Arrhenius (1906b, pp. 154 and 225) still clung to the aether hypothesis, which refers to the unspecified material medium of space. Arrhenius’ adherence to this hypothesis remained firm in spite of its sound refutation by Michelson & Morley (1887). This leaves the conceptual underpinning of radiation in Arrhenius’ “Greenhouse Effect” to Tyndall (1864, pp. 264-265; 1867, p. 416), who ascribes communication of molecular vibration into the aether and communication of aethereal vibration to molecular motion. This interaction conceptually separates radiated heat from conducted heat so that radiation remains separate and distinct from conductive heat flow – effectively isolating conductive heat flow from the radiative mode of heat transfer. Thus no consideration is made for internal radiative transfer as a part of conductive transfer, in the context of aethereal wave propagation. However, Arrhenius’ contemporaries, having moved beyond the debunked aether hypothesis, had a much more realistic perspective of the interactions between radiation, heat, and subatomic particles.
During the life of Arrhenius’ “Greenhouse Effect”, the scientific community understood that radiation was electromagnetic (Maxwell, 1864; Heaviside, 1881; Hertz, 1888), and by the time Arrhenius first published on the subject of the “Greenhouse Effect”, Thompson (1896) had extended his idea of electrons to photoelectric effects on gases due to ionizing radiation, known then as röntgen rays. The photoelectric effect, by which a current or charge could be generated in certain materials by their exposure to electromagnetic radiation, was a matter of inquiry at the time. The emission of radiation in discrete quanta, though first suggested by Boltzmann in 1877, was mathematically formalised by Planck (1901). Einstein (1905) experimentally confirmed Planck’s Equation after adapting it to the photoelectric effect, which was the subject of his study. However, ideas concerning the internal structure of the atom and it’s relationship to ionisation, magnetism, photoelectric interactions, and discrete quanta of electromagnetic radiation were under intense development at the time (Thomson, 1902; Thomson, 1903; Thomson, 1904). By the time Bohr (1913) corrected the problems in Thomson’s atomic model, the relationship between changes in electron shell (i.e. orbit) potential and photoelectric emission of radiation were a foregone conclusion. The relevance of these discoveries to the question of heat transfer is that unlike the notion of aethereal heat transfer, emission of electromagnetic energy quanta by atoms and molecules in materials confirmed that the radiative mode of heat transfer was as much a part of thermal conduction as any other mode of heat transfer.
In order to understand how heat moves through materials, we must first examine the structure and behaviour of the material media at a sub-atomic level. An atom comprises a nucleus within a shell. The shell is due to “Thomson’s corpuscles”, later known as electrons, which are negatively charged particles that orbit a nucleus with a positive charge corresponding to the number of these electrons. These orbital paths are also known as electron shells and, when shared by more than one atom, electron shells form the chemical bond between those atoms. When a “photon”, or rather an electromagnetic wave pulse, passes through the electron shell -which is the region defined by the corresponding mathematical function called an orbital- one of a number of things may occur. It may pass through the “shell”, it may be deflected by the “shell”, or it may be absorbed by an electron in the “shell”. When an electromagnetic wave pulse or ‘photon’ of light or heat is absorbed by an electron, the energy imparted to the electron is converted to kinetic energy, which moves the electron out to an orbital level commensurate with the energy gained. If we consider, from the mass of both electron and nucleus, that the centre of mass is somewhere between the electron and the nucleus, then this centre of mass does not coincide with the centre of positive charge, about which the electron orbits. Imagine a circumstance in which this centre of mass remains static, while the nucleus revolves around it. As the electron shell is centred on the nucleus, then in this case the shell and the entire atom or molecule is thus seen to wobble or vibrate about a particular point. The higher the electron shell, the more intense this wobble or vibration becomes. As a consequence, the absorption of electromagnetic radiation by a material manifests itself as what appears to be a corresponding net increase in the kinetic energy of constituent molecules.
If we take the processes we have just examined and apply them to more than one molecule, we may then perceive as Waterson (1843, 1846, 1892) did, that through collisions between molecules, the material must either expand or its internal pressure will increase. By this we may infer the kinetic propagation of heat through a medium by the collision of its molecules, as the momentum of one molecule is transferred to another in the collision. This is not the only consequence of molecular collision. Such a collision may transfer the kinetic energy from an electron of the inbound molecule to an electron of the outbound molecule. It is also possible that the collision may destabilise one or both electron shells resulting in the corresponding drops to lower electron potentials. When an electron falls to a lower orbit or electron shell of lesser potential, a “photon” or pulse of electromagnetic radiation is emitted. That electromagnetic wave pulse then propagates through the material until it is either absorbed by another molecule or escapes from the material. However short-lived, such radiation quanta carry a proportion of heat flow in all materials. Whether we are talking about air, glass, or steel, a component of internal heat transfer is via internal radiation, however short the path of that radiation may be. Ergo, thermal conduction is not solely the kinetic transfer of heat, but also the transmission and reception of radiation within a material or materials in thermal contact. This is confirmed by the fact that conductive heat transfer, as defined by Fourier (1822), is only concerned with total heat flow and therefore describes the sum of both radiative and kinetic transfer without addressing either specifically. This differs markedly from the separation of radiative and kinetic transfer implicit in the ethereal model of heat transfer proposed by Tyndall and favoured by Arrhenius. This divergence of Arrhenius’ idea of heat transfer from the facts of contemporary science forecasts a major error in Arrhenius’ thermodynamics.
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