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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+ values. The conversion may be made accurately to the second decimal place.

The Calculator is very quick and convenient in operation. It is made of heavy celluloid, and measures 6 inches by 6 inches.

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SCIENCE

WANTED_Position as Professor of Biology in college

of moderate size. Applicant holds the degrees A Weekly Journal devoted to the Advancement of

M.S., and Ph.D:, in Entomology and Botany, and Science, publishing the official notices and pro

has experience in teaching and museum work. ceedings of the American Association for the Advancement of Science

Address “L,” care of Science Published every Friday by

PLANT BREEDER. (Ph.D., Asst. Professor) will THE SCIENCE PRESS consider position here or abroad requiring teachLANCASTER, PA. GARRISON, N. Y. ing Genetics, Crop Improvement and Agronomy, NEW YORK, N. Y.

with part-time research Entered in the post-office at Lancaster, Pa as second-class

Address X Y, care of Science. matter, under the act of March 3, 1879

NOW READY

The Physics of the Air

BOTANIST AND PLANT PATHOLOGIST, with three

years' teaching and research experience and with all of his residence requirements for Ph.D. completed, will be available next September. Initial salary or place of employment immaterial, but positions with a future and associates with a vision are essential.

Address “G," care of Science, Garrison, N. Y.

By WILLIAM J. HUMPHREYS, Ph.D.
Professor of Meteorological Physia, United States

Weather Bureau
A rational and deductive treatise on the physics of
the atmosphere. Of great value to meteorologists,
physicists and the aviator.
678 Pages, Illustrated, Index, 8vo. Price, $5

Address orders and inquiries to
THE FRANKLIN INSTITUTE

PHILADELPHIA, U. S. A.

ZOOLOGIST, M.S., in Bacteriology, Entomology, Embryology, Histology; A.M., in Cytology, Protozoology, Neurology: (Dissertation for Ph. D. on Invertebrate Morphology, nearly complete ), has studied abroad, seeks post as Professor or Head of Department, with part time for research. Eight years teaching experience, (in Zoology, Comparative Anatomy, Histology, Embryology and Biology). Address “K” care of Science, Garrison, N. Y.

ANATOMIST wants position. Eminently prepared to
teach all branches pertaining to medicine (histology,
embryology, neurology, gross anatomy and cross sec-
tions). Present salary $2400. Will not change for
less than $3000. Best of recommendations.
Address "Q" Care of Science, Garrison, N. Y.

THE COCCIDAE
Tables for the Identification of the Subfamilies
and Some of the More Important Genera
and Species Togethor with Discussions
of Their Anatomy and Life History

By
ALEX. D. MacGILLIVRAY Ph.D.
Prof. of Systematic Entomology in the University of Nlinois

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

8vo. Cloth. IX+502. $6.00 net Sent prepaid upon receipt of price. Onder direct from publisher

SCARAB COMPANY 603 Michigan Ave.

Urbana, Illinois

WANTED, Instructor in Mathematics and Astronomy

to take the place of a professor on leave of absence for the college year 1921-22. Salary $2,000. Instructor will have charge of new Warner & Swasey Observatory and will be expected to teach practical astronomy as well as some classes in mathematics. President, CASE SCHOOL OF APPLIED SCIENCE

Cloveland, Ohio

Cameras

Lenses
for Naturalists

WANTED, in an Eastern college, a young man as Assistant in Zoology (B.S. or M. S.). Must have a fair knowledge of vertebrate morphology. State qualifications. Salary $1500.

Addross U, Care of Scienno, Garrison, N.Y.

Graflex, Kodak, Foreign Cameras, Zeiss,
Goerz, Cooke, and all other high grade Lenses
and Motion Picture Cameras. Save 25% to 50%.
Get Our Price Write at once for Com.

plete Catalog and month. ly Bargain List. Experts on equipping expeditions for still or Motion Picture work. Write today to Graflex Headquarters of America.

The Microscope

BASS CAMERA COMPANY

By SIMON H. GAGE of Cornell University

13th Edition, Published December, 1920
in this edition, special emphasis is put upon the Dark-Field Microscope

Postpaid $3.00
COMSTOCK PUBLISHING CO., Ithaca, N. Y.

109 E. Dearbora St.

DBP. IS3

Chicago, nu.

THIRD EDITION-NOW READY

AMERICAN MEN OF SCIENCE

A BIOGRAPHICAL DIRECTORY

EDITED BY J. McKEEN CATTELL AND DEAN R. BRIMHALL

The third edition of the Directory contains about 9,600 sketches as compared with 4,000 in the first edition and 5,500 in the second edition. The work should be in the hands of all those who are directly or indirectly interested in scientific work.

(1) Men of Science will find it indispensable. It gives not only the names, addresses, scientific records and the like of their fellow workers, but also an invaluable summary of the re, search work of the country, completed and in progress.

(2) Those interested in science, even though they may not be professionally engaged in research work, will find much of interest and value to them in the book.

(3) Executives in institutions of learning and others brought into relations with scientific men will use the book constantly.

(4) Editors of Newspapers and periodicals will find it to be one of the works of reference that they will need most frequently. (5) Libraries will find the book to be a necessary addition to their reference shelves.

Price, Teo Dollars, Net, Postage Paid

THE SCIENCE PRESS GARRISON, N. Y.

LANCASTER, PA.

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A. D. L. FILTER PAPER

After two years of careful research and experimentation, Arthur D. Little, Inc., announce the contribution to the list of American-made chemical products of

A. D. L. Quantitative Filter Paper

A. D. L. PAPER IS MADE WITH DISTILLED WATER, is doubleacid washed, and is subjected to a system of rigid inspection and testing which insures a high degree of uniformity. It is made by chemists for chemists, to meet the most exacting demands of accurate analysis.

A. D. L. PAPER RETAINS BARIUM SULFATE and other fine precipitates without sacrificing the advantages of rapid filtration.

A. D. L. PAPER HAS AN EXCEPTIONALLY LOW ASH CONTENT. It is strong and easily folded.

A standardized series of tests, developed in our own laborao tories and under the control of which our manufacture is conducted, show A. D. L. Quantitative Filter Paper to be equal, or superior, in the more important physical characteristics to the best imported filter papers. Correspondence with chemists as to the performance of our filter paper or its suitability for specific purposes is solicited.

The underlying motive in this undertaking has been to utilize our intimate professional experience in the scientific development of paper making in the United States by adding a new and important item to the production of American chemical industry. The distribution of the A. D. L. Quantitative Filter Paper has, therefore, been placed in the hands of reliable dealers throughout the United States, to whom orders, requests for samples, and prices should be addressed.

Made at 30 Charles River Road, Cambridge 39. Mass., U.S.A.

in the laboratories of

Arthur D. Little, Inc.

Chemists :: Engineers :: Managers

Repeated object lessons have demonstrated that nearly all progress in science has resulted in important advances in industry

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G-E Research Laboratory

Schenectady. N. Y.

Among the many products developed by the General
Electric Company's research laboratories the following
are of special interest to manufacturers:

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For further information address Supply Department, Schenectady Office.

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General Electric

General Office
Schenectady. N.Y.

Company

Sales Offices in all large cities

35B-48

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