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WORKS OF JOSEPH RACE
PUBLISHED BY JOHN WILEY & SONS, Inc.
=Examination of Milk for Public Health Purposes.=
A practical handbook for those engaged in the chemical and
bacteriological examination of milk for public health purposes. vi
+ 224 pages, 5-1/4 x 8, 4 diagrams. Cloth, $1.75 net.
=Chlorination of Water.=
In this book the various aspects and methods of chlorination are
discussed with a view to stimulating research work in this field of
science. viii + 158 pages, 5-1/4 x 8, 12 figures and 16 diagrams.
Cloth, $1.50 net.
Chlorination of Water
BY
JOSEPH RACE, F.I.C.
_City Bacteriologist and Chemist, Ottawa; Capt. Canadian Army
Hydrological Corps; Associate Member of Committee on Water Supplies,
American Public Health Association; Member of Committee on Water
Standards and Standard Methods of Analysis, American Water Works
Association; Chairman of Committee on Standard Methods of Analysis,
Canadian Public Health Association_
_FIRST EDITION_
NEW YORK
JOHN WILEY & SONS, INC.
LONDON: CHAPMAN & HALL, LIMITED
1918
Copyright, 1918
BY
JOSEPH RACE, F.I.C.
PRESS OF
BRAUNWORTH & CO.
BOOK MANUFACTURERS
BROOKLYN, N. Y.
DEDICATED
TO
~Sir Alexander Houston, K.B.E., D.Sc., M.B., C.M.~
PREFACE
No apology is necessary for the publication of a book on the
chlorination of water. This method of treatment, practically unknown
fifteen years ago, has advanced in popularity during the last decade in
a most remarkable manner, and in 1918 over forty millions of people are
being supplied with chlorinated water.
It may justifiably be said that no other sanitary measure has
accomplished so much at so small a cost; and that civilization owes a
deep debt of gratitude to the pioneers in municipal water chlorination:
Dr. A. C. Houston in England, and Mr. G. A. Johnson and Dr. Leal in
America.
In this volume I have endeavoured to collect and correlate the
information hitherto scattered in various journals and treatises and to
present it in a comprehensible manner. The various aspects and methods
of chlorination are discussed and suggestions have been made which, I
hope, will stimulate research work in this fertile field of science.
I wish to acknowledge my indebtedness to the engineering staff of the
Ottawa Water Works Department and to Lieut. W. M. Bryce for the
preparation of diagrams.
JOSEPH RACE.
OTTAWA, ONT.,
April, 1918.
CONTENTS
CHAPTER PAGE
I. HISTORICAL 1
Sodium Chloride. Chlorine. Bleach. Eau de Javelle. Antiseptics.
Hermite fluid. Webster's process. Electrozone. Chlorination of
sewage in Germany, U. S. A., and England. Chlorination of
water. Lincoln installation. Oxychloride. German experiments.
European practice. Inception of chlorination in America.
II. MODUS OPERANDI 14
Composition of bleach. Bleaching action. Nascent oxygen
hypothesis. Hydrolysis of bleach. Effect of acids and salts on
hydrolysis and germicidal action. Effect of ammonia. Direct
toxic action. Hypochlorous acid. Sodium hypochlorite. Chlorine
water. Nature of action.
III. DOSAGE 30
Organic matter. Initial count. Viability of organisms. Mineral
matter. Colour. Temperature. Admixture. Contact period.
Turbidity. Light. Determination of dosage.
IV. BACTERIA SURVIVING CHLORINATION 50
Disinfectants. Antiseptics. Viability of bacteria. New York
results. Reversed ratio of counts. Coliform organisms.
Aftergrowths in water and sand.
V. COMPLAINTS 62
Auto-suggestion. Tastes and odours. Sludge problem. Colic.
Effect on fish and birds. Effect on plants and flowers.
Corrosion of iron and lead pipes.
VI. BLEACH TREATMENT 72
Storage of bleach. Mixing tanks. Storage tanks. Dosing
apparatus. Control. Analysis of liquor. Detection and
estimation of free chlorine. Chlorometer. Cost of construction
and operation. Antichlors. DeChlor filters.
VII. LIQUID CHLORINE 89
Historical. Leavitt-Jackson machine. Electro Bleaching Gas
Co.'s types. Wallace and Tiernan's manual control types.
Effect of temperature on gas pressure. Impurities in gas.
Advantages. Comparison of liquid chlorine and bleach. Cost of
treatment. Popularity. Chlorine water.
VIII. ELECTROLYTIC HYPOCHLORITES AND CHLORINE 105
Hermite fluid. Eau de Javelle. Chloros. Non-diaphragm cells:
Dayton, Hermite, Mather and Platt, Haas and Oettel. Diaphragm
cells: Hargreaves-Bird, Nelson, Allen-Moore. Montreal
installation. Costs.
IX. CHLORAMINE 115
Preparation. Absorption by water. Experimental results. Works
results. Ratio of chlorine to ammonia. Economics. Advantages.
Operation. Other chloramines. Halazone.
X. RESULTS OBTAINED 132
Object of chlorination. Effect on filter rates and algae.
Hygienic results. Typhoid rates. Typhoid reduction at
Philadelphia, Chicago, and Ottawa. Abortive epidemics. Use and
abuse of chlorination.
APPENDIX 147
Estimation of free chlorine in water.
NAME INDEX 151
SUBJECT INDEX 155
CHLORINATION OF WATER
CHAPTER I
HISTORICAL
Chlorine, although one of the most widely distributed elements known to
chemists, is never found in the free condition in nature; it exists in
enormous quantities in combination with sodium, potassium, calcium,
magnesium, etc. As sodium chloride, common salt, it occurs in
practically inexhaustible quantities in sea water together with smaller
quantities of other chlorides. In mineral form, enormous deposits of
sodium chloride are found in Galicia, Transylvania, Spain, in England
(particularly in Cheshire), and in sections of North America. The most
important deposits of potassium chloride are those at Stassfurt,
Germany, where it occurs either in the crystalline condition as sylvine
or combined with magnesium chloride as carnallite.
Chlorine was discovered by the Swedish chemist Scheele in 1774, but he,
like Lavoisier and his pupil Berthollet, who declared it an oxygenated
muriatic acid, was unaware of the elemental nature of the new substance.
Sir Humphrey Davy investigated this body in 1810 and definitely proved
it to be an element; Davy designated the element chlorine from the Greek
[Greek: chloros] = green.
The first attempt to utilise chlorine, or its compounds, for bleaching
purposes, appears to have been due to James Watt, who noticed the
decolourising properties of chlorine during a visit to Berthollet. This
attempt ended in failure because of the destructive effect on the
fibres, but, in later trials, this was prevented by first absorbing the
gas in a solution of fixed alkali. These experiments proved the
possibility of bleaching by means of chlorine compounds but the high
cost of soda made the process unprofitable, and it was not until Henry
succeeded in preparing a combination with lime that could be reduced to
a dry powder that this mode of chemical bleaching became a commercial
success.
The manufacture of chloride of lime (hypochlorite of lime, bleaching
powder, bleach) was taken up by Charles Tennant in 1799 at St. Rollox
near Glasgow, and in 1800 about 50 tons were sold at a price of $680
(L139) per ton.
Chlorine is produced as a by-product in the manufacture of soda by the
Leblanc process, but until 1865, when the British Alkali Act stopped the
discharge of hydrochloric acid vapours into the atmosphere, the
development of the bleaching powder industry was not rapid. The
hydrochloric acid that was formerly discharged into the air as a waste
product afterwards became a valuable asset that enabled the Leblanc
process to successfully compete with the newer ammonia-soda process. In
1890 another competitor to the Leblanc process was introduced when
caustic and chlorine were produced in Germany by electrolytic methods.
After the successful development of this method in Germany, it was taken
up in the United States of America and in 1912 more than 30,000
electrical horse-power were daily used in this industry. In 1914 the
almost complete cessation of exports of bleach from Europe raised the
price, which attained phenomenal heights in 1916 (cf. page 125), and
stimulated the production of bleach both in the U. S. A. and Canada.
TABLE I.--BLEACH STATISTICS. NORTH AMERICA
-------------+----------------------------+---------------------
Year. | Bleach Manufactured, | Selling Price
| Short Tons. | Per 100 Lbs.
-------------+----------------------------+---------------------
1904 | 19,000 |
1909 | 58,000 |
1914 | 155,000 | $1.63
1915 | 180,000[A] | 2.63
1916 | 230,000[A] | 6.56
1917 | 260,000[A] | 2.44
-------------+----------------------------+---------------------
[A] Estimated.
As a disinfectant, chlorine was first used about the year 1800 by de
Morveau, in France, and by Cruikshank, in England, who prepared the gas
by heating a mixture of hydrochloric acid and potassium bichromate or
pyrolusite; this is essentially the same as the original mixture used by
Scheele.
During the early part of the last century the efficacy of chlorine of
lime as a disinfectant, and particularly as a deodourant, was well
recognised and as early as 1854 an English Royal Commission used this
substance for deodourising the sewage of London. A committee of the
American Public Health Association reported in 1885 that chloride of
lime was the best disinfectant available when cost and efficiency were
considered.
Eau de Javelle, first made by Percy at the Javelle works near Paris in
1792, is another chlorine compound that has enjoyed a considerable
reputation as a disinfectant and deodouriser for over a century; it is
essentially a mixture of sodium chloride and sodium hypochlorite.
The discovery of electrolytic hypochlorites dates back to 1859, when
Watt found that chlorides of the fixed alkalies and alkaline earths
yielded hypochlorites on being submitted to the action of an electrical
current.
Until the middle of the last century disinfection was regarded as a
process that arrested or prevented putrefactive changes but the nature
of these changes was imperfectly comprehended and micro-organisms were
not associated with them.
In 1839 Theodor Schwann,[1] who might be regarded as the founder of the
school of antiseptics, reported that "Fermentation is arrested by any
influence capable of killing fungi, especially by heat, potassium
arseniate, etc...."; but his results were not accepted by the adherents
of the theory of spontaneous generation and it was not until the
publication of the work of Schroder and Dusch[2] that Schwann's views
were even partially accepted. The final refutation to the spontaneous
generation theory was given by the monumental researches of Pasteur who,
in 1862, proved the possibility of preparing sterile culture media and
demonstrated the manner in which they could be protected from
contamination. Bacteria and other micro-organisms were shown to be
responsible for the phenomena that had been attributed previously to the
"oxygen of the air," and from this period the development of
bacteriology as a science proceeded rapidly.
The next important step, from the public health standpoint, was the
discovery by Koch, in 1876, that a specific bacterium (_B. anthracis_)
was the cause of a specific disease in cattle (anthrax or splenic
fever). In 1882 Koch made a further advance by developing a solid
culture medium which permitted disinfectants and antiseptics to be
studied quantitatively with a greater degree of accuracy than had been
possible previously.
Since 1845, when Semmelweiss succeeded in stamping out puerperal fever
in Vienna, where it had been so long established as to be endemic,
chlorine has been very generally employed in sanitary work and the
conditions necessary for obtaining successful results have been
partially elucidated. Baxter was the first to state that the
disinfecting action depended more upon the nature of the pabulum than
upon the specific organism present and this was confirmed later by Kuhn,
Bucholtz, and Haberkorn. The latter found that urine consumed large
quantities of chlorine before any disinfection occurred.
One of the earliest preparations used in sanitary work was an
electrolysed sea water, usually known as Hermite Fluid. This was
introduced by M. Hermite in 1889 and was employed for domestic purposes
and for flushing sewers and latrines. It was used at Brest for the
dissolution of faecal matter and a prolonged trial was given to it at
Worthing in 1894. The report of Dupre and Klein, who conducted the
bacteriological examinations, was against the process, but Ruffer and
Roscoe reported more favourably and further trials were carried out at
Havre, l'Orient, and Nice. The _Lancet_ (May 26, 1894) reported at
length upon the Worthing experiments: it was found that during the
electrolysis of the sea water, the magnesium chloride was also partially
converted into hypochlorite, which then dissociated into magnesium
hydrate and hypochlorous acid; the former deposited in the electrolyser
and left the solution acid and unstable; urine was found to act upon it
at once with a consequent loss in strength of over 50 per cent.
Another electrolytic method was that of Webster,[3] who installed an
experimental plant at Crossness, near London, in 1889. A low-tension
direct current was passed between iron electrodes placed in the sewage
and although the process was largely one of chemical precipitation,
Webster noted the disinfecting value of the hypochlorite formed from the
chlorides normally present in the sewage. He also directed the attention
of sanitarians to the possibility of using sea water as a cheap source
of chlorides and a plant based on this principle was erected in Bradford
in 1890 and reported upon by McLintock.[4]
Strong salt solutions were substituted for sea water by Woolf and the
product was commercially known as "Electrozone." A plant of this
description was installed at Brewster, N. Y., in 1893[5] for
chlorinating the sewage from a small group of houses. The sewage was
discharged into a small creek which polluted Croton Lake. Successful
results led to a similar treatment near Tonetta Creek.[6] This was
apparently the first occasion on which the specific object was the
destruction of bacteria.
Electrozone was used at Maidenhead, on the Thames, in 1897 and the
installation was reported upon by Robinson, Kanthack, and Rideal in
1898. Kanthack found that a dosage 3-3.6 p.p.m. reduced the organisms in
a sewage effluent to 10-50 per c.cm. whilst Rideal found that about 18
p.p.m. of chlorine produced a condition of sterility in 1 c.cm.
Chloride of lime had previously been used in the London sewage as a
deodourant by Dibden in 1884 but the treatment was not successful and
was abandoned in favour of other oxidisers.
During the last decade of the twentieth century the use of bleach for
the disinfection of both sewage and water received the attention of many
well-known German sanitarians and many important results were obtained.
In the earlier experiments made at Hamburg, Proskauer and Elsner[7]
obtained satisfactory results with 3-4 p.p.m. of chlorine on a clarified
sewage with 10 minutes contact. Dunbar and Zirn (_ibid._) used crude
sewage and found that 17 p.p.m. of available chlorine were required to
remove _B. typhosus_ and cholera vibria with a contact period of two
hours. A striking feature of all the German work on chlorination is the
very high degree of purification aimed at: quantities as large as one
litre were tested for specific organisms and in many of the experiments
with sewage _B. coli_ was found to be absent from a considerable
percentage of the samples.
The importance of previously removing suspended matter, which could not
be penetrated by the germicide, was emphasised by Schwartz[8] although
it had been previously noted by Schumacher.
At the Royal Testing Station in Berlin, numerous experiments on sewage
chlorination were made by Kranejuhl and Kurpjuivut.[9] The results were
judged by the _B. coli_ content, which was taken as an index of
pathogenicity because this typical intestinal bacillus was found to be
more frequent and less viable than the majority of the pathogenic
organisms.
Other important work on this subject was carried out, in connection with
the pollution of the Hooghly River, by a Bengal Government Commission in
1904; and by the State Board of Health of Ohio in co-operation with the
Bureau of Plant Industry of the United States Department of Agriculture
in 1907. The chlorination experiments of the latter were reported by
Kellerman, Pratt, and Kimberly.[10]
The most valuable contribution to the disinfection of sewage was that of
Phelps,[11] who critically examined the work of previous experimenters
and directed attention to the unnecessary stringent standards adopted in
European practice. His work at Boston in 1906, at Red Bank, N. J., and
at Baltimore in 1907, demonstrated in an indubitable manner the economic
possibilities of sewage chlorination. The dosages necessary for crude
sewage and filter effluents were indicated and also the necessary
contact periods. This work marks the commencement of a new era in
sanitary science.
The first occasion on which chlorine compounds were first used for the
disinfection of water cannot be definitely ascertained. It has been
stated to the author that bleach was used for treating wells as early as
1850 but this treatment was apparently made without definite knowledge
of the destruction of micro-organisms.
In 1897, Sims Woodhead employed bleach solutions for the sterilisation
of the distribution mains at Maidstone, Kent, subsequent to an epidemic
of typhoid fever.
The credit for the first systematic use of chlorine in water
disinfection is due to A. C. Houston with whom McGowan was associated in
the work carried out at Lincoln in 1904-1905.[12] The reservoirs,
filters, and distribution system, owing to flood conditions, became
infected with typhoid bacilli which caused a severe epidemic amongst the
consumers. The storage and purifications works were thoroughly treated
with a solution of "chloros" (sodium hypochlorite containing
approximately 10 per cent of available chlorine) which was regulated to
give an approximate dosage of 1 part per million. The bacteriological
results were entirely satisfactory but many complaints were received
that the treatment had imparted a mawkish taste to the water. This was
attributed to the action of the alkaline chloros on the organic
impurities in the water. It was also stated that the water injured
plants, fish, and birds and extracted abnormal amounts of tannin from
tea but no substantiating evidence was produced in support of these
complaints. Houston made a continuous physiological test of the effect
of the chlorinated water on small fish by suspending a cage of gold fish
in the filter effluent chamber and also proved that the treatment had no
appreciable effect on the plumbo-solvency of the supply.
Nesfield, of the Indian Army Medical Service,[13] reported in 1903 the
results of numerous experiments on the destruction of pathogenic
organisms by chlorine compounds and suggested their use in military work
to prevent a recurrence of the appalling loss of life from water-borne
diseases (especially enteric fever) such as took place during the Boer
War. Nesfield proposed to use about 100 p.p.m. of available chlorine and
to remove the excess after a contact period of 10 minutes. This work is
especially interesting from the historical standpoint because it
contains the first suggestion of the possibilities of compressed
chlorine gas in steel cylinders.
A few years later, electrolytic hypochlorite (oxychloride) was used at
Guildford by Rideal and various chlorine compounds were tried on the
water of the Seine and Vanne, in France, and at Middlekerke and Ostend,
in Belgium. Experimental work on water chlorination was also reported by
Thresh and by Moor and Hewlett.[14]
During the nineties many experiments on water chlorination were made by
Traube, Sickenberger, Kauffman, Berge, Bassenge, and others. Traube[15]
was able to completely sterilise water rich in bacteria in 2 hours by
the addition of bleach equal to 1.06 p.p.m. of available chlorine. At
the end of the contact period about 90 per cent of the added chlorine
was unabsorbed and was destroyed by the addition of sodium bisulphite.
Bassenge[16] followed up the work of Traube and that of Sickenberger and
Kauffman, who had shown that it was possible to destroy cholera vibrio
in Nile water by means of sodium hypochlorite. Bassenge used higher
concentrations than Traube and found it possible to destroy _B.
typhosus_ and _B. coli_ in ten minutes with 60-90 p.p.m. of available
chlorine. The excess was destroyed by adding calcium bisulphite.
Lode[17] experimented with waters seeded with _B. coli_, _B. typhosus_,
and _B. tetani_ and found, contrary to Traube, that 1-2 p.p.m. of
chlorine did not sterilise in two hours. _B. coli_ was usually destroyed
by 4 p.p.m. of chlorine in ten minutes and even better results were
obtained with _B. typhosus_ and cholera vibrio: the former was destroyed
in one hour by 1 p.p.m. and in ten minutes by 2 p.p.m.; the latter
organism required 1-2 p.p.m. with a twenty-minute contact period. Lode
noted that organic matter lowered the bactericidal activity of chlorine
and recommended the use of 30 p.p.m. of chlorine to ensure rapid and
complete sterilisation. Berge[18] used chlorine peroxide, generated by
the action of hydrochloric acid on potassium chlorate, for the
sterilisation of water and this process was afterwards used at Ostend at
a plant having a capacity of about 1,300,000 gallons per day. The dosage
was equal to 0.53 p.p.m. of available chlorine and coke filters were
used to destroy the excess although they were not found to be
indispensable as the free chlorine disappeared spontaneously. This
process appears to have been tried on the Brussels supply and also for
the treatment of a hospital supply at Petrograd.
The object of German sanitarians seems to have been to obtain
practically instantaneous sterilisation of water for the use of
travellers and troops in the field. Until the commencement of the
European War they did not have a high opinion of chlorination and
generally regarded it as inefficient. Schumberg[19] expressed the
opinion that no chemical method of disinfection could be absolutely
relied upon, under all circumstances, to prove fatal to bacteria.
Plucker[20] stated that several investigators, particularly Schuder, had
shown that chlorine, even in the proportion of 40 p.p.m. did not
invariably destroy cholera vibrio and _B. typhosus_; and that with
smaller doses the destruction was still less complete. He also stated
that the bacteriological experiments of American workers were open to
criticism and that they employed antiquated methods.
By 1916 the German sanitarians appeared to have realised that their
bacteriological standards were too stringent (Langer[21]) and that the
process had proved its value in an indisputable manner.
European practice, in the comparatively few instances in which it has
been used, has been to employ large doses of chlorine and to remove the
excess by chemicals or by filtration through special media. In 1916,
however, London commenced to chlorinate a portion of its supply and the
following year practically the whole supply was chlorinated. A dosage of
approximately 0.5 p.p.m. is used and the bleach solution is added to
the pre-filtered water. Worcester is also proposing to chlorinate the
supply to maintain the purity of the water without extending the slow
sand filtration plant.
In North America, hypochlorite of soda and chlorine were used on the
Jewell Filter at the Louisville Experimental Station in about 1896 by
George W. Fuller and a year later they were used at Adrian by Jewell.
The first commercial successful attempt was made by G. A. Johnson. In
1908 the Union Stock Yards Company of Chicago were proceeded against by
the City of Chicago regarding the condition of the effluent of the
Bubbly Creek filter plant. Copper sulphate had been previously used in
conjunction with the filters but stock shippers complained that the
water had a deleterious effect upon the animals consuming it. Johnson
eliminated the copper treatment and substituted bleach which was added
seven and a half hours previous to filtration, with a dosage of 1.5
p.p.m. The results were very satisfactory.
About the same time, Johnson and Leal commenced the treatment of the
Boonton supply of Jersey City, N. J., consumed about 40 million gallons
per day. The water was first treated with 36 pounds of bleach per
million gallons (1.4 p.p.m. of available chlorine) but this quantity was
gradually reduced until only 5 pounds per million gallons (0.2 p.p.m. of
chlorine) were being used in April, 1909. The ability of the process to
adequately purify water became the cause of a lawsuit and the decision
of the Court was:
"From the proofs taken before me, of the constant observation of the
effect of this device, I am of the opinion and find that it is an
effective process which destroys in the water the germs, the presence of
which is deemed to indicate danger, including the pathogenic germs, so
that the water after this treatment attains a purity much beyond that
attained in water supplies of other municipalities. The reduction and
practical elimination of such germs from the water was shown to be
substantially continuous.
"Upon the proofs before me, I find that the solution described leaves no
deleterious substances in the water. It does produce a slight increase
in the hardness but the increase is so slight as in my judgment to be
negligible.
"I do therefore find and report that this device is capable of rendering
the water delivered in Jersey City pure and wholesome, for the purposes
for which it is intended and is effective in removing from the water
those dangerous germs which were deemed by the decree to possibly exist
therein at certain times."[22]
During the next few years the use of hypochlorite in water purification,
both alone and in conjunction with filtration, became very popular and
in 1911 over 800 million gallons per day were treated in this manner.
Amongst the users were some of the largest cities in North America,
including Brooklyn, Albany, and New York City, N. Y., Cincinnati and
Columbus, Ohio, Harrisburg, Philadelphia, Pittsburg, and Erie, Pa.,
Hartford, Conn., Nashville, Tenn., St. Louis and Kansas City, Mo.,
Montreal, P. Q., Toronto and Ottawa, Ont., Baltimore, Md., and
Minneapolis, Minn. At present (1918) over 3,000 million gallons per day
are being chlorinated in North America and more than 1,000 cities and
towns are employing this process.
BIBLIOGRAPHY
[1] Schwann. Microskopische Untersuchungen ueber die Uebereinstimmung in
der Textur und dem Wachstum der Tiere und Pflanzen. Berlin. 1839
[2] Schroder and Dusch. Ann. der Chem. u. Pharm., 1854, 89, 232.
[3] Webster. The Engineer. 1889, 67, 261.
[4] McLintock. Brit. Med. Jour., 1890, 11, 498.
[5] Eng. News. 1893, 30, 41.
[6] Eng. Record. 1894, 29, 110.
[7] Proskauer and Elsner. Vierteljahresschr. ger. Med. u. oeff.
Sanitaetswesen. 1898, 16, Supp. Heft.
[8] Schwartz. Gas. Eng., 1906, 29, 773.
[9] Kranejuhl and Kurjuivut. Mitteilungen aus der Koeniglichen
Pruefungsanstalt fuer Wasserversorgung und Abwaesserbeseitigung zu Berlin,
1907, 9, 149.
[10] Kellerman, Pratt, and Kimberly. Bull. 115, Bur. Plant Ind., U. S.
Dept. of Agr., 1907.
[11] Phelps. Water Supply Paper 229, Dept. of Int., U. S. Geo. Survey.
[12] Houston and McGowan. 5th Rpt. Royal Commission on Sewage Disposal.
[13] Nesfield. Public Health. 1903, 15, 601.
[14] Moor and Hewlett. Rpt. of M. O. to L. G. B., 1909-10.
[15] Traube. Zeit. f. Hyg., 1894, 16, 149.
[16] Bassenge. Zeit. f. Hyg., 1895, 20, 227.
[17] Lode. Archiv. f. Hyg., 1895, 24, 236.
[18] Berge. Rev. d'Hyg., 1900, 22, 905.
[19] Schumburg. Zeit. f. Hyg., 1903, 45, 125.
[20] Plucker. J. Gasbeleucht., 1911, 54, 385.
[21] Langer. Zeit. f. Hyg., 1916, 81, 296.
[22] Johnson. Jour. Amer. Pub. Health Assoc., 1911, 1, 566.