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)

Based largely on unpublished manuscript material from the Kelvin papers, and especially on a series of letters exchanged in 1867 between Fleeming Jenkin and William Thomson , this paper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)