A point-particle model universe

The observable cosmos is modeled as a set of point-particles, representing the galaxies, which perturb a dust-filled, Robertson-Walker space-time. The analysis proceeds only to first order in=8G/c ² and employs a metric suggested by McVittie [General Relativity and Cosmology (Chapman and Hall, London, 1965)], whose original work this paper seeks to develop. Necessary and sufficient conditions are found for the metric to give rise to an energy tensor of a chosen form appropriate to the modeling. In particular, a second-order equation is found which governs a certain time-independent potential. A class of solutions to this equation is established, and the associated singularities of the mass density are shown to be of a Dirac type.     

The conditions of water absorption when employing an external absorbent in the microdetermination of carbon and hydrogen

Summary The conditions of water absorption when using an external nitrogen dioxide absorbent in the carbon and hydrogen microdetermination are very critical. Two sources of error are considered and effectively eliminated or reduced to insignificance. The resultant modified procedure has an accuracy of % C ±0.35, % H ±0.25.

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Zusammenfassung Die Bedingungen der Wasserabsorption bei Absorption der Stickstoffoxyde außerhalb des Verbrennungsrohres sind für die Mikrobestimmung von Kohlenstoff und Wasserstoff sehr kritisch. Zwei Fehlerquellen wurden erfaßt und wirksam beseitigt bzw. soweit ausgeschaltet, daß sie bedeutungslos sind. Die Genauigkeit des modifizierten Verfahrens beträgt ±0,35% C und ±0,25% H.

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Résumé Lorsque l'on utilise un absorbant externe au peroxyde d'azote les conditions d'absorption de l'eau sont très critiques dans le cas du microdosage du carbone et de l'hydrogène. On considère deux sources d'erreurs et on les élimine effectivement ou on les rend négligeables. Le procédé modifié qui en résulte a une précision en % C de ±0,35 et en % H de ±0,25.

     

Stripping chronopotentiometric detection of copper using screen-printed three-electrode system—application to acetic-acid bioavailable fraction from soil samples

Single-use sensors, incorporating a three-electrode configuration (graphite carbon-working electrode; carbon-counter electrode and silver/silver chloride-reference electrode), have been fabricated on a polyester substrate using low cost screen-printing (thick-film) technology. These electrodes coupled with constant current stripping chronopotentiometry (CCSCP), has provided a convenient screening tool for on-site detection of trace levels of copper. Modification of the graphite carbon surface based on in situ deposition of mercury film has been carried out. By appropriate choice of supporting medium and applied constant stripping current, well-resolved and reproducible response for copper was obtained. The stripping response for copper following 2 min deposition was linear over the concentration range examined (10-2000 ppb) with detection limit of 6 ppb using 2 M hydrochloric acid (HCl). Successful applications of the sensing device to acetic-acid bioavailable fraction of a certified reference material (CRM 601, a lake sediment) and soil samples are demonstrated.     

Modular properties of the hard hexagon model

The physical quantities (or powers thereof) in the hard-hexagon model that were computed exactly by Baxter are shown to be modular functions with respect to the number-theoretic group ₁[N]. This allows us to determine the analytic structure of, the partition function per site in the thermodynamic limit, and, the density, as functions of the activityz.     

Polyperfluorobutadiene. II. Fractionation and crosslinking

Perfluorobutadiene was polymerized in bulk under low pressure and temperature in the presence of diisopropyl peroxydicarbonate, bis(trifluoromethyl) peroxide, and benzoyl peroxide. Fractional solvent extraction of polyperfluorobutadiene by n-hexane and hexafluorobenzene into three polymeric fractions was described. The hexafluorobenzene-soluble fraction was used for crosslinking studies. Direct fluorination of solid polyperfluorobutadiene required no catalyst. The fluorine–polyperfluoropolyene reactions furnished a mechanism for crosslinking the unsaturated chains. Fluorination by gaseous fluorine formed free radical sites on the polymer chains besides saturation reactions. The presence of excess monomer in contact with these partially fluorinated solid polyperfluorobutadienes under controlled conditions were capable of crosslinking and grafting to the linear chains. Direct fluorination of solid polyperfluorobutadiene tends to involve reaction with the internal double bonds rather than the pendant perfluorovinyl group; the converse is true for crosslinking with hexamethylenediamine. The mechanisms are discussed.