CONTENTS Page PHYSICS. THE ATTACHMENT OF ELECTRONS to Neutral MoleculeS IN AIR. By F. G. Benedict, M. F. Hendry and M. L. Baker 10 PHYSICS. ON THE REFLECTION AND RE-EMISSION OF ELECTRONS FROM METAL SURFACES: AND A METHOD OF MEASURING THE IONIZING POTENTIAL OF By R. A. Millikan and I. G. Barber 13 ANTHROPOLOGY.-CHRONOLOgy of the SaN JUAN AREA . . By E. H. Morris 18 GEOLOGY.-Some Geologic Conclusions from Geodetic DATA..By W. Bowie 23 CHEMISTRY.-EXPERIMENTS ON THE ELECTRICAL CONDUCTION OF A HYDROGEN The HOME SECRETARY and the FOREIGN SECRETARY of the ACADEMY The CHAIRMAN and the PERMANENT SECRETARY of the NATIONAL RESEARCH COUNCIL WILLIAM DUANE, '23 A. L. DAY, '22 J. M. CLARKE, '21 F L. RANSOME INFORMATION TO SUBSCRIBERS SUBSCRIPTIONS TO THE PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES at the rate of $5.00 per annum should be made payable to the National Academy of Sciences, and sent either to Easton, Pa., or to C. G. 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The following publications are issued by the NATIONAL RESEARCH COUNCIL. Orders and inquiries should be addressed: Publication Office, National Research Council, 1701 Massachusetts, Ave.. Washington, D. C. (A) THE BULLETIN OF THE NATIONAL RESEARCH COUNCIL published at irregular intervals: Price $5.00 per volume of about 500 pages; individual numbers priced variously (B) The Reprint AND CIRCULAR SERIES OF THE NATIONAL RESEARCH COUNCIL, individual numbers, variously priced. POSITIVE RAY ANALYSIS OF MAGNESIUM BY ARTHUR J. DEMPSTER RYERSON PHYSICAL LABORATORY, UNIVERSITY OF CHICAGO Communicated by A. A. Michelson, Dec. 7, 1920 In the Physical Review for April, 1918, I described an apparatus for positive ray analysis and gave examples of hydrogen, oxygen, sodium and potassium rays, showing that at least the lighter elements could be readily analyzed so as to separate any molecules differing in molecular weight by unity. I have recently resumed the experiments and will give in this paper an account of experiments with magnesium. As stated in the Physical Review article, the experimental difficulties are largely in obtaining a steady source of the rays desired. Magnesium rays have been obtained from a piece of the metal which was heated electrically by a coil of wire, and at the same time bombarded by electrons from a Wehnelt cathode. The occluded gases are first driven off, and then the heating current is increased till the magnesium lines appear due to the metal vaporizing slightly. The positively charged molecules formed pass through a hole in a plate below the cathode and are then accelerated by a strong variable field of several hundred volts. The plate has been added to the apparatus described in the Physical Review to prevent the strong electrical field influencing or even inhibiting the low voltage discharge. The first slit, about 1 mm. wide, separates out a bundle of rays which is bent into a semicircle by a strong magnetic field and refocussed, if their speed is right, on the second slit, below the detecting electrode. The charge carried by the rays, which is proportional to the number of the molecules of different kinds, is measured by a Wilson electroscope used as a null instrument with a special compensating device for rapid measurements. The charged atoms of different atomic weights are successively brought on to the detecting electrode by keeping the magnetic field constant and varying the potential which accelerates the rays, the potential required being inversely proportional to the mass of the particles. Thus, if one atomic weight is known the others may be found. Due to the finite width of the slits, each element gives a curve, on the atomic weight scale, which 45 is theoretically a linear increase to a maximum and then a linear decrease. 2S The width half way to the maximum is given by m. where m is the d atomic weight, S the slit width and d the diameter of the circle in which the rays travel. Under good vacuum conditions this theoretical sharpness is practically obtained. For 1 mm. slits this width of the curves should thus be one-half a unit on the atomic weight scale. The former measurement with the apparatus and the magnetic field determinations sufficed to locate elements between 20 and 30 within one unit, and identified the strong nitrogen rays (possible carbon monoxide) of molecular weight 28 which are given off when the metal is first heated. One series of experiments was as follows: After heating the magnesium slightly and pumping, till a MacLeod gauge gave no pressure indication, the nitrogen molecule was the only particle present. The heating current was then increased by steps to vaporize the magnesium. With 0.7 ampere, 28 alone was present, with 0.75 ampere an arc apparently struck as the cathode-anode current jumped suddenly to five times its value. The electron current was decreased to its former value by cooling the cathode and the rays were measured. It was found that three strong new lines had appeared. The new lines which are undoubtedly due to magnesium were compared with the nitrogen rays which were still faintly present and found to have atomic weights 24, 25 and 26. The observations are illustrated in figure 1, which gives the current or number of particles for different atomic weights. The nitrogen line had its maximum at 817 volts, and the atomic weight abscissae are 28 X 817 divided by the volts applied. The ordinates of the 28 line are multiplied by 10 in plotting to make them comparable with the other three lines. The dotted continuation to the axis indicates the slight overlapping of the lines. We conclude that magnesium consists of three isotopes of atomic weights 24, 25 and 26. Later curves made with steadier discharge conditions are more suitable |