The evolution of organic mantles on interstellar grains

By laboratory simulation of the chemical processes on dust grains it was investigated how solid organic materials can be produced in the interstellar medium. The ice mantles that accrete on grains in molecular clouds, consisting primarily of H2O, CO, H2CO, NH3, and O2, are irradiated by the internal UV field, resulting in the storage of radicals upon photodissociation of the original molecules. Transient heating events lead to the production of oxygen-rich organic species by recombination reactions. The experiments indicated that in this way the observed amount of organic material can be produced if a grain passes a few times through a molecular cloud during its life. After the destruction of the cloud the grains enter a more diffuse medium. Here they are subjected to the interstellar UV field as well as to collisions with atomic hydrogen. Experiments show that the intense photoprocessing results in the removal of small species like H2O and NH3 as well as in carbonization of the organic molecules. Contrary to this, the atomic H flux will maintain a certain hydrogen level in the mantle. These processes likely convert the original, oxygen-rich organics into an unsaturated hydrocarbon type material such as that observed towards IRS 7 and in Comet Halley grains

The organic component of interstellar grains

The 3.4 micron absorption feature observed in the spectrum of a number of Galactic Center (GC) sources indicates the presence of organic molecules in the interstellar medium. It is ascribed to the C-H stretch vibration of tetrahedrally bonded carbon. From the observed features due to the interstellar organic material, an estimate was made of its composition and abundance. The ratio of the number of C-H groups of tetrahedrally to those of trigonally bonded carbon was 1.5, the cosmic abundance of carbon was .00037, and the depth of the silicate absorption toward the GC was taken equal to 3.6

Ontological commitment and mathematics : inaugural lecture delivered at Rhodes University

Inaugural lecture delivered at Rhodes UniversityRhodes University Libraries (Digitisation

Oor die Begrip wese

In die wetenskap moet noodwendig van abstraksies gebruik gemaak word by die sistematisering en korrelering van kennis

Navorsing aan ons universiteite

Die taak van ’n universiteit is tweeledig, navorsing en dosering, of verwerwing en oordrag van kennis

Patent Law: Controversy in the Federal Circuit over Product-by-Process Claims

Atlantic Thermoplastics Co. v. Faytex Corp., 970 F.2d 834 (Fed. Cir.), reh\u27g en banc denied, 974 F.2d 1279 (1992)

Die formalisme in die wiskunde

Die kernprobleeni in die Wysbegeerte van die Wiskunde is die vraag na die aard van mateniatiese entiteite en hulle verhoudings. As die Wiskunde sy aanspraak om erkenning as deel van die wetenskap wil handhaaf, moet dit bestaan uit oordele aangaande ’n deel van die werk- likheid. Die wetenskap soek immers kontak met die werklikheid waaroor dit geldige oordele wil formuleer. Die Wiskunde maak dus aanspraak op waarheid, d.w.s. die oordele van die Wiskunde moet ooreenstem met die gegewens wat daardie bepaalde deel van die werklikheid bied. Voordat bepaal kan word wat hierdie waarheid, waarop die Wiskunde aanspraak maak, eintlik is, moet ons weet waaroor die oordele van die Wiskunde gaan, m.a.w. die vraag 11a die aard, en in besonder die syns- wyse van die wiskundige entiteite en die verhoudings wat daartussen hestaan, moet beantwoord word

Geestigheid en A.G. Visser

Dit is ’n bekende verskynsel in die literatuur dat verskillende tye verskillende menings oor ’n digter se werk kan he. Hiervoor bestaan daar baie redes. Dit beteken dat ’n digter se werk ’n boeiendheid besit wat eers later, miskien lank na die digter se dood, duideliker word. By Visser het daar ’n herwaardering ingetree ten opsigte van onder meer sy geestigheid. Hiervoor is daar ’n verskeidenheid redes, wat ek hieronder sal aansny. Dit gaan terug na een oorkoepelende rede wat teweeggebring het dat Visser se geestigheid in die soeklig staan. En dit het te doen met die veranderde werklikheidsbelewing in die modeme tyd, wat die plek en waarde van geestigheid raak. Hierop sal ek ingaan. Tot op hede is daar twee duidelik onderskeibare benaderings van Visser se geestigheid

Invloed van wiskundige denke

In this article the author indicates how mathematical thought has exerted influence in various fields. The questions around the nature of mathematical knowledge and how this might be obtained had their foundation in philosophy. According to Plato man comes lo the truth by means of pure thought, independent of sensory perception. Bergson feels that truth if obtained through “intuition" and not by means of the intellect. Russel reduced mathematical truths to logical truths. According to him logical truths are true exclusively on the basis of the meanings adduced to words and sentences. The extreme criteria laid down by the logical positivists for the meaningfulness of a statement have led to the situation of statements such as “God exists" having no sense any more. Mathematical thought has exerted influence also via philosophy and the mathematical sciences on man’s sense of values and views on the meaning of human existence. The descript ion of man purely in terms of concepts, which was so successful in physics leads to superficiality in the concept of the meaning of human existence, and has alienated man from those things in which he had to find inspiration and tranquillity. The way to oppose nihilism lies in the fact that all aspects of reality and of being human have to he places in a balanced relationship and the correct perspective, and that all these have to be evaluated in the right light