The velocity and absorption of ultrasound in dry CO2 have been measured along the 50.8 ø isotherm with a Hubbard-type, variable path, recording acoustic interferometer at six frequencies from 300 kc to 7 mc and from 0.3 to 250 atmos, i.e., up to liquid densities. Velocity dispersion is clearly shown in addition to the change in velocity attributable to the nonideality of the gas. The values of the frequency/density ratio at which the transition from V02 to V• 2 is half-completed as the pressure is lowered, and at which the maximum absorption per wavelength occurs, were measured wherever possible. From these determinations the relaxation time of the gas has been determined as a function of the density. It proves to be inversely proportional to the density up to the highest density reached (0.8 g/ml), indicating that ternary collisions have not become important and that the number of binary collisions required to excite the internal vibrations does not vary with density. At 50.8 ø and one atmosphere the relaxation frequency 1/2,rr is about 26 kc and the number of collisions required to excite the molecule is 48 500. The extra absorption (in excess of the classical) remains a consequence of thermal relaxation up to liquid densities, and no new mechanism need be postulated to explain it. Further work at higher densities and other temperatures is in progress.
The pressurized Kundt's tube developed in this laboratory has been used to measure the napier (relaxation) frequency of nitrogen in both binary and ternary mixtures with carbon dioxide and water vapor. Carried out at about 175°C, where the relaxing specific heat of nitrogen becomes readily measurable, the separate contaminants shift the napier frequency upward by amounts proprotional to their respective mole fractions: water vapor by 310 Hz/mole-percent, carbon dioxide by 300 Hz/mole-percent. When both are present, the shift is notably greater and is not linear in the mole fraction. This synergistic effect is explicable on theoretical grounds. Preliminary results with heavy water vapor (D2O) as impurity are also given. Its effect is also nonlinear in the mixed impurities.
Although the problem of transmuting chemical elements into each other is much older than a satisfactory definition of the very concept of chemical element, it is well known that the first and most important step towards its solution was made only nineteen years ago by the late Lord Rutherford, who started the method of the nuclear bombardments. He showed on a few examples that, when the nucleus of a light element is struck by a fast α-particle, some disintegration process of the struck nucleus occurs, as a consequence of which the α-particle remains captured inside the nucleus and a different particle, in many cases a proton, is emitted in its place. What remains at the end of the process is a nucleus different from the original one; different in general both in electric charge and in atomic weight. The nucleus that remains as disintegration product coincides sometimes with one of the stable nuclei, known from the isotopic analysis; very often, however, this is not the case. The product nucleus is then different from all "natural" nuclei; the reason being that the product nucleus is not stable. It disintegrates further, with a mean life characteristic of the nucleus, by emission of an electric charge (positive or negative), until it finally reaches a stable form. The emission of electrons that follows with a lag in time the first practically instantaneous disintegration, is the so-called artificial radioactivity, and was discovered by Joliot and Irene Curie at the end of the year 1933. These authors obtained the first cases of artificial radioactivity by bombarding boron, magnesium, and aluminium with α-particles from a polonium source. They produced thus three radioactive isotopes of nitrogen, silicon and phosphorus, and succeeded also in separating chemically the activity from the bulk of the unmodified atoms of the bombarded substance. The neutron bombardment Immediately after these discoveries, it appeared that α-particles very likely did not represent the only type of bombarding projectiles for producing
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