Showing papers by "University of Leeds published in 1984"

Chemistry of the elements

Abstract: Origin of the elements, isotopes and atomic weights Chemical periodicity and the periodic table Hydrogen Lithium, sodium, potassium, rubidium, caesium and francium Beryllium, magnesium, calcium, strontium, barium and radium Boron Aluminium, gallium, indium and thallium Carbon Silicon Germanium, tin and lead Nitrogen Phosphorus Arsenic, antimony and bismuth Oxygen Sulfur Selenium, tellurium and polonium The halogens: fluorine, chlorine, bromine, iodine and astatine The noble gases: helium, neon, argon, krypton, xenon, and radon Coordination and organometallic compounds Scandium, yttrium, lanthanum and actinium Titanium, zirconium and hafnium Vanadium, niobium and tantalum Chromium, molybdenum and tungsten Manganese, technetium and rhenium Iron, ruthenium and osmium Cobalt, rhodium and iridium Nickel, palladium, and platinum Copper, silver and gold Zinc, cadmium and mercury The lanthanide elements The actinideand transactinide elements (Z=90-112).

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract: A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD=Δ (XCa,g) where KD is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low aO2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.

C/S method for distinguishing freshwater from marine sedimentary rocks

Abstract: In organic-rich sediments laid down in fresh water, much less diagenetic pyrite is formed than in analogous marine sediments because of the much lower concentrations of dissolved sulfate found in most fresh waters as compared to seawater. As a result, modern organic-rich freshwater sediments exhibit a much higher organic carbon-to-pyrite sulfur ratio (C/S) than marine sediments with similar organic contents. On this basis, C/S ratios can be used to distinguish ancient marine from freshwater (or slightly brackish) sedimentary rocks. This is demonstrated here for several Carboniferous shales and siltstones. The C/S technique cannot distinguish brackish-water sediments deposited under salinities greater than half that of seawater from marine sediments, as demonstrated by analyses of modern Chesapeake Bay sediments. Also, the method is not applicable to nearly pure limestones or to rocks low in organic matter (less than about 1% organic carbon). Saline (high sulfate) phases of ancient lakes can be distinguished from nonsaline phases using the C/S method.

Walking and Running

Abstract: The human body is a complicated machine whose movements involve many different joints, operated by a great many muscles. For that reason it is easy to get bogged down in detail when thinking about walking and running from a mathematical point of view. Any position of the human body (or of any other jointed mechanism) can be described by giving the angles of joints. The number of angles needed for an unambiguous description is the number of degrees of freedom of the mechanism. For example, the position of a hinge joint is described by just one angle: a hinge allows only one degree of freedom. The human knee is a hinge. The ankle, however, allows rotation about two axes – you can tilt your foot toes up or toes down, and you can also rock it sideways so that the sole faces inwards towards the other foot – so it gives two degrees of freedom. The hip is a ball and socket joint allowing rotation about any axis through the centre of the ball, but any position can be described by just three angles (measured, for example, in three planes at right angles to each other), so it allows three degrees of freedom. In total, there are six degrees of freedom in each leg, making twelve in all, and suggesting that we need twelve equations of motion to describe walking. If we took account of the flexibility of the foot and the movements of the arms, we would need more.

The Gaits of Bipedal and Quadrupedal Animals

Abstract: The gaits of reptiles, birds, and mammals are reviewed. It is shown that mammals of different sizes tend to move in dy namically similar fashion whenever their Froude numbers u2/gh are equal: here ...

Calcium as a plant nutrient

Abstract: . Calcium occurs as a relatively large, divalent ion which readily enters the apoplast and is bound in exchangeable form in cell walls and on the exterior surface of the plasmalemma. It occurs in only very low concentrations in the cytoplasm and chloroplasts and appears to have a limited role as an enzymatic cofactor. The soil solution usually provides an adequate supply of Ca to plants. The well known physiological disorders resulting from localized Ca deficiencies within the plant are thus attributable to poor Ca distribution rather than restriction in uptake. Calcium is moved largely in the xylem and only to a very limited extent in the phloem. The phloem/xylem ratio of the solute input can be particularly critical in organs which are naturally low in Ca, such as fruits and young leaves. Factors which influence the distribution of Ca such as humidity, root pressure, phytohormone activity, can also affect the occurrence of these disorders.

Turbulent Burning Velocities and Flame Straining in Explosions

Abstract: Turbulent burning velocities have been measured in an explosion bomb equipped with four high speed fans. Turbulent parameters were measured by laser doppler anemometry. The turbulent Reynolds numbers were significantly higher than in most previous measurements and high rates of strain were achieved until, ultimately, several of the flames quenched. Results are presented in terms of previously used dimensionless parameters plus a Lewis number and a dimensionless activation energy. The two-eddy theory of burning can allow for flame straining reductions in laminar burning velocity and experimental values of u t / u 1 were compared with those from such a theory.

The Variogram and Kriging: Robust and Resistant Estimators

Abstract: Previously published techniques for robust and resistant variogram estimation are reviewed and several alternative methods are introduced. The performance of various estimators is assessed on two typical practical examples.

Interspecific competition is not a major organizing force in many insect communities

Abstract: Part of the current dogma in ecology is that competition between species for limited resources is not only common but also a major organizing force in many communities1,2 largely because studies on vertebrates, particularly birds, have played a major role in creating the traditional framework of niche theory and resource partitioning3–9. Other workers, particularly those studying insect communities, have suggested that significant interspecific competition is too rare and sporadic to be of major significance and have placed more emphasis on autecological processes10–13. Efforts to resolve the controversy have concentrated on the question of whether or not competition is common in nature1. Here we show that even where competition can be demonstrated, it need not have a major role in community organization.

Properties and applications of liquid-crystalline main-chain polymers

Abstract: The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings.

The effect of grain size on the permittivity of BaTiO3

Abstract: Grain size effects in polycrystalline BaTiO3 are reviewed in terms of the “internal stress model”. By considering polarization dependent terms in the Devonshire free energy expression, the dielectric constant may be calculated for the whole ferroelectric temperature region, under various stress systems. Encouraging comparisons with measured values may be made.

On roll waves down an open inclined channel

Abstract: Flow down an open inclined channel is considered. Dressier (Communs pure appl. Math. 2, 149-194 (1949),) using the equations of the shallow water theory augmented by the Chezy formula for drag, has shown that the uniform flow becomes unstable when the Froude number F exceeds 4, and in this case he was able to construct a one-parameter family of discontinuous periodic solutions by piecing together continuous sections of wave profile and a series of hydraulic jumps. Here the work of Dressler is extended by the inclusion of a further term that accounts for energy dissipation by shearing normal to the flow. It is shown that the inclusion of such a term does not alter the condition for stability of the uniform flow, and that when the uniform flow is unstable, a one-parameter family of quasi-steady periodic solutions exists (parametrized by the propagation speed U), appearing as a Hopf bifurcation out of the uniform flow at the critical value $U_c = 1 + F^{-\frac{1}{2}$. After the existence of these periodic solutions has been shown, uniformly valid expansions for the periodic solutions are obtained by using the Krylov-Bogoliubov-Mitropolski averaging method, and the results are also extended to larger amplitudes by numerical integration.

A phenomenological gibbs function for BaTiO3 giving correct e field dependence of all ferroelectric phase changes

Abstract: The elastic Gibbs function for BaTiO3 introduced by Buessem, Goswami and Cross1 has been modified by including the last symmetry permitted sixth order term HP12P22P32 with a coefficient H = 4.91 × 109 Vm9 C-5. The function predicts the correct high electric field behavior of the low temperature ferroelectric: ferroelectric phase changes in the single crystal and suggests an interesting change of sign of the pyroelectric effect at high field levels in the induced tetragonal ferroelectric phase.

Stride length and speed for adults, children, and fossil hominids

Abstract: Research workers studying the relationship between stride length (L) and speed (u) in human walking have often expressed their results as multiples of stature (h): they have given values of L/h and u/h They have claimed or implied that this takes account of differences of body size and that L/h should be the same function of u/h for people of all sizes It is shown that this is not true for comparisons of children with adults Further, it is argued by dimensional analysis that u/square root gh is a more appropriate speed parameter that u/h (g is the acceleration of free fall) It is shown that L/h is approximately the same function of u/square root gh for children aged 4 or more years as for adults The empirical relationship between L/h and u/square root gh is used to make new estimates of walking speed for the early hominid footprints found at Laetoli, Tanzania The speeds obtained are equivalent to mean speeds of human walking observed in small towns (ie, they give approximately equal values of u/square root gh)

Creep behavior of ultrahigh‐modulus polyethylene: Influence of draw ratio and polymer composition

Abstract: The creep behavior of ultrahigh-modulus polyethylene monofilaments has been studied over the temperature range 20–70°C. A wide range of samples was examined in an attempt to determine the influence of draw ratio, molecular weight, copolymerization, and crosslinking by γ irradiation Prior to drawing. Results are also presented for a solution-spun fiber. It is proposed that the permanent flow creep arises from a combination of two creep processes, one of which is associated with the crystalline regions of the oriented structure and the other with a molecular network.

Transient Elastohydrodynamic Lubrication Models for the Human Ankle Joint

Abstract: An equivalent bearing was proposed to represent the normal human ankle joint. The geometry was based on measurements of dissected ankle joints and tissue properties were obtained from the work of previous investigators. Theoretical models were developed to estimate the cyclic variation in lubricant film thickness and coefficient of friction during repetitive activities such as walking. Solutions were obtained for various combinations of input parameters. For the conditions representing the walking cycle, film thicknesses of about 0.7 μm were calculated. Although this value was smaller than most previous measurements of the rms roughness of cartilage, it was not much smaller and suggested that transient elastohydrodynamic lubrication played a role in synovial joint lubrication. The possibility of full fluid film lubrication was supported only when a very high input viscosity was employed, based on values estimated from the previous experimental studies of the boosted lubrication mechanism. Also, an attempt...

Magnesium absorption from the digestive tract of sheep.

Abstract: Factors affecting absorption of Mg from the ovine rumen have been studied using either a pouch constructed from part of the dorsal rumen or by an isolated washed rumen technique in vivo. Net absorption of Mg against the prevailing electrochemical gradient was observed. An increase in the K/Na ratio within the rumen led to an increase in the potential difference across the rumen wall, blood positive, and to a decrease in the net efflux of Mg from the rumen. This decrease was due to an increase in Mg influx into the rumen. The addition of ammonium chloride (30 mmol/l) to the rumen contents also led to a reduction in net Mg absorption but to no significant change in potential difference. The effects of high K/Na ratio and high ammonium ion concentration within the rumen were additive in causing decreases in net effluxes of both Mg and Na. An inverse relationship was demonstrated between the Ca concentration in the rumen and the net absorption rate of Mg. It was concluded that the efflux of Mg across the rumen wall depends at least in part on a functional system for Na transport.

Calcite–dolomite geothermometry in the system CaCO3–MgCO3–FeCO3: an experimental study

Abstract: An experimental study of the system CaCO3–MgCO3–FeCO3 was undertaken in order to calibrate the iron correction to the calcite–dolomite geothermometer, which is based on the solubility of magnesium in calcite in the assemblage calcite + dolomite. The experiments, at 450°C and lower temperatures, resulted in products with a very small grain size and incomplete equilibration. However, application of a carefully-devised automatic data processing algorithm to analyses of the phases in experimental charges, combined with a thermodynamic analysis, results in geothermometer diagrams which should be preferred to previous theoretical predictions.