As a natural philosopher, Count Rumford is best known for his vehement advocacy of a motion or mechanical hypothesis of heat and for the dramatic experiments that he performed to support this hypothesis. Although a motion hypothesis which held that heat was merely the motion of the ultimate particles of a body had a distinguished history, with advocates that included Bacon, Boyle, Hooke, and Newton, most British natural philosophers by the beginning of the nineteenth century believed that the phenomena associated (...) with heat were caused by an imponderable fluid. William Henry reflected contemporary opinion in 1803, when he wrote:The former of these opinions [i.e. the existence of caloric], though far from being universally admitted, is now generally received; and the peculiar body, to which the phenomena of heat are referred, has been denominated by M. Lavoisier [as] caloric. (shrink)

In June 1849 William Thomson wrote to Michael Faraday suggesting that the concept of a uniform magnetic field could be used to predict the motions of small magnetic and diamagnetic bodies. In his letter Thomson showed how Faraday's lines of magnetic force could represent the effect of the ‘conducting power’ for magnetic force of matter in the region of magnets. This was Thomson's extension to magnetism of an analogy between the mathematical descriptions of the distribution of static electricity and of (...) the diffusion of heat through uniform bodies. In 1850 Faraday published his first comprehensive theory of the magnetic properties of matter. He explained the behaviour of matter in the field by four assumptions: that matter has a specific disturbing effect on the normal distribution of lines of magnetic force; that this effect depends on its ability to conduct or transmit the magnetic action; and that material bodies tend to move so as to cause the least possible disturbance of the lines from their normal distribution. Faraday also assumed that diamagnetics transmit magnetic action less well than empty space, while paramagnetics transmit it more readily than space. This implied that space must have a specific conductivity between that of paramagnetic and diamagnetic materials. In order to preserve a distinction between matter and space Faraday defined ‘matter’ as either the source of action or as a conductor which is able to influence the lines of action; space was the absence of such powers. While space could conduct, it differed from matter in that it could neither originale lines of force nor influence their course and distribution. (shrink)

We continue our discussion of the competing arguments in favour of the unified theory and the pluralistic theory of radiation advanced by three nineteenth-century pioneers: Herschel, Melloni, and Draper. Our narrative is structured by a consideration of the epistemic criteria relevant to theory-choice; the epistemic focus highlights many little-known aspects of this relatively well-known episode. We argue that the acceptance of light-heat unity in this period cannot be credibly justified on the basis of common evaluative criteria such as simplicity and (...) standard notions of explanatory power. Whether the consensus was justified by some other criteria remains an open question.Keywords: Theory-choice; Epistemic values; Radiation; Simplicity; Explanatory power; Causal explanation. (shrink)

Based largely on unpublished manuscript material from the Kelvin papers, and especially on a series of letters exchanged in 1867 between Fleeming Jenkin and William Thomson , this paper aims to examine the background and content of the Thomson-Jenkin speculations on the nature of matter. The letters formed an interlude in a long collaboration over electrical patents and raise the fundamental question of whether these speculations, involving the construction of a variety of conceptual models, derive primarily from older traditions of (...) speculative matter theory, or from contemporary problems within natural philosophy qua physics. The correspondence also provided the immediate background to Jenkin's North British review article of 1868 on ‘Lucretius and the atomic theory’. (shrink)

In the late eighteenth century Newton's Principia was studied in the Scottish universities under the influence of the local school of ‘Common Sense’ philosophy. John Robison, holding the key chair of natural philosophy at Edinburgh from 1774 to 1805, provided a new conception of ‘mechanical philosophy’ which proved crucial to the emergence of physics in nineteenth century Britain. At Cambridge the emphasis on ‘mixed mathematics’ was taken to a new level of refinement and application by the introduction of analytical methods (...) in the 1820s. The fusion of these two schools, with emphasis on conceptual unity on the one hand, and mathematisation on the other, came about from the 1830s onwards, and reached full expression in the new framework of a unified mathematical physics based on the energy principle. (shrink)

In 1859 Charles Darwin in chapter nine of the Origin of Species showed how he had calculated that the age of the Weald was three hundred million years and that consequently the age of the earth was considerably greater than that. Darwin of course needed such a long period of time for the process of evolution by natural selection to occur. Arguments which showed that the earth could not be that old would therefore cast serious doubt on his theory. Such (...) views were advanced in 1862 by William Thomson, later Lord Kelvin, professor of Natural Philosophy at Glasgow. He specifically challenged the result of Darwin's calculation of the age of the Weald by arguing that the sun could not have emitted its heat and light for that length of time. The consequences of this assertion for the biological and geological sciences for the remainder of the nineteenth century have already been delineated by Burchfield. What I wish to do in this paper is to show that the theoretical basis of Thomson's 1862 assertion had not been specifically developed as a response to Darwin, but that it was a consequence of the formulation of the first two laws of thermodynamics. I shall also show that Thomson's work was not done in isolation but that the question of the maintenance of solar energy was a serious concern of a number of physicists who had formulated the laws of thermodynamics. (shrink)