INDUSTRIAL AND ENGINEERING CHEMISTRY
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(36) Kopac, M. J., AnaL Record, 70 (Suppl. 1), 71 (1937). (37) Kopac, M. J., Biol. Bull., 75,351 (1938). {Continued from page J5) (38) Ibid., 75, 372 (1938). (28) Hecht, S., and Pickels, E. G., Proc. (39) Kopac, M. J., and Chambers, R.( Ibid., 75. 372 (1938). Natl. Acad. Sci. U. S., 24.172 (1938). (29) Holter, H.( and Doyle, W. L., J. Cellu- (40) Langmuir, I., and Waugh, D. F., J. Gen. Physiol., 21, 745 (1938). lar Comp. Physiol., 12, 295 (1938). (30) Holtfreter, J., Arch. Entwicktungsmech. (41) Lillie, F. R., and Juhn, M.. Physiol. Zool., 11, 434 (1938). Organ., 138, 163 (1938). (42) McClung, C. £., Proc. Natl. Acad. Sci. (31) H6rstadius..S.f I&u*., 138,197(1938). U. S., 24, 439 (1938). (32) Jennings, H. S.f Proc Nad. Acad. Sri. (43) Marks, G. W.t Biol. Bull., 75, 224 U. S., 24, 117 (1938). (1938). (33) Johnson, F. H., J. Cellular Comp. (44) Melland, A. M.. Ibid., 75. 348 (1938). Physiol.. 12, 281 (1938). (45) Needham, J., Boell, E. J., and Rogers, (34) Johnson, F. H., and Anderson, R. S., V., Nature, 141, 973 (1938). Ibid.. 12, 273 (1938). (35) Kimball, R. F., Proc. Nail. Acad. Sri. (46) Rudnick, D., J. ExpU. Zool., 79, 399 (1938). U. S., 23, 469 (1937).
Zoology
Physics Karl K. Darrow, 230 West 105th S t . , New York, N. Y. EDITOR'S NOTE:
We
are
deeply
in-
debted to Mr. Darrow for the following paragraphs on physics which he kindly prepared for us, almost at a moment 8 notice, when the review for 1938 on this subject which we had been counting upon failed to materialize. HESE paragraphs are in no sense a review of the progress of physics during T 1938; they consist merely of allusions to
a few advances in subatomic or nuclear physics which of late have merited the attention of physicists and possibly that of chemists also. The particle variously named "heavy electron/' "barytron," "Yukawa particle," and (preferably) "mesotron" is steadily winning its way to acceptance. A constituent of the cosmic rays, it has not as yet been produced by any artifice of the laboratory, and whatever is known or conjectured about it is a portion of the science of cosmic rays. The enormous energies with which cosmic-ray particles are endowed—energies ranging upward to the billions of electron volts—transcend by several orders of magnitude the greatest energies which can be imparted to recognized electrons or protons in the laboratory. This means that we cannot compare a cosmic-ray particle with a known electron of equal energy and thereby settle the question of identity vs. difference. The question poses itself in the form: Do cosmic-ray particles behave as theory predicts that electrons of such great energy should behave, or do they not? When the question is posed in this form, it is seen to be an interesting one per se; but one may feel that it renders meaningless the original question "is there a heavy electron?" For, suppose that the cosmicray particles were found to depart from the predictions of theory: This might be taken as meaning that they are particles of a different nature from electrons, but it might also be taken as meaning that they are true electrons and that the theory is wrong. The answer to the question does not, however, suffer from this ambiguity; for it is found that there are two categories of particles, one of them behaving (roughly, at least) in concordance with the theory, the other departing from it definitely. The category of particles classified as true electrons is distinguished by the feature that they produce "cosmic-ray showers." These, the most striking and beautiful of all the cosmic-ray phenomena, are composed of quantities of electrons and of photons of great energy, bursting out of
pieces of dense matter. The initiating electron of colossal energy is believed to radiate its energy in the form of photons; as it cleaves through the dense matter, these photons convert themselves into pairs of electrons of opposite sign, themselves endowed with large kinetic energies; the electrons lose their kinetic energies in the same manner, and photons are created along their courses and also where positive electrons merge with negatives; the processes alternate, and showers are built up in cumulative fashion. These are processes which are attributed by theory to electrons, and accordingly the showerproducing particles and the charged particles of the showers are identified with electrons. The category of particles classified as mesotrons is distinguished by the negative feature that they do not produce showers, and by the positive feature that they penetrate many centimeters, even a meter or more, of materials as dense as lead; theory says definitely that no electron of even the cosmic-ray energies could get through as much as 10 centimeters of lead. They may also be distinguished from electrons by the ionization along their paths through air or other gases in the expansion chamber of Wilson. For making the distinction, a strong magnetic field must be applied to the chamber, curving the tracks of the particles which fly through. In a field of known strength the ionization density appropriate to electrons is a known function of the curvature of the track. Should one find a track of which the ionization density was definitely several times too high to agree with this function, one would identify it as the track of a mesotron (unless there should be reason to fear that the curvature had been distorted, a possibility which must be carefully examined). A dozen or so such tracks have been reported. If they have been correctly interpreted, the mass of the mesotron is a couple of hundred times as great as that of the electron, its charge the same in magnitude, and either positive or negative in sign. The names to be mentioned in the history of the mesotron are those of C. D. Anderson and S. Neddermeyer, in the first line; then, those of Street, Stevenson, Blackett, Auger, Ehrenfest, and several others. An ingenious method for determining the magnetic moments of nuclei has been invented in the past few months by Rabi, and applied already to half-a-dozen nuclear types. The molecules containing the nuclei (the electron systems of these molecules are so constituted that their magnetic
VOL. 17, NO. 1
(47) Slifer, E. H., Quart. J. Micro. Sci., SO, 437 (1938). (48) Sonneborn, T. M., Proc. Natl. Acad. Sou, 23, 378 (1937). (49) Svedberg, T., and Andersson. EL, Nature, 142, 147 (1938). (50) Tyler, A., and Horowits, N. H., BioL Bull., 75, 209 (1938). (51) Whitaker, D. M., Anat. Record, 70 (Suppl. 1), 70 (1937). (52) Whitaker, D. M., Proc. Natl. Acad. Sci. U. S., 24. 85 (1938). (53) Willier, B. H., and Rawlea, M. E., Anat. Record, 70 (Suppl. 3), 81 (1938). (54) Willier, B. H., and Rawlea, M. E., Bid. Bull., 75, 340 (1938). (55) Willier, B. H., and Rawlea, M. E., Proc. Natl. Acad. Sci. U. 8., 24. 446 (1938).
moments vanish) pass through an inhomogeneous magnetic field. The nuclei distribute themselves among their two (or more) permitted orientations in this field, and those which share a particular orientation are deflected by the field along a certain path. Midway in this path is interposed a small region over which an oscillating magnetic field is applied to the flying molecules. There is a critical frequency of oscillation, for which this field is able to impel the nuclei to change over into another of their permitted orientations. When this happens to any nucleus the deflecting force upon the molecule is changed, the molecule no longer continues along its original path, and a measuring device placed in the prolongation of the original path reports a minimum in the strength of the arriving beam. The critical frequency depends upon the magnetic moment of the nucleus, which accordingly is computed from the data. Values are now available for the magnetic moments of the nuclei of hydrogen, deuterium, sodium, cesium, and the isotope 39 of potassium. While the numerous great cyclotrons lately completed or in process of building attract most attention, the electrostatic generator is also proving its merits for the production of high-energy protons, deuterons, and alpha-particles for use in transmutation. The earliest generators stood in large halls or in the open air, but the newer ones are smaller in scale and are enclosed in tanks containing air mixed with freon and raised to a pressure of several atmospheres. Herb and his colleagues, to take one example, have worked at voltages up to 2,500,000 with proton streams forming currents of the order of a milliampere. With this they have explored the fields in the immediate vicinity of protons, by projecting them against other protons and making a statistical study of their deflections. They have also studied the excitation of gamma-rays—i. e., of highfrequency light, occurring when protons are projected against fluorine nuclei: the curves of radiated energy vs. energy of incident protons consist ofpeaks wonderfully sharp and narrow, speaking strongly for the controllability of the apparatus itself as well as for the sharpness of these "resonances" at which the radiation process occurs. To analyze the emitted radiation is a task of another sort, requiring that the rays be sent through expansion chambers: in these are seen the tracks of electrons ejected from the atoms by the photons, and also the tracks of electron pairs created by the photons. The spectrum of the gamma-rays must be deduced from the energy distribution of these electrons and these electron pairs, a difficult operation which has been carried out chiefly in California by Lauritsen, Crane, Fowler, J. R. Richardson, and others.
