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Development and Improvement of Scientific Instruments and Equipment go hand in hand

with the development of science, and their progress is mutually beneficial

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Among the many new instruments developed by W. M. Welch Scientific Company may be mentioned the following:

SOURCE OF LIGHT FOR OPTICAL DISC. Parallel

rays for use with the Optical Disc. Avoids setting up an arc. Attaches to 110 volt circuit either A.C. or D.C. Cat. No. 3680.

UNIVERSAL MOTOR ROTATOR for Direct Current

or Alternating Current. Continuous speed chang-
ing. Produces rotation in vertical or horizontal
direction. With speed counter and 6 ft. of cord.

Cat. No. 905.
MODEL OF THE ATOM. Design suggested by Prof.

R. R. Ramsay, Indiana University. Shows stable
arrangement of rotating bodies similar to atoms or
solar system. Illustrates radio-activity. Suitable

for projection. Cat. No. 610. DEMONSTRATION ORGAN PIPE. Design sug

gested by Prof. Lunn, University of Chicago.
Shows the effect of mouth piece upon fundamental.
Enables holding fundamental regardless of way of
blowing. Shows the finer features of the organ
pipe design. Cat. No. 3268.

CHOKE COIL shows impedance factor and that Ohm's

law does not hold with Alternating Current. May be used either as a demonstration piece or as a student experiment piece. Light glows more dimly when iron core is inserted in solenoid. Cat No. 2610.

AIR BRAKE MODEL. Triple valve air brake as used

on railway cars, with safety device in which reserve supply uf air releases brake. Cat. No. 1560.

Full Information regarding these developments, and others will be sent upon request. Ask to be put on our mailing list to receive the first notices of the new Welch Specialties.

QUALITY

A Sign of Quality

W

A Mark of Service

SERVICE

W. M. WELCH SCIENTIFIC COMPANY

Manufacturers, Importers and Exporters of

Scientific Apparatus and School Supplies 1516 Orleans Street

CHICAGO, ILL

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In making a determination of the H-ion concentration of a solution by the electrometric method, the actual measurement is one of potential difference, or voltage, between a hydrogen and a calomel electrode in contact with the solution. The H-ion concentration, or the Ph value, must then be found by reference to conversion tables or by calculation from an equation.

The H-ion Calculator is a slide rule, which, with one setting converts the measured voltage into the corresponding Ph or Ch+ value. Everyone concerned with H-ion concentrations should have this handy tool. . Price

50 cents
Partially covering cost of manufacture.
Have you a copy of our Catalogue S75, “Electrometric
Methods and Apparatus for Determining Hydrogen Ion Concentra-
tions"? We will be glad to send you one.

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Leeds & Northrup Company

Electrical Measuring Instruments 4901 Stenton Ave.

Philadelphia, Pa.

SCIENCE | The University of Adelaide A Weekly Journal devoted to the Advancement of

Applications are invited for the position of ProScience, publishing the official notices and pro

fessor of Zoology in the university of Adelaide, ceedings of the American Association for

South Australia. .

The Professor will be required to offer a course the Advancement of Science

of Lectures and Practical Work in Elementary Published every Friday by

Animal Biology, suitable for students of Science,

Medicine and Agriculture, together with an adTHE SCIENCE PRESS

vanced course in Zoology for students in Science. LANCASTER, PA.

GARRISON, N. Y. | A demonstrator and a collector will be provided. NEW YORK, N. Y.

| Salary, £1,100 per annum.

Applications with testimonials, statement of Entered in the post-office at Lancaster, Pa as second-class matter, under the act of March 3, 1879

academic qualifications, experience, copies of pub

lications, and a recent photograph, should be forZOOLOGIST, M.S., in Bacteriology, Entomology,

warded to the Registrar, The University of AdeEmbryology, Histology; A.M., in Cytology, Protozoology, Neurology; (Dissertation for Ph. D. on In

laide, South Australia, to reach him not later than vertebrate Morphology, nearly complete ), has August 1st, 1921. studied abroad, seeks post as Professor or Head of Candidates must indicate the earliest date at Department, with part time for research. Eight years

which they can commence duties. teaching experience, (in Zoology, Comparative Anatomy, Histology, Embryology and Biology). Ad

Further information may be obtained from : dress “K” care of Science, Garrison, N. Y.

(a) The Agent-General for South Australia,

Australia House. Strand, London. MATHEMATICIAN, with several years of expe

(b) The Commissioner for Australia, 61 Broadrience in university and college teaching, seeks

way 3012, New York City, U. S. A. new opening with genuine opportunity for the

(c) Any University in Australia or New Zeafuture. Address "D,” care "Science"

land. Garrison, N. Y.

EMPLOYMENT OFFERED

SCIENCE PROFESSORS WANTED An Exceptional Opportunity is offered to those who

can sell our goods (for which a market has already been made) to Physicians and Dentists. Exclusive

Applications are invited for vacant territory. Permanent work is guaranteed to successful salesmen. Address, Sales Manager, Lea & positions in Physics, Biology, and Modern Febiger, 706-10 Sansom Street, Philadelphia.

Languages at Mount Allison University, THE COCCIDAE

Sackville, New Brunswick, Canada. Tables for the Identification of the Subfamilies and Some of the More Important Genera

Apply to and Species Together with Discussions of Their Anatomy and Life History

B. C. BORDEN, President By ALEX. D. MacGILLIVRAY Ph.D. Prof. of Systematic Entomology in the University of Illinois

Chapters devoted to near relatives of Coccids, Prep An Opening For An ASSISTANT PROFESSOR aration, External Anatomy, Classification, and to each of the Subfamilies and Tribes, Twenty-seven in all.

OF PHYSICS in a University in the South. Oppor8vo. Cloth. IX+502. $6.00 net.

tunity for growth for a high type, trained man with Sent prepaid upon receipt of price. Order direct from publisher teaching experience. Address S, care of SCIENCE, SCARAB COMPANY

Garrison, N. Y. 603 Michigan Ave.

Urbana, Illinois

UNIVERSITY OF ALBERTA, CANADA

Junior Demonstrator required for Department of OSMIUM SOLUTIONS

Physics. Demonstrator expected to engage in Our special packages designed for your convenience,

research. Beginning salary $1,200 per annum. carries our guarantee of purity and exact weight.

Duties to commence October 1st. Allowance for AMERICAN CHEMICAL PRODUCTS CO.

passage. Apply with recommendations to ProWrito for Cir. 200

Rochester, N. Y. | fessor Boyle.

OSMIC ACID

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Important Scientific Books

The Grand Strategy of Evolution

By WILLIAM PATTEN
PROFESSOR OF BIOLOGY, DARTMOUTH COLLEGE

“One of the most significant and original contributions to general theoretical biology that has appeared in years."-Raymond Pearl, Professor of Biometry, Johns Hopkins University.

A book which it is an inspiration to read and a duty to recommend. It is the sort of book which carries mot the burr or the shell or the boxes or burlaps in which nature or man has packed the makings of knowledge and wisdom; but really in shape to be converted into immediate understanding. It is the kind of book which is a liberal education in itself.

No book in the entire post-Darwinian literature equals this volume as a guide to the congruity between the constructive processes of nature and the moral economics of 'the psychic factors.' It would be difficult to overstate the service which Professor Patten has performed in teaching the lesson that the problem of life, personal and public, is not to be solved by 'fighting the cosmic process, but by 'accepting nature's constructive rightness as the ethical standard, and by adopting her constructive methods as the moral code.'”—Albion W. Small, Professor of Sociology, University of Chicago, in “ The American Journal of Sociology."

LARGE 8VO CLOTH, $5.00 NET

SANITARY ENTOMOLOGY

Illus. $10.00 net Edited by W. Dwight Pierce, M.D., Con

sulting Entomologist, Formerly of U. S. Bureau

of Entomology. A volume by ten expert entomologists on how insects cause or carry disease, how these insects live, and how to prevent disease by controlling the insects. THE UNITY OF THE ORGANISM

Illus. 2 vols, $5.00 net By William E. Ritter, Ph.D., Director of the

Scripps Institute for Biological Research, University of California. The author reviews the history and doctrine of “elementalism," and opposes to it his doctrine of the unity of the living being BIOLOGY IN AMERICA Illus. $10.00 net By R. T. Young

A comprehensive consideration of the progress of biology in this country.

LIFE AND DEATH, HEREDITY AND

EVOLUTION IN UNICELLULAR OR-
GANISMS

Illus. $2.50 net
By H. S. Jennings, Professor of Zoology in the

Johns Hopkins University. One-celled organisms presented as an opportunity for solving some of the deeper problems of life. CHEMISTRY AND CIVILIZATION

Illus. $2.50 net By Allerton S. Cushman, A.M., Ph.D., Di

rector, Institute of Industrial Research, Wash

ington, D. C. A general view of all that chemistry has done, is doing, and hopes to accomplish for mankind in the future. MARS

Illus. $2.50 net By William H. Pickering, Harvard College Ob

servatory, Jamaica. The results of a lifelong study of the planet Mars—its snows, its seas, its marshes, and its canals.

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