Showing papers by "University of Cambridge published in 1970"

Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors

Abstract: The experimental evidence concerning the density of states in amorphous semiconductors and the ranges of energy in which states are localized is reviewed; this includes d.c. and a.c. conductivity, drift mobility and optical absorption. There is evidence that for some chalcogenide semiconductors the model proposed by Cohen, Fritzsche and Ovshinsky (1969) should be modified by introducing a band of localized states, near the centre of the gap. The values of C, when the d.c. conductivity is expressed as C exp (- E/kT), are considered. The behaviour of the optical absorption coefficient near the absorption edge and its relation to exciton formation are discussed. Finally, an interpretation of some results on photoconductivity is offered.

The Singularities of gravitational collapse and cosmology

Abstract: A new theorem on space-time singularities is presented which largely incorporates and generalizes the previously known results. The theorem implies that space-time singularities are to be expected if either the universe is spatially closed or there is an ‘object’ undergoing relativistic gravitational collapse (existence of a trapped surface) or there is a point p whose past null cone encounters sufficient matter that the divergence of the null rays through p changes sign somewhere to the past of p (i. e. there is a minimum apparent solid angle, as viewed from p for small objects of given size). The theorem applies if the following four physical assumptions are made: (i) Einstein’s equations hold (with zero or negative cosmological constant), (ii) the energy density is nowhere less than minus each principal pressure nor less than minus the sum of the three principal pressures (the ‘energy condition’), (iii) there are no closed timelike curves, (iv) every timelike or null geodesic enters a region where the curvature is not specially alined with the geodesic. (This last condition would hold in any sufficiently general physically realistic model.) In common with earlier results, timelike or null geodesic incompleteness is used here as the indication of the presence of space-time singularities. No assumption concerning existence of a global Cauchy hypersurface is required for the present theorem.

Mountain belts and the new global tectonics

Abstract: Analysis of the sedimentary, volcanic, structural, and metamorphic chronology in mountain belts, and consideration of the implications of the new global tectonics (plate tectonics), strongly indicate that mountain belts are a consequence of plate evolution. It is proposed that mountain belts develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins. These assemblages result from the events associated with the rupture of continents and the expansion of oceans by lithosphere plate generation at oceanic ridges. The earliest assemblages thus developed are volcanic rocks and coarse clastic sediments deposited in fault-bounded troughs on a distending and segmenting continental crust, subsequently split apart and carried away from the ridge on essentially aseismic continental margins. As the continental margins move away from the ridge, nonvolcanic continental shelf and rise assemblages of orthoquartzite-carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites (rise) accumulate. This kind of continental margin is transformed into an orogenic belt in one of two ways. If a trench develops near, or at, the continenal margin to consume lithosphere from the oceanic side, a mountain belt (cordilleran type) grows by dominantly thermal mechanisms related to the rise of calc-alkaline and basaltic magmas. Cordilleran-type mountain belts are characterized by paired metamorphic belts (blueschist on the oceanic side and high temperature on the continental side) and divergent thrusting and synorogenic sediment transport from the high-temperature volcanic axis. If the continental margin collides with an island arc, or with another continent, a collision-type mountain belt develops by dominantly mechanical processes. Where a continent/island arc collision occurs, the resulting mountains will be small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop on the oceanic side of the arc. Where a continent/continent collision occurs, the mountains will be large (e.g., the Himalayas), and the single trench zone of plate consumption is replaced by a wide zone of deformation. Collision-type mountain belts do not have paired metamorphic belts; they are characterized by a single dominant direction of thrusting and synorogenic sediment transport, away from the site of the trench over the underthrust plate. Stratigraphic sequences of mountain belts (geosynclinal sequences) match those asciated with present-day oceans, island arcs, and continental margins.

The Contact of Two Nominally Flat Rough Surfaces

Abstract: Most models of surface contact consider the surface roughness to be on one of the contacting surfaces only. The authors give a general theory of contact between two rough plane surfaces. They show that the important results of the previous models are unaffected: in particular, the load and the area of contact remain almost proportional, independently of the detailed mechanical and geometrical properties of the asperities. Further, a single-rough-surface model can always be found which will predict the same laws as a given two-rough-surface model, although the required model may be unrealistic. It does not seem possible to deduce the asperity shape or deformation mode from the load-compliance curve.

The stress system in a suspension of force-free particles

Abstract: The purpose of the paper is to consider in general terms the properties of the bulk stress in a suspension of non-spherical particles, on which a couple (but no force) may be imposed by external means, immersed in a Newtonian fluid. The stress is sought in terms of the instantaneous particle orientations, and the problem of determining these orientations from the history of the motion is not considered. The bulk stress and bulk velocity gradient in the suspension are defined as averages over an ensemble of realizations, these averages being equal to integrals over a suitably chosen volume of ambient fluid and particles together when the suspension is statistically homogeneous. Without restriction on the type of particle or the concentration or the Reynolds number of the motion, the contribution to the bulk stress due to the presence of the particles is expressed in terms of integrals involving the stress and velocity over the surfaces of particles together with volume integrals not involving the stress. The antisymmetric part of this bulk stress is equal to half the total couple imposed on the particles per unit volume of the suspension. When the Reynolds number of the relative motion near one particle is small, a suspension of couple-free particles of constant shape is quasi-Newtonian; i.e. the dependence of the bulk stress on bulk velocity gradient is linear. Two significant features of a suspension of non-spherical particles are (1) that this linear relation is not of the Newtonian form and (2) that the effect of exerting a couple on the particles is not confined to the generation of an antisymmetrical part of the bulk stress tensor. The role of surface tension at the particle boundaries is described.In the case of a dilute suspension the contributions to the bulk stress from the various particles are independent, and the contributions arising from the bulk rate of strain and from the imposed couple are independent for each particle. Each particle acts effectively as a force doublet (i.e. equal and opposite adjoining ‘Stokeslets’) whose tensor strength determines the disturbance flow far from the particle and whose symmetrical and antisymmetrical parts are designated as a stresslet and a couplet. The couplet strength is determined wholly by the externally imposed couple on the particle; but the stresslet strength depends both on the bulk rate of strain and, for a non-spherical particle, on the rate of rotation of the particle relative to the fluid resulting from the imposed couple. The general properties of the stress system in a dilute suspension are illustrated by the specific and complete results which may be obtained for rigid ellipsoidal particles by use of the work by Jeffery (1922).

Unique Biosynthesis by Kidney of a Biologically Active Vitamin D Metabolite

Abstract: BEFORE the molecule of vitamin D acts in the control of calcium metabolism it is converted to a functional form in at least two metabolic steps. The first, a side chain hydroxylation at carbon 25, was shown by the elegant experiments of DeLuca and his co-workers1–4. This, metabolite is synthesized in the liver from vitamin D5,6 and is the major form of the circulating vitamin in blood plasma, not only of experimental rats1 and chickens7, but also of man8. A second more polar metabolite, localized in intestinal cell nuclei7,9 and having at least three times the biological activity of an equivalent amount of cholecalciferol10,11, is derived from 25-hydroxycholecalciferol (25-HCC)12. When it is obtained from chickens that have been given [4-14C,l-3H] cholecalciferol, there is a loss of tritium from carbon 1 (refs. 7 and 12). The tritium deficiency and the greater polarity are compatible with oxygen insertion at carbon 1 of 25-HCC.

The correlation of indentation experiments

Abstract: The theory of rigid perfectly-plastic solids predicts indentation pressures, using wedge-shaped or conical indenters, which depend only on the geometry of the indenter and the yield stress of the material. With blunt wedges or with materials having a low ratio of Young's modulus, E, to yield stress, Y, the material displaced by the indenter is accommodated by an approximately radial expansion of the surrounding material. The indentation pressure then falls below the rigid perfectly-plastic value. In these circumstances, measurements of indentation pressure for a variety of indenter geometries are shown to correlate with the single parameter (E/Y) tan β, where β is the angle of inclination of the indenter to the surface at the edge of the indentation. This parameter may be interpreted as the ratio of the strain imposed by the indenter to the yield strain of the material. A simplified theoretical model of this behaviour is obtained by extending R. Hill's theory of expanding a cylindrical or spherical cavity in an elastic-plastic material to ensure compatibility between the volume of material displaced by the indenter and that accommodated by elastic expansion.

Slender-body theory for particles of arbitrary cross-section in Stokes flow

Abstract: A rigid body whose length (2l) is large compared with its breadth (represented by R0) is straight but is otherwise of arbitrary shape. It is immersed in fluid whose undisturbed velocity, at the position of the body and relative to it, may be either uniform, corresponding to translational motion of the body, parallel or perpendicular to the body length, or a linear function of distance along the body length, corresponding to an ambient pure straining motion or to rotational motion of the body. Inertia forces are negligible. It is possible to represent the body approximately by a distribution of Stokeslets over a line enclosed by the body; and then the resultant force required to sustain translational motion, the net stresslet strength in a straining motion, and the resultant couple required to sustain rotational motion, can all be calculated. In the first approximation the Stokeslet strength density F(x) is independent of the body shape and is of order μUe, where U is a measure of the undisturbed velocity and e = (log 2l/R0)−1. In higher approximations, F(x) depends on both the body cross-section and the way in which it varies along the length. From an investigation of the ‘inner’ flow field near one section of the body, and the condition that it should join smoothly with the ‘outer’ flow which is determined by the body as a whole, it is found that a given shape and size of the local cross-section is equivalent, in all cases of longitudinal relative motion, to a circle of certain radius, and, in all cases of transverse relative motion, to an ellipse of certain dimensions and orientation. The equivalent circle and the equivalent ellipse may be found from certain boundary-value problems for the harmonic and biharmonic equations respectively. The perimeter usually provides a better measure of the magnitude of the effect of a non-circular shape of a cross-section than its area. Explicit expressions for the various integral force parameters correct to the order of e2 are presented, together with iterative relations which allow their determination to the order of any power of e. For a body which is ‘longitudinally elliptic’ and has uniform cross-sectional shape, the force parameters are given explicitly to the order of any power of e, and, for a cylindrical body, to the order of e3.

Development of the Brain depends on the Visual Environment

Abstract: IN a normal cat, neurones of the visual cortex are selective for the orientation of lines and edges in the visual field, and the preferred orientations of different cells are distributed all around the clock1. Hirsch and Spinelli2 have recently reported that early visual experience can change this organization. They reared kittens with one eye viewing vertical stripes, the other horizontal, and found that out of twenty-one neurones with elongated receptive fields all were monocularly driven, and in all but one case the orientation of the receptive field closely matched the pattern experienced by that eye.

Perseverative interference in monkeys following selective lesions of the inferior prefrontal convexity.

Abstract: Monkeys with lesions of the inferior frontal convexity were impaired relative to controls in retaining an auditory frequency differentiation (although subsequent thresholds were normal) and in learning object and spatial reversals. Performance was characterized by perseverative interference, a frontal symptom which now seems attributable to damage to the inferior convexity.

Aquatic animal propulsion of high hydromechanical efficiency

Abstract: This paper attempts to emulate the great study by Goldstein (1929) ‘On the vortex wake of a screw propeller’, by looking for a dynamical theory of how another type of propulsion system has evolved towards ever higher performance. An ‘undulatory’ mode of animal propulsion in water is rather common among invertebrates, and this paper offers a preliminary quantitative analysis of how a series of modifications of that basic undulatory mode, found in the vertebrates (and especially in the fishes), tends to improve speed and hydromechanical efficiency.Posterior lateral compression is the most important of these. It is studied first in ‘pure anguilliform’ (eel-like) motion of fishes whose posterior cross-sections are laterally compressed, although maintaining their depth (while the body tapers) by means of long continuous dorsal and ventral fins all the way to a vertical ‘trailing edge’. Lateral motion of such a cross-section produces a large and immediate exchange of momentum with a considerable ‘virtual mass’ of water near it.In § 2, ‘elongated-body theory’ (an extended version of inviscid slender-body theory) is developed in detail for pure anguilliform motion and subjected to several careful checks and critical studies. Provided that longitudinal variation of cross-sectional properties is slow on a scale of the cross-sectional depth s (say, if the wavelength of significant harmonic components of that variation exceeds 5s), the basic approach is applicable and lateral water momentum per unit length is closely proportional to the square of the local cross-section depth.The vertical trailing edge can be thought of as acting with a lateral force on the wake through lateral water momentum shed as the fish moves on. The fish's mean rate of working is the mean product of this lateral force with the lateral component of trailing-edge movement, and is enhanced by the virtual-mass effect, which makes for good correlation between lateral movement and local water momentum. The mean rate of shedding of energy of lateral water motions into the vortex wake represents the wasted element in this mean rate of working, and it is from the difference of these two rates that thrust and efficiency can best be calculated.Section 3, still from the standpoint of inviscid theory, studies the effect of any development of discrete dorsal and ventral fins, through calculations on vortex sheets shed by fins. A multiplicity of discrete dorsal (or ventral) fins might be thought to destroy the slow variation of cross-sectional properties on which elongated-body theory depends, but the vortex sheets filling the gaps between them are shown to maintain continuity rather effectively, avoiding thrust reduction and permitting a slight decrease in drag.Further advantage may accrue from a modification of such a system in which (while essentially anguilliform movement is retained) the anterior dorsal and ventral fins become the only prominent ones. Vortex sheets in the gaps between them and the caudal fin may largely be reabsorbed into the caudal-fin boundary layer, without any significant increase in wasted wake energy. The mean rate of working can be improved, however, because the trailing edges of the dorsal and ventral fins do work that is not cancelled at the caudal fin's leading edge, as phase shifts destroy the correlation of that edge's lateral movement with the vortex-sheet momentum reabsorbed there.Tentative improvements to elongated-body theory through taking into account lateral forces of viscous origin are made in §4. These add to both the momentumandenergyof the water's lateral motions, but mayreduce the efficiencyof anguilliform motion because the extra momentum at the trailing edge, resulting from forces exerted by anterior sections, is badly correlated with that edge's lateral movements. Adoption of the ‘carangiform’ mode, in which the amplitude of the basic undulation grows steeply from almost zero over the first half or even two-thirds of a fish's length to a large value at the caudal fin, avoids this difficulty.Any movement which a fish attempts to make, however, is liable to be accompanied by ‘recoil’, that is, by extra movements of pure translation and rotation required for overall conservation of momentum and angular momentum. These recoil movements, a potentially serious source of thrust and efficiency loss in carangiform motion, are calculated in § 4, which shows how they are minimized with the right distribution of total inertia (the sum of fish mass and the water's virtual mass). It seems to be no coincidence that carangiform motion goes always with a long anterior region of high depth (possessing a substantial moment of total inertia) and a region of greatly reduced depth just before the caudal fin.The theory suggests (§5) that reduction of caudal-fin area in relation to depth by development of a caudal fin into a herring-like ‘pair of highly sweptback wings’ should reduce drag without significant loss of thrust. The same effect can be expected (although elongated-body theory ceases to be applicable) from widening of the wing pair (sweepback reduction). That line of development of the carangiform mode in many of the Percomorphi leads towards the lunate tail, a culminating point in the enhancement of speed and propulsive efficiency which has been reached also along some quite different lines of evolution.A beginning in the analysis of its advantages is made here using a ‘twodimensional’ linearized theory. Movements of any horizontal section of caudal fin, with yaw angle fluctuating in phase with its velocity of lateral translation, are studied for different positions of the yawing axis. The wasted energy in the wake has a sharp minimum when that axis is at the ‘three-quarter-chord point’, but rate of working increases somewhat for axis positions distal to that. Something like an optimum regarding efficiency, thrust and the proportion of thrust derived from suction at the section's rounded leading edge is found when the yawing axis is along the trailing edge.This leads on the present over-simplified theory to the suggestion that a hydromechanically advantageous configuration has the leading edge bowed forward but the trailing edge straight. Finally, there is a brief discussion of possible future work, taking three-dimensional and non-linear effects into account, that might throw light on the commonness of a trailing edge that is itself slightly bowed forward among the fastest marine animals.

Plate tectonics of the Mediterranean region.

Abstract: The seismicity and fault plane solutions in the Mediterranean area show that two small rapidly moving plates exist in the Eastern Mediterranean, and such plates may be a common feature of contracting ocean basins. The results show that the concepts of plate tectonics apply to instantaneous motions across continental plate boundaries.

Effects of 6-hydroxydopamine on catecholamine containing neurones in the rat brain.

Abstract: After the intraventricular injection of 6-hydroxydopamine (6-OHDA), there was a long lasting reduction in the brain concentrations of noradrenaline (NA) and dopamine (DA). The brain concentration of NA was affected by lower doses of 6-OHDA than were required to deplete DA. A high dose of 6-OHDA which depleted the brain of NA and DA by 81 per cent and 66 per cent respectively, had no significant effect on brain concentrations of 5-hydroxytryptamine (5-HT) or γ-aminobutyric acid (GABA). The fall in catecholamines was accompanied by a long lasting reduction in the activities of tyrosine hydroxylase and DOPA decarboxylase in the hypothalamus and striatum, areas in the brain which are rich in catecholamine containing nerve endings. There was, however, no consistent effect on catechol-O-methyl transferase or monamine oxidase activity in these brain regions. The initial accumulation of [3H]NA into slices of the hypothalamus and striatum was markedly reduced 22–30 days after 6-OHDA treatment. These results are consistent with the evidence in the peripheral sympathetic nervous system that 6-OHDA causes a selective destruction of adrenergic nerve endings and suggest that this compound may have a similar destructive effect on catecholamine neurones in the CNS.

Plate Tectonics of the Red Sea and East Africa

Abstract: The relative motion between the plates on each side of the East African Rift Valley can be obtained from the opening of the Red Sea and the Gulf of Aden. The calculated direction of relative motion agrees well with fault plane solutions for earthquakes north of the equator.

Growth Theory: An Exposition

Abstract: Nobel Lecture, December 8, 1987: Growth Theory and After 1. Characteristics of Steady States 2. A Variable Capital/Output Ratio 3. A Model without Direct Substitution 4. A Model with Two Assets 5. Economic Policy in a Growth Model 6. Aspects of Economic Policy Intermezzo 7. The Standard Model Once More 8. Human Capital: The Lucas Model 9. Endogenous Technology: The Romer Model 10. New Consumer Goods: Grossman and Helpman 11. Schumpeterian Ideas: Aghion and Howitt 12. Lessons and Suggestions for Aggregative Growth Theory Bibliography

Wavetrains in Inhomogeneous Moving Media

Abstract: When a slowly varying wavetrain of small amplitude propagates in a general medium, changes of frequency and wavenumber are determined along definite paths known as rays. It is shown that, for a wide class of conservative systems in fluid dymamics changes in amplitude along the rays may be computed from conservation of wave action, which is defined as the wave energy divided by the intrinsic frequency. The intrinsic frequency is the frequency which would be measured by an observer moving with the local mean velocity of the medium. This result is the analogue for continuous systems of the adiabatic invariant for a classical simple harmonic oscillator.

Lateral inhibition between orientation detectors in the human visual system

Abstract: Experiment has shown that two lines of different orientation interact with each other so that they seem to be displaced from one another in orientation. This could be explained in terms of mutual inhibition between neighbouring columns in the visual cortex.

Low-temperature oxidation

Abstract: Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces “dangling” bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.

Dynamics of extended bodies in general relativity. I. Momentum and angular momentum

Abstract: Definitions are proposed for the total momentum vector p$^\alpha$ and spin tensor S$^{\alpha\beta}$ of an extended body in arbitrary gravitational and electromagnetic fields. These are based on the requirement that a symmetry of the external fields should imply conservation of a corresponding component of momentum and spin. The particular case of a test body in a de Sitter universe is considered in detail, and used to support the definition p$\_\beta$S$^{\alpha\beta}$ = 0 for the centre of mass. The total rest energy M is defined as the length of the momentum vector. Using equations of motion to be derived in subsequent papers on the basis of these definitions, the time dependence of M is studied, and shown to be expressible as the sum of two contributions, the change in a potential energy function $\Phi$ and a term representing energy inductively absorbed, as in Bondi's illustration of Tweedledum and Tweedledee. For a body satisfying certain conditions described as 'dynamical rigidity', there exists, for motion in arbitrary external fields, a mass constant m such that $M = m + \frac{1}{2}S^\kappa\Omega\_\kappa + \Phi$, where $\Omega_\kappa$ is the angular velocity of the body and S$^\kappa$ its spin vector.

A Theory for Cerebral Neocortex

Abstract: It is proposed that the learning of many tasks by the cerebrum is based on using a very few fundamental techniques for organizing information. It is argued that this is made possible by the prevalence in the world of a particular kind of redundancy, which is characterized by a `Fundamental Hypothesis’.

Lithosphere Plate-Continental Margin Tectonics and the Evolution of the Appalachian Orogen

Abstract: In terms of plate tectonic theory, and by analogy with modern continental margins, the Appalachian orogen evolved through a sequence of interrelated sedimentation-deformation-metamorphism patterns within a tectonic belt situated along the eastern margin of the North American continent. As exemplified by the northern part of the orogen, Appalachian stratigraphic-tectonic zones and deformation sequences are related to Late Precambrian to Ordovician expansion, followed by Ordovician through Devonian contraction, of a Proto-Atlantic ocean. This oceanic opening and closing was achieved by initial extensional necking of a single North American/African continent and by lithosphere plate accretion, followed by contractional plate loss along a trench, or complex of trenches, marginal to the drifted North American continent. A lithosphere plate model for the evolution of the orogen incorporates spatial and chronologic relations within and between bulk stratigraphic units and tectonic events. Pre-orogenic Appalachian sedimentation patterns were essentially the same as those found along modern continental margins; that is, shelf/slope/rise/abyss. Appalachian tectonic patterns are also analogous with modern tectonic patterns of continental margins, island arcs, and trenches, and involved continent-ward driven thrust sheets and ancillary exogeosynclines.

Calorie conversion factors. An experimental reassessment of the factors used in the calculation of the energy value of human diets

Abstract: 1. The intake and excretion of total nitrogen, fat and the various forms of carbohydrate, and the heats of combustion of the diet, urine and faeces were measured in groups of young men, young women, elderly men and elderly women. 2. Each group was studied while the subjects were eating two diets in turn, which differed in their contents of unavailable carbohydrate; the young women were also studied on a third diet which was rich in unavailable carbohydrate. 3. Increasing the intake of unavailable carbohydrate resulted in a greater faecal loss of energy, and in most instances of nitrogen and fat. 4. There was no significant effect of sex or age on the apparent digestibility of protein, fat or available carbohydrate. 5. The results are used to evaluate the use of calorie conversion factors for calculating the metabolizable energy content of mixed diets. 6. These show that for practical purposes the classical Atwater factors can be used to calculate the metabolizable energy of a diet with reasonable accuracy, provided that when available carbohydrate (as monosaccharides) values are used in the calculation a factor of 3.75 kcal/g (15.7 kJ/g) is used. 7. The studies demonstrate that the accuracy of any method for calculating the metabolizable energy of a diet is largely determined by the accuracy with which the method is capable of predicting the gross energy of the diet.

Deep vein thrombosis of the leg. Is there a "high risk" group?

Abstract: Summary Two hundred and three patients undergoing elective surgery were investigated to determine the group of patients who are at a “great risk” of developing deep vein thrombosis. It was found that the patients who formed this group included those who had a history of previous deep vein thrombosis or pulmonary embolism, those who had varicose veins or underwent operation for malignant disease, and elderly patients (over sixty-one years) having major operations. All of these patients are at a “great risk” of developing thrombosis during the postoperative period.

Discreteness of Conductance Change in Bimolecular Lipid Membranes in the Presence of Certain Antibiotics

Abstract: NET ion movements across biological or synthetic lipid membranes may take place by various mechanisms, underlying all of which there is a rather ill-defined and small ion leakage or background conductance. Most ions permeate by means of pathways involving either permanent or transient modifications of the basic structure of the membrane. If permanent pathways are involved, a given membrane conductance may be accounted for by routes which are either numerous and of low conductance or few and of high conductance. For transient pathways, duration must also be considered. Thus, if a carrier is invoked, the duration will be the time the carrier, complexed with an ion, spends shuttling across the membrane. For a pore, the duration is the time for which it remains open to ions. At present little is known concerning the number, conductance and duration of the ionic pathways in any membrane of the types mentioned. Limited information is available for the nerve membrane, although this is rather imprecise and indirect1.

The Fitting of Pseudopotentials to Experimental Data and Their Subsequent Application

Abstract: Publisher Summary This chapter summarizes results from the theory of pseudopotentials that relate to choosing suitable mathematical forms forfitting. The procedures used are described. It is reported that it is important to distinguish between what is exact and what is approximate, and to discuss where various approximations might be reasonably appropriate and where not. The chapter reviews all the experimentally fitted pseudopotentials. Primarily they come from Fermi surface studies in the case of metals and analysis of optical properties for semiconductors, but phonon spectra, resistivities of liquid metals, and other data have also been important. It is suggested that by making use of all the theory, all the fitted results by interpolating, extrapolating, cajoling, and cudgeling, one should be able to produce a useful pseudopotential for many elements in many situations.

Exogenous inorganic carbon sources in plant photosynthesis

Abstract: CONTENTS