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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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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 research 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.

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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 labora 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.

General Electric

General Office
Schenectady, NY.

Company

Sales Offices in
all large cities 35B-48

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