The freezing point of platinum

The International Temperature Scale, which has been in force since 1927, is based on certain values assigned to the boiling and freezing points of pure substances and on specified means of interpolation between, or extrapolation beyond, these points. The highest basic point of the scale is the freezing point of gold, defined as 1063·0° C, while for extrapolation from this temperature use is made of the Wien law of radiation, with a certain value of the constant C 2 . Though any temperature above 1063° C is thus completely defined without reference to further fixed points, determinations of such points are of considerable value. In particular, they serve to indicate the degree of reproducibility of the scale by the various users of it, and, when well authenticated, to provide secondary standards for its realization. Of such fixed points the most important has been the freezing point of palladium (1555° C), but the latest developments in furnace technique and refractory materials should now enable the freezing point of platinum to be used with equal, if not greater, advantage. The qualities of platinum which render it especially valuable in this connection are as follows: its freedom from oxidation; its high standard of purity, for which a convenient electrical test is available; its high freezing point (about 1775° C) which approaches the important zone of temperature covered by the electric fighting industry These qualities also make the platinum point especially suitable as the basis for a standard of fight, as has been proposed by a number of experimenters. It is with the two objects indicated above that the National Physical Laboratory has undertaken an investigation concerning the freezing point of platinum the precise scope of which may be defined as follows:— (1) To determine the value of the freezing point in terms of the International Temperature Scale.

The annihilation of fast positrons by electrons in the K-shell

The probability of the simultaneous of a positron and an electron, with the emission of two quanta of radiation, has been calculated by Dirac and several other authors. From considerations of energy and momentum it follows that an electron and positron can only annihilate one another with the emission of one quantum of radiation in the presence of a third body. An electron bound in an atom could, therefore, annihilate a positron, represented by a hole on the Dirac theory, by jumping into a state of negative energy which happens to be free, the nucleus taking up the extra momentum. The process is now mathematically analogous to the photoelectric transitions to states of negative energy in the sense that the matrix elements concerned are the same, and we might expect that the effect would be most important for the electrons in the K-shell. Fermi and Uhlenbeck have calculated the process approximately, for the condition where the kinetic energy of the positron is of the order of magnitude of the ionization energy of the K-shell. The result they obtained was very small compared with the two quantum process, which is to be explained by the fact that for these small energies, the positron does not get near the nucleus. In view of the fact that positrons of energies of the order 100 mc 2 occur in considerable quantities in the showers produced by cosmic radiation, and that the primary cosmic radiation itself may consist, in part, of positrons, it becomes of interest to calculate the cross-section for the annihilation of positrons of high energy by electrons in the K-shell, and their absorption in matter, and also to compare this process with the two quantum process for high energies. In the photoelectric effect for hard γ -rays, the electron the electron leaves the atom in states of different angular momentum (described by the azimuthal quantum number l ), and the terms which give the largest contribution are roughly those for which l is of the order of the energy of the γ -ray in terms of mc 2 . For high energies, therefore, a calculation by the method of Hulme, in which the last step is carried out numerically, is out of the question, and we must find some approximate method of effecting a summation. We shall use an adaptation of Sauter's method, in which we shall treat as small the product of the fine structure constant and the nuclear charge. This method may be expected to give a good approximation for small nuclear charge. Our method has the further restriction that it is valid only when the kinetic energy of the positron is not small compared with mc 2 .

Intensity measurements in a fine structure multiplet of As II

The present work was undertaken with the object of testing the fine structure intensity formulæ deduced by Hill. Up to the present very few intensity measurements have been made on the fine structures arising from nuclear spin. The principal difficulty in such measurements arises from the smallness of the structures which are usually incompletely resolved by the interferometers employed. The use of the interferometer in any event necessitates careful corrections for the instrumental intensity distribution. Schüler and Keyston have made photometric determinations of the intensity ratios in the fine structures of two Cdl lines and have verified the intensity rules for these lines. An inherent difficulty in the examination with a FabryPerot interferometer of Cdl structures lies in the presence of an intense evei isotope line within the pattern due to the nuclear spin of the odd isotopes The even isotope component contributes 77% of the intensity of the line and the remaining 23% is distributed amongst the members of the nuclear spin multiplet. The authors do not describe their method of coping with this difficulty which, judging from the experience of the present writers, must have been serious.

The physical basis of the biological effects of high voltage radiations

Although both the physical properties of penetrating X-rays and gamma rays and their biological effects have been carefully studied, the mechanism of the action of the rays is little known. The question of the relative effects of the same absorbed energy per cubic centimetre of tissues when different wave-lengths are used is a particularly important and obscure one. The present paper is attempt to apply recent theories of high-speed electron production to this problem. Radiations, such a high voltage X-rays or gamma rays, on suffering real absorption give rise to high speed negative electrons, either in photoelectric absorption whereby nearly the whole of the quantum is transferred to the electron, or in a Compton recoil process in which only part of the energy is transferred. The mean fraction given to the electrons rises gradually as the radiations become more penetrating. The relative importance of these two types of process varies in a complex manner with the wave-length and absorbing materials, but in this paper it is proposed to confine discussion to the absorption of “hard” radiations in light elements, of which living materials are mostly constructed.

Fluorescent radiation from N 2 O

The action of light on N 2 O was studied by Leifson and continued in recent years by Wulf and Melvin, Dutta, and the present authors from experiments on its absorption spectra. The experimental data so far known indicate that under the action of light quanta of suitable wave-length N 2 O dissociates into a normal NO and N which may be in different excited metastable states as shown below. N 2 O + hv 1 = NO + N ( 4 S) (1) N 2 O + hv 2 = NO + N ( 2 D) (2) N 2 O + hv 3 = NO + N ( 2 P) (3) These processes are inferred from the various starting points of continuous absorptions from a long wave-length limit, and with retransmitted patches of light occurring between these beginnings. The energies corresponding to hv 1 , hv 2 , hv 3 are given by the different long wave beginnings of absorption at λ 2750, λ 1850, and λ 1580; the differences of energy between of these light quanta are respectively the values of 4 S - 2 D 2 D - 2 P of N.

Spectrum of the afterglow of sulphur dioxide

Following a previous investigation of the afterglow of carbon dioxide it was decided to examine sulpur dioxide under similar conditions of experiment. An afterglow of considerable intensity and duration had, in fact, already been noted by Professors Sir J. J. and G. P. Thomson as occurring when sulphur dioxide was excited in a ring discharge, but no observations on its spectrum appear to have been recorded. Strutt has recorded an afterglow when ozone is passed over sulphur, but no glow was recorded with sulphur dioxide. The spectrum yielded by SO 2 in vacuum tubes varies greatly according to the conditions of excitation. With sufficiently powerful condensed discharges and a rather low pressure of the gas the molecules are dissociated into atoms and the spectrum consists of lines of oxygen and sulphur. With uncondensed discharges of moderate intensity and a suitable pressure of gas, the spectrum shows a strong system of bands degraded to the red which have been analysed by Henry and Wolff and attributed to the diatomic molecule SO; these bands are most intense in the region λ 2442 to λ 3941. Still weaker excitation yields an entirely different system of bands extending from the blue to about λ 2000, and there is evidence that these bands are due to undissociated molecules of SO 2 . The absorption of SO 2 is characterized by a large number of bands, which are most intense in the region λ 2800 to 3150. Owing to the continuous spectrum emitted by the gas during electrical excitation, this absorption may appear superposed on the emission spectrum in some forms of discharge tube. Recently Lotmar has reported on a band system excited in SO 2 by fluorescence.

The kinetics of the reaction between hydrogen and nitrous oxide. Part II

An analysis of the mechanism of the thermal hydrogen-nitrous oxide reaction in silica vessels by the kinetic method has shown that it is a chain process. The experiments were confined to a comparatively narrow pressure range and the evidence for chain propagation, although quite definite, required confirmation. The present paper is therefore concerned with the kinetics under a much wider variety of conditions. First, the experiments have been extended to pressures below 30 mm; second, photochemical methods have been employed to she more light on the individual steps of the reaction and to demonstrate unequivocally its chain character; third, in view of the close similarity of the hydrogen-nitrous oxide and hydrogen-oxygen reactions, a detailed study has been made of the effect of small amounts of oxygen on the former reaction. The results of these experiments all lend additional strong support to the chain hypothesis. Small alterations to the apparatus were made. A glass spring gauge was employed for measuring low pressures. One end of the furnace was provided with quartz lens in order to focus the light from the mercury lamp on the reaction bulb; the cathode of the lamp was water cooled. Arrangements were also made for inserting a hollow silica cell between the lamp and the lens so that filters could be used for controlling the intensity and wave-length of the light reaching the bulb. Direct photo dissociation of the nitrous oxide molecule was not attempted since ( a ) absorption of photochemically active light at low pressures in small bulbs is not complete, ( b ) the intensity of the lines of the mercury arc in the absorption region of nitrous oxide is weak. Recourse was therefore made to mercury sensitization in spite of a little additional complication.

Inert gas effects in the photosynthesis of hydrogen bromide

Recent studies of many chemical gas reactions which involve the production and behaviour of atoms, have shown that the effect of the surface of the reaction vessel on the atom concentration can seldom be regarded as a small disturbing factor to be allowed for by a semi-empirical correction, but must be adequately considered in relation to the other processes determining the velocity of reaction. In this connection, therefore, alteration in the pressure of any one reactant must involve an effect depending on the diffusion coefficient of the atom concerned, with regard to the expected from the mass action principles of the chemical kinetics. Examples of this simple diffusion effect are well known. The results to be expected on these lines are sometimes complicated by the existence of other factors consequent on increase in pressure in the reacting system. In the photosynthesis of hydrogen bromide, where bromine atoms are involved, it has been shown that increase in total pressure, by the introduction of an otherwise inert gas, produces a relative decrease in reaction velocity, where the application of the simple diffusion theory as above would lead one to expect an increase in velocity by prevention of the removal of bromine atoms by the walls. This decrease has been attributed to the stabilization of the bromine “quasi-molecule” by the added gas molecule, the removal of bromine atoms being thus facilitated by what is virtually a triple collision. Similar considerations apply to the photochemical formation of posgene.

The kinetics of the reaction between hydrogen and nitrous oxide III—Effect of oxygen

One of the most striking dissimilarities between the hydrogen-oxygen and hydrogen-nitrous oxide reactions is the absence in the latter of sharp explosion limits, a feature characteristic of the former. Another important difference is that propagation of chains in the H 2 -N 2 O mixtures is rather less easy than in H 2 -O 2 , for the photochemical chain length is smaller for H 2 -N 2 O than for H 2 -O 2 at the same temperatures and pressures (see below). It has, however been postulated that the carriers in the two reactions are identical and that at least one step, viz., OH+H 2 →H 2 O+H, is common to both reactions. The differences in the propagation factors would therefore be due to these reactions H+N 2 O=OH+N 2 and H+H 2 +O 2 = OH+H 2 O, or H+O 2 = HO 2 HO 2 +H 2 = OH + H 2 O. It may be anticipated that termination processes will be somewhat similar, and consequently the observed differences in the thermal reactions will also be partly due to initiation reactions.

The exchange of energy between a platinum surface and gas molecules

Recent experiments by Roberts have shown, not only that the accommodation coefficients of helium and neon atoms impinging on a clean heated tungsten surface are extraordinarily low, but also that these values increase with time after cleaning the surface. To explain this increase he suggests the gradual formation of adsorbed films on the surface of the tungsten due to residual impurities in the gas. The primary object in starting the investigations to be described in this paper was to gain some information as to the nature of these films, the existence of which has also been postulated by Blodgett and Langmuir. For this purpose experiments have been carried out in which the emission of energy from electrically heated wires of platinum, a metal relatively resistant to contamination, has been investigated under varied conditions. Apparatus A pure platinum wire about 20 cm long and 0·025 mm diameter was silver-soldered to platinum leads of 0·4 mm diameter and mounted loosely in a vertical glass tube of approximately 5 cm diameter. The wire was connected, by double leads, to a Thomson bridge, by means of which the resistance could be determined and also controlled by suitable adjustment of the current. The potential drop along the wire was measured by means of a Siemens and Halske potentiometer. The mean temperature of, and the heat developed in, the wire could thus be determined.