MORE THAN ONE HUNDRED

scientific laboratories are using the Type K Potentiometer method for measuring hydrogen-ion concentrations. We have yet to learn of a single case where, with proper manipulation, a Type K Outfit has not unfailingly given as highly accurate results as the experimental conditions permitted.

IF YOU WANT FURTHER PROOF

IF

of the excellence of the Type K Potentiometer as applied to hydrogen-ion measurements, you will find them in Catalogue S75, which also contains much valuable information about such measurements.

Leeds & Northrup Company

Electrical Measuring Instruments

4901 Stenton Ave.

Philadelphia, Pa.

FOR SALE

Scientist retiring from active work will sell The Microscope

the following apparatus, which would form an excellent adjunct to any institution of learning: One complete X-ray outfit, adaptable to

By SIMON H. GAGE of Cornell University 13th Edition, Published December, 1920

any current, with fluoroscopes. One Demon- in this edition, special emphasis is put upon the Dark-Field Microscope stration Apparatus, with Polariscope, Spectro

scope, Pencil Apparatus, Lenses, and every COMSTOCK
attachment on optical bench all complete,
including a large collection of rare demonstra-
tion specimens. One Projection Microscope,
with lenses. One Projection Lantern, with
slides and lenses. One complete Moving Pic-
ture Machine, and other apparatus, in excellent
condition. To be sold either in one lot or
separate, at low price.

Must be seen to be appreciated.

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

Professor in a Western College desires exchange professorship with Zoologist in university or college in Eastern States. Address

Apply R, care of Science, Garrison, N. Y. | P., Science, Garrison, N. Y.

New Books That Have a Message For You

Hygiene

By Clair E. Turner, Asst. Prof. of Biology and
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Myers presents a discussion of the chemical blood determinations which have been found of definite value in the treatment and diagnosis of disease. Complete literature references,

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169. Callaway, Morgan. The Infinitive in Anglo-Saxon. A Textbook for Colleges and

The simple explanations of biological CARNEGIE INSTITUTION OF WASHINGTON facts and the pleasing style in which the