Development and Improvement of Scientific Instruments and Equipment go hand in hand with the development of science, and their progress is mutually beneficial Among the many new instruments developed by W. M. Welch Scientific Company may be mentioned the following: UNIVERSAL MOTOR ROTATOR for Direct Current or Alternating Current. Continuous speed chang- MODEL OF THE ATOM. Design suggested by Prof. gested by Prof. Lunn, University of Chicago. 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. 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 of 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. A Sign of Quality WELCH A Mark of Service W. M. WELCH SCIENTIFIC COMPANY Manufacturers, Importers and Exporters of 1516 Orleans Street CHICAGO, ILL. L &NORTHRUP LEEDS & This calculator can H-ION CALCULATOR verts the voltage, messured at 25°C betwem a calomel electrode and a hydrogen the wlution under test is based upon the relation patial difference between the calomel cell used in the me and hydrogen electrode in a normal solution af H The values of f atural normal and teach-normal electrodes are marked on the wal This calculator is currect for a temperature of 25°C. A compion of formula and its application appears in a colo 009 N 店 350 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 Partially covering cost of manufacture. 50 cents Have you a copy of our Catalogue S75, "Electrometric Methods and Apparatus for Determining Hydrogen Ion Concentrations"? We will be glad to send you one. Leeds &Northrup Company Electrical Measuring Instruments 4901 Stenton Ave. Philadelphia, Pa. The Professor will be required to offer a course of Lectures and Practical Work in Elementary Animal Biology, suitable for students of Science, Medicine and Agriculture, together with an advanced course in Zoology for students in Science. A demonstrator and a collector will be provided. Salary, £1,100 per annum. Applications with testimonials, statement of academic qualifications, experience, copies of publications, and a recent photograph, should be forwarded to the Registrar, The University of Ade ZOOLOGIST, M.S., in Bacteriology, Entomology, Embryology, 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 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. Candidates must indicate the earliest date at which they can commence duties. Further information may be obtained from : (b) The Commissioner for Australia, 61 Broad- SCIENCE PROFESSORS WANTED Applications are invited for vacant positions in Physics, Biology, and Modern Languages at Mount Allison University, Sackville, New Brunswick, Canada. Apply to B. C. BORDEN, President An Opening For An ASSISTANT PROFESSOR OF PHYSICS in a University in the South. Opportunity for growth for a high type, trained man with teaching experience. Address S, care of SCIENCE, Garrison, N. Y. UNIVERSITY OF ALBERTA, CANADA Junior Demonstrator required for Department of Physics. Demonstrator expected to engage in research. Beginning salary $1,200 per annum. Duties to commence October 1st. Allowance for passage. Apply with recommendations to Professor Boyle. 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. 66 A book which it is an inspiration to read and a duty to recommend. It is the sort of book which carries not 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 A volume by ten expert entomologists 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 A comprehensive consideration of the progress of biology in this country. LIFE AND DEATH, HEREDITY AND 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, Washington, D. C. A general view of all that chemistry has done, is doing, and hopes to accomplish for mankind in the future. |