ANALYSIS OF THE POTENTIOMETRIC TITRATION OF NATURAL AND SYNTHETIC POLYCARBOXYLATES

Potentiomctric titrations of a variety of natural and synthetic poly-COOH's varying in molecular weight and COOH density gave pKm values varying from 3.5 to 5.2. The pattern of carboxylic acid group dissociation is discussed in terms of purely electrostatic assumptions and of COOH group clustering. Synthetic polymaleic acid showed many of the characteristics of both fulvic and humic acids, and therefore seems to be a reasonable representation of an average, hypothetical soil organic matter polycarboxylic acid.     

Radio-caesium fixation dynamics: measurement in six Cumbrian soils

Five peat soils and a mineral soil were artificially contaminated with ¹³⁷Cs. Soil solution activity and radio–lability of ¹³⁷Cs were monitored over 709 days to quantify progressive ¹³⁷Cs fixation. The peat soils fixed large amounts of ¹³⁷Cs, but less than the mineral soil did. Distribution coefficients (K_d, cm³ g^?1) ranged from 30 to 5000 at the end of equilibration. A labile ¹³⁷Cs distribution coefficient, K_dt, was estimated by a method involving solid ? solution equilibration in dilute solution. In a separate study several concentrations of KCl were added to soils in increasing concentration both before and after the addition of ¹³⁷Cs. Differences in apparent adsorption strength of radiocaesium indicated that K⁺ induced the collapse of expanded mineral interlayers, thereby trapping ions. It seemed that ^I37Cs adsorbs at sites in the small micaceous clay fraction of the peat soils. The different rates of ¹³⁷Cs adsorption and fixation in the peat and mineral soils, in which the rate of access of ¹³⁷Cs to fixation sites in peat soils is less, seems to have been caused partly by lack of K, and partly by the scarcity of fixation sites.     

The origin and loss of fractal scaling in simulated soil aggregates

A growing body of evidence supports the use of fractals to approximate the heterogeneous structure of soil. Fractal models of transport processes in soil indicate that the fractal dimension and the upper limit to fractal scaling (denoted ξ) determine flow rates and scaling properties. A two-dimensional Monte Carlo simulation of soil aggregation is used to identify the key processes which may lead to fractal-like scaling in the soil matrix. A bonding law is developed which parameterizes the relative importance of chemical bonding to binding by microbial by-products and fungi. We investigate the sensitivity of the fractal dimension and ξ to the nature of the bonding law, and to aggregate fragmentation, initial particle size distribution and mechanical restructuring. Fractal-like structures result from the aggregation process. However, the measured fractal dimension is insensitive to the detailed form of the bonding law. The factor that most sensitively influences the fractal dimension is found to be mechanical restructuring such as would arise from the action of roots, soil fauna and natural weathering. By contrast, ξ is most sensitive to the bonding law. The results suggest that the fractal dimension and characteristic diameter of aggregates should be uncorrelated, and that increased binding by microbial by-products and fungi should produce larger, less dense, aggregates.     

Hyperactivated Motility in Sperm

Hyperactivation is a movement pattern seen in sperm at the site and time of fertilization in mammals. It may be critical to the success of fertilization, because it enhances the ability of sperm to detach from the wall of the oviduct, to move around in the labyrinthine lumen of the oviduct, to penetrate mucous substances and, finally, to penetrate the zona pellucida of the oocyte. The movement of hyperactivated sperm appears different under different physical conditions and in different species, but basically it involves an increase in flagellar bend amplitude and, usually, beat asymmetry. Presumably, a signal or signals exist in the oviduct to initiate hyperactivation at the appropriate time; however, none has yet been identified with certainty. While the signal transduction cascade regulating hyperactivation remains to be completely described, it is clear that calcium ions interact with the axoneme of the flagellum to switch on hyperactivation. Although hyperactivation often occurs during the process of capacitation, the two events are regulated by somewhat different pathways.     

Cardiopulmonary effects of clonidine, diazepam and the peripheral α2 adrenoceptor agonist ST-91 in conscious sheep

The cardiopulmonary effects of the intravenous administration of clonidine (15 μg/kg), ST-91 (30 μg/kg) and diazepam (0.4 mg/kg) were compared in five healthy sheep using a randomized cross-over design, to determine whether the hypoxaemic effects of α₂ adrenoceptor agonists are due to sedation, or to peripheral α₂ adrenoceptor stimulation. All three drugs significantly lowered arterial oxygen tension (PaO₂) levels within 2 min of their administration; however, clonidine and ST-91 produced long lasting and severe hypoxaemia with mean PaO₂ levels of ≈40 mm Hg and 50 mm Hg (5.3 kPa and 6.6 kPa), respectively. The fall in PaO₂ was considerably less with diazepam (63 mm Hg or 8.4 kPa at 2 min) and by 15 min the values did not differ from placebo treated animals. None of the drugs increased arterial carbon dioxide tension (PaCO₂) levels when compared to saline treatment and the acid base variables did not show any significant change. A significant increase was recorded in the packed cell volume of the ST-91 treated group throughout the study. Within 2 min of their administration, all drugs caused a significant increase in mean arterial pressure (MAP) as compared to the placebo treated group. The MAP remained significantly increased for 5 and 60 min after clonidine and ST-91 treatment, respectively. The study shows that ST-91 and clonidine produce a greater degree of hypoxaemia than occurs with diazepam sedation, and that the hypoxaemic effect of α₂ adrenoceptor agonists in sheep are mainly mediated by peripheral α₂ adrenoceptors.     

MAGNETIC RESONANCE IMAGING FEATURES OF INTRACRANIAL ASTROCYTOMAS AND OLIGODENDROGLIOMAS IN DOGS

Astrocytomas and oligodendrogliomas represent one third of histologically confirmed canine brain tumors. Our purpose was to describe the magnetic resonance (MR) imaging features of histologically confirmed canine intracranial astrocytomas and oligodendrogliomas and to examine for MR features that differentiate these tumor types. Thirty animals with confirmed astrocytoma (14) or oligodendroglioma (16) were studied. All oligodendrogliomas and 12 astrocytomas were located in the cerebrum or thalamus, with the remainder of astrocytomas in the cerebellum or caudal brainstem. Most (27/30) tumors were associated with both gray and white matter. The signal characteristics of both tumor types were hypointense on T1‐weighted images (12 each) and hyperintense on T2‐weighted images (11/14 astrocytomas, 12/16 oligodendrogliomas). For astrocytomas and oligodendrogliomas, respectively, common findings were contrast enhancement (10/13, 11/15), ring‐like contrast enhancement (6/10, 9/11), cystic regions within the mass (7/14, 12/16), and hemorrhage (4/14, 6/16). Oligodendrogliomas were significantly more likely to contact the brain surface (meninges) than astrocytomas (14/16, 7/14, respectively, P=0.046). Contact with the lateral ventricle was the most common finding, occurring in 13/14 astrocytomas and 14/16 oligodendrogliomas. No MR features were identified that reliably distinguished between these two tumor types. Contrast enhancement was more common in high‐grade tumors (III or IV) than low‐grade tumors (II, P=0.008).     

Large-Scale Synthesis of Aligned Carbon Nanotubes

Large-scale synthesis of aligned carbon nanotubes was achieved by using a method based on chemical vapor deposition catalyzed by iron nanoparticles embedded in mesoporous silica. Scanning electron microscope images show that the nanotubes are approximately perpendicular to the surface of the silica and form an aligned array of isolated tubes with spacings between the tubes of about 100 nanometers. The tubes are up to about 50 micrometers long and well graphitized. The growth direction of the nanotubes may be controlled by the pores from which the nanotubes grow.     

Chromosome number of the chimpanzee,Pan troglodytes

The chromosome numbers of nine chimpanzees (Pani troglodytes) have been determined by the bone marrow technique. The diploid number in this species is 48, with a probable XX-XY sex chromosome constitution.