From Current Literature

Structure of Matter

The influence of a magnetic field on the internal friction of oxygen. In investigating the thermal conductivity of paramagnetic gases O₂, NO, Zeeman (Phys. Zs. 31, 832, 1930; 32, 460, 1931; 33, 117, 1932) found that, when a magnetic field is applied to them, directed perpendicular to the heat flow, the coefficient of thermal conductivity decreases. If one makes the most natural assumption, that this effect is due to the influence of the magnetic field on the mean free path, then one should expect that there must also be an analogous change in the internal friction in a magnetic field. The experiments of Engelhardt and Sack have indeed revealed this effect. It turned out that in O₂ at a pressure of 110 mm Hg a decrease in viscosity is observed, so that at 2000 gauss the quantity \(-\Delta\eta/\eta\) reaches \(5\cdot10^{-3}\). In air the same effect is considerably smaller (at \(p = 112\) mm Hg and at 2000 gauss, \(-\Delta\eta/\eta\ 10^{-3}\)); in nitrogen it is not observed at all. (H. Engelhardt und H. Sack, Phys. Zs. 33, 724–727, 1933.)

Determination of molecular weight in the gaseous state at very low pressures. For the usual determination of molecular weight (m. w.) it is necessary to know the weight \(g\), pressure \(p\), volume \(v\), and temperature of the evaporating substance. The lower the pressure, the more difficult it is to make the measurements. Nevertheless, for a number of substances it is required to determine the m. w. precisely at low pressures; such substances include, for example, polymeric substances which, in the temperature interval of their stable state, possess a low vapor elasticity. For the direct determination of m. w. in such cases, Föhlmer proposed and developed, together with collaborators Fölmer and Hofmann, the following extremely elegant method, which evidently may also have wider application. A small box made of thin-walled sheet metal (see Fig. 1) is filled with the substance under investigation. The box has at the top a tube for introducing the substance and two openings situated on the front and rear walls. After the substance has been placed inside the box, the tube is closed, and the box is suspended on a fine thread in a pumped-out, high-vacuum vessel. Evaporating molecules, flying out of the box, will produce a recoil, as a result of which a couple of forces will appear and the box will rotate through a certain angle. By twisting the thread through an angle \(\alpha\), the box can be brought back to its original position.

The molecular weight in such a case may be found from the formula:

\[ M=\frac{\pi R T g^{2} a^{2}}{8 k \alpha^{2}}, \]

where \(g\) is the weight of the evaporating substance, \(a\) is the arm of the couple of forces (i.e., the distance between the straight parallel walls of the box through which...

through the centers of the holes), \(\alpha\) is the angle of twist, and \(k\) is the directing force of the thread (the moment of the couple \(=k\alpha\)). All quantities entering into the formula are comparatively easily accessible to measurement: \(k\) is determined from the period of torsional oscillations of a body of known moment of inertia suspended on the thread; \(\alpha\) is found by direct reading; \(a\) is measured with a comparator; and, finally, for measuring \(\theta\), microbalances are built into Fohlmer’s apparatus itself.

To test the apparatus, measurements were made of the molecular weights of substances with already known molecular weights. Thus, for example, for benzophenone (\(M=182\)) the values 194, 174, 186.5, 176 were obtained; for azophenone (\(M=182\)) the values 178, 182, 183. After testing, the apparatus was used to find a number of unknown molecular weights. (M. Volmer, Z. Physikal. Ch. Bodenstein-Festband, 863–873, 1931.)

Rotation of Molecules in Solid Hydrogen Chloride.

Recently, data have been accumulating which testify to the possibility of rotation of molecules in solids. Rotation of molecules in liquids had already been discovered earlier by investigation of the Raman effect. The possibility of rotation of molecules in solids was pointed out by Bonhoeffer and Harteck, who, in particular, indicated that the high dielectric constant of ice near its melting temperature indicates the possibility of orientation of the dipole axes of molecules of solid ice. Pauling (Phys. Rev., 36, 430, 1930) proposed the same hypothesis of rotation of molecules in solids for the explanation of certain transition points in crystals. He assumes that at low temperatures the molecules in crystals execute oscillatory motion about certain positions of equilibrium, whereas at higher temperatures the store of kinetic energy of the molecules is sufficient for them to perform more or less uniform rotation. Direct convincing proof of the rotation of molecules in solids was recently given by Gettner through investigation of the infrared absorption band in solid HCl, lying in the interval 3.5–3.8 \(\mu\). It turned out that this band, lying approximately at the same place as the rotational-vibrational band of gaseous HCl, is double at a temperature of \(87^\circ\) abs., whereas at \(20^\circ\) abs. its long-wave part disappears completely. Discussion of the results obtained leads Gettner to the conclusion that the band observed in solid HCl can be interpreted, just as for gaseous HCl, as a rotational-vibrational band. Hence it follows that in the solid state rotation of molecules, though strongly hindered, is possible. It should be noted that it follows from Gettner’s experiments that the solid crystal forms an atomic, and not an ionic, lattice. As is known, from experiments with optical dissociation it follows that in the gaseous state too the HCl molecules are atomic, not ionic, as one could only naturally have expected. (C. Gettner, Z. Physik, 78, 141–155, 1932.)

Fig. 1.

Disintegration of Lithium by Fast-Moving Protons.

The experiments of Cockcroft and Walton have been repeated in America by Lawrence, Livingston, and White, who used, to produce a stream of fast protons, the jet method of Lawrence and Livingston (see the article by Mysovskii, “Advances in the Physical Sciences,” issue 5, 1932). Crystals of lithium fluoride were bombarded with protons having velocities of 360,000, 510,000, and 710,000 V. The particles liberated were counted with a Geiger counter.

tip with a point. The observations confirmed not only the very fact of the disintegration of lithium, but also the order of magnitude of the number of particles obtained proved to be the same as in the experiments of Cockcroft and Walton. At the authors’ request, Oppenheimer calculated the probabilities of liberation of $\alpha$-particles from the lithium nucleus on the basis of Gamow’s theory and compared the values obtained with the authors’ experimental results. According to Oppenheimer’s calculation, at $500\,000$ V there should be liberated from 1 to 10 $\alpha$-particles per $10^7$ incident protons; from the experimental results the following figures are obtained: 1.0 at 360 kV, 2.6 at 510 kV, and 5.2 at 710 kV. Thus the agreement is excellent. (E. O. Lawrence, M. S. Livingston, M. C. White, Phys. Rev., 42, 151, 1932.)

Method of obtaining fast canal particles and its application to the artificial disintegration of atoms. To obtain fast canal particles without the aid of ultrahigh voltages, Ch. Gerthsen proposed the following method. Canal particles which have passed

Fig. 2.

through the drilled cathode $K$ (Fig. 2) are additionally accelerated in the field $E_1$ and enter the grounded gas chamber $U_{\mathrm{I}}$ (hydrogen at a pressure of several hundredths of mm Hg). In this chamber the process of recharging of the particles takes place, and the neutral particles thus obtained prove capable, despite the retarding field $E_2$, of passing from $U_{\mathrm{I}}$ into the second gas chamber $U_{\mathrm{II}}$, where the process of recharging of the particles again takes place. The new canal particles thereby produced are again accelerated by the field $E_3$, so that the fastest of them, entering the grounded chamber $B$, already have twice the velocity. By attaching to the apparatus still more such chambers for recharging, one can further increase the energy of the particles. The magnet after $M$ makes it possible to resolve these particles into a velocity spectrum and to obtain monochromatic proton beams. In this way the author obtained monochromatic proton beams with an intensity of $10^{-8}$ A.

With the aid of this method, the experiments on the disintegration of lithium by proton beams of a definite constant velocity were repeated. A curious result obtained by the author is that the disintegration of lithium is observed at velocities considerably below 120 kV, at which Cockcroft and Walton began their experiments. The author was able to observe the disintegration of atoms already at 70 kV, with the number of particles increasing sharply between 90 and 100 kV. (Chr. Gerthsen, Naturwiss., H. 40, 743, 30 Sept. 1932.)

E. Shpolsky