) is inserted in the brass tube of the microscope imme-
diately above the objective, and the other (the polarizer) is fitted
beneath the stage of the microscope, so that the specimen
examined on the slide stage of the microscope is now between
two Nicol prisms, the lower one of which is the polarizer. Such
an instrument will be found of assistance in distinguishing
between certain starches, some of which polarize better than
others, in detecting the addition of starchy matter (such as rice)
to pepper or mustard (which do not polarize in the mass), and in
distinguishing between pure butter and margarine. When, for
instance, a specimen of pure pepper is examined, it is possible to
obtain, by rotating the analyzer, a completely darkened field;
whereas this is impossible when ground rice is the article under
examination. Hence the addition of ground rice to pepper can
readily be detected from the circumstance that it is not possible
to obtain a completely obscured field. Similarly, with pure
butter a completely dark field cannot be obtained, whereas with



margarine from fat which has been melted it can; and in the
case of mixtures it is impossible to completely obscurr; tiio field
by rotating the analyzer.

Graduated Burettes.
In working with delicate standard solutions it is best to
employ a mounted burette fitted with a stopcock at the bottom,
rather than an unmounted one controlled by the finger; as in the
former case the possibihties of contamination are reduced, and
there is no risk of any loss from the burette while the operator
is mixing or colour-matching in the intervals of the addition
of further quantities of the standard solution. When a hand-
burette is employed the index-finger which controls its delivery
must be quite dry.




Unmounted burettes should not be blown out, but allowed to
drain, and the drop at the dehvery end removed by touching the
side of the vessel into which the contents are emptied.

In judging the height to which fluid stands in a burette,
always take the level of the convex lower border of the meniscus
which forms upon its upper surface, and make this rest upon the
Hne to which the fluid is required to reach. Water standing to
the level of 10 c.c. in a burette will appear, therefore, as in Fig. 9.
The eye must always be on a level with the upper surface of the
liquid when a reading is made.

The burette just holds 10 c.c. of water if at a temperature of
about 15° C. the water weighs 9-99 grammes. Similarly, with a
100 c.c. measuring flask the graduation is correct if the 100 c.c.
of water, at about 15° C, weigh 99-9 grammes.



For cleaning glass burettes, etc., and porcelain apparatus,
especially from fatty matter, the commercial trisodium phosphate
is useful.

International Atomic Weights (1914). 0=i6.
















































































































Weights and Measures upon the j\Ietrical System.

The metrical system is founded upon the " metre," which is
divided or multiplied by ten to represent different measures, as
follows :


inVu part of a metre,
ji- part of a metre.
jV part of a metre.

I millimetre
I centimetre
I decimetre
I metre
I decametre
I hectometre
I kilometre*

39-37 inches.
10 metres.
100 metres.
1,000 metres.

* The Latin prefix therefore indicates division, the Greek multiplication.



I cubic centimetre
28'35 cubic centimetres
1,000 cubic centimetres

or I cubic decimetre
I litre = 35-3 ounces
I pint
1,000 litres

O'oGi cubic inch.
I fluid ounce.

I litre.

1-765 pints.

568 cubic centimetres,

1 cubic metre.


One c.c. of distilled water at 4° C, and 760 millimetres baro-
metric pressure, weighs i gramme, which is the standard of weight.


I milligramme =

I centigramme =

I decigramme =

I gramme =

I decagramme =

I hectogramme =

I kilogramme =

I ounce =
I pound (16 ounces) =

I gallon of water =

I litre of hydrogen at 0° C, and

O'oSgS gramme.

I litre of oxygen at 0° C, and

O'oSqGx 16 grammes.

tttVo part of a gramme.

jItj part of a gramme.

^^ part of a gramme.

15-432 grains.

10 grammes.

100 grammes.

1,000 grammes.

28-35 grammes = 437-5 grains.

453*6 grammes =7,000 grains.

4-536 litres = 10 pounds.

760 millimetres pressure, weighs
760 millimetres pressure, weighs

Thermometer Scales.




Freezing-point = o


Boiling point =100

Centigrade Reaumur Fahrenheit — 32

5 4 9 ■

To convert Centigrade to Fahrenheit, x 9-^5, and add 32.
Fahrenheit to Centigrade, subtract 32, -=-9x5.
Reamur to Fahrenheit, -f 4 x 9, and add 32.
grains to grammes, x 0-0648.
cubic feet to cubic metres, x 0-0283.
cubic feet to litres, x 28-3.







The sample should always be collected for analysis just as it is
ordinarily obtained for drinking purposes. It is obvious, since
our object is to discover all the possibilities of danger, that an
endeavour should be made to ascertain the maximum amount of
pollution to which the water is liable. For instance, in the case of
streams, lakes, etc., the point of entrance of any drains, should
only be avoided to the same extent as it is by those who may
come to collect their drinking-water.

When there is a general system of water-supply, an effort must
be made to meet the same ends by choosing samples from the
street fountains and street mains, rather than from storage, etc.,
reservoirs. But since impurities may gain access during domestic
storage and distribution, it would not be fair in all cases to judge
a public supply from the tap-water of any particular dwelling.

With regard to shallow wells from which the water is removed
by pumping, it is advisable to continue the process for some time,
but no longer than it is judged that the water may be pumped
during any one day, under the prevailing circumstances of
demand. This is done because the last " pumpings " will often
furnish the maximum evidence of any pollution present.

To ascertain whether the water may have been contaminated
during its domestic storage and distribution, the sample should
be taken from the lowest draw-off tap (gener ally the scullery sink



tap), as then the water will have run the maximum risk of

When the fact is borne in mind that the water from many
shallow wells is materially influenced both as to quantity and
quality by the rainfall, it will be understood how samples from
the same well may vary in purity according as to whether a long
dry period may have preceded the collection, or a heavy rainfall,
which may be the means of conveying to the well, water impreg-
nated with surface washings, or water which may have washed
accumulated impurities out of the interstices of the soil. These
facts as to rainfall should always be ascertained ; and it may be
desirable to examine a further sample after prolonged and heavy

The fact as to whether a cesspool or drain contaminates a well
can readily be decided either by introducing a considerable
quantity of sodium chloride, followed by plenty of water, and
estimating the chlorine in the well-water every morning and
evening for several days; or by introducing a strongly alkaline
solution of fluorescine, and endeavouring to detect the green
colour in the well-water.

Water is customarily collected for analysis in a large glass-
stoppered bottle, called a " Winchester quart," which holds about
twice the amount which is implied by its description {i.e., about
half a gallon). Stout wicker covers are made to protect them in
transit by parcel post or rail. These bottles have become gener-
ally adopted because, in addition to holding an amount which
meets all the requirements of an ordinary analysis (even though
it be necessary to repeat some of the estimations), they are
strongly made; but obviously any stout glass bottle of similar
dimensions, fitted with a glass stopper, will serve the same end.
If a mineral analysis should be required, it is necessary to have
quite 2 gallons of the water. It is well to avoid the employment
of stoneware bottles.

The bottle must be thoroughly cleansed by first well rinsing
with a little dilute hydrochloric acid, and then by washing in
good water until the washings are no longer acid.

In collecting a sample the bottle is first quite filled with the
water, and then emptied; it is again almost completely filled up
(in a manner which will not favour the aeration of the water),
and the glass stopper, having been found to fit accurately and
tightly, is well rinsed in the water before it is inserted, when it is


tied down firmly on to the neck of the bottle and the knots are
protected with seahng-wax. Care is taken to keep the sample
cool and unexposed to light until the analysis is commenced; and
under no circumstances should some of the estimations be un-
necessarily delayed, as important chemical changes may occur —
i.e., organic matter may suffer a very slight reduction, free
ammonia may increase or decrease in amount, nitrates may be
reduced or even increased, calcium or magnesium carbonates
and iron, which were held in solution by carbonic acid, may,
owing to the escape of the carbonic acid, be partially deposited.
Therefore the figures of the two ammonias, the oxidizable organic
matter and of the oxidized nitrogen, together with the physical
characters, should always be ascertained as soon as possible (and
certainly within forty-eight hours) after the sample has been

Information required as to Samples.

It is often difficult to form a correct opinion upon the purity
of a sample without the knowledge of some of the circumstances
of its source; and if the water is held to run risk of harmful
pollution this should suffice for its condemnation, although the
chemical analysis at the time may prove satisfactory; natural
agencies may suffice to purify water for a time, but there is always
a possibility of their purifying powers being exhausted at any
moment, and the danger of drinking such water is a constant one.

Thus, information as regards the risks of pollution may be of
great value as indicating possibilities of danger, when such danger
may not be manifest at the time by analysis; it is also of value
to ask, in every instance, the motive for requiring an analysis.

Information bearing upon the constitution of the strata
through or over which the water has passed is most valuable,
since the soluble mineral constituents of certain strata are similar
to those which may result from previous organic pollution.
Anyone is able to furnish information as to whether the surface
consists of such familiar substances as clay, gravel, sand, chalk,
or vegetable mould, and whether the subsoil, exposed as it is by
railway or road cuttings, is of chalk, sandstone, etc.

It is very desirable that labels should be given to those col-
lecting samples, and that these should be affixed to the bottle.
The subjoined label, when filled in, would convey all necessary
information to the analyst :


Sample of Water for Analysis.

Name and address of sender

Place, date, and hour of collection

Source of sample and method of collection

If from well, give approximate depth

and geological characters of the soil and subsoil of the district

If from shallow well, give the rainfall during the previous week, in

such terms as " nil," " small," or " great " in amount

Nature and distance of any evident or possible source of pollution

Reason for tlcsiring an analj'sis.

The following is the usual form of report upon the chemical
examination of a sample of water:

Report on the Analysis of a Sample of Water received on.
from and labelled . . .

Number of sample . .

Date of examination . . . . . . ■ . .

Physical characters

Reaction . .

Saline and free ammonia

Organic (or " albuminoid ") ammonia. .

Oxygen absorbed from permanganate in two hours at 27° C.


Nitrogen as nitrates

Total solid matter . .

(a) Volatile
(fe) Fixed

Appearance on ignition
Total hardness

{a) Temporary..

(b) Permanent..
Poisonous metals

Microscopical examination of the sediment





Where a series of analyses are to be brouf^ht into comparison,
the following form of report is to be preferred :

Results of Analysis expressed in Parts i^kr 100,000.




c 2

Organic or

Oxygen absorbed

in Two Hours

at 27° C.




Nitrogen as

Nitrates and
















The result of every analysis should be carefully entered in a
book kept for the purpose, for such a record becomes most
valuable for reference purposes and for making comparisons
with future samples of water from the same locality.

The results of the estimations made in water analysis are still

Online LibraryHenry R. (Henry Richard) KenwoodPublic health laboratory work → online text (page 2 of 36)