Electricity and Matter¹

Abstract

IT has been customary in the earlier Kelvin lectures to give an account of some, phase of Kelvin's work. I could easily follow this custom by concentrating on the publications of Kelvin that deal with the proof of the atomic nature of matter and the dimensions of atoms and molecules, including the first suggestions of the mechanism of atomic constitution. This was a subject in which Kelvin was permanently interested. In his Royal Institution lecture of 1883, reprinted in “Popular Lectures and Addresses,” vol. 1, he gives an illuminating account of the different lines of evidence that all converge to a cumulative proof that matter is coarse-grained or atomic in structure and set a definite minimum limit to the dimensions of the atom. His deduction of the diameter of the water molecule from the cooling effect observed when a water film is stretched, is one of the most notable of these examples. In his later papers he accepts Stoney's arguments in support of the atomic nature of electricity, and in a paper of curious title, “Æpinus Atomised,”² he restates the old theory of ^Epinus of the nature and relation of positive and negative electricity in a more modern form, by assuming that the negative electricity in an atom is distributed in the form of definite units called “electrions”—or electrons, as we should now term them—held in equilibrium embedded in a sphere of uniform positive electrification. This was the first type of model atom put forward. A similar type of atom, developed and worked out in detail by Sir J. J. Thomson, played a notable part in giving a concrete view of atomic structure which was directly amenable to mathematical calculation. In some of his later papers, Kelvin devised types of atoms which, under certain disturbances, broke up with explosive violence, simulating in behaviour the atoms of radium. While keenly interested in such speculations, there remained the curious anomaly that he did not accept entirely the current explanation that radio-activity was a consequence of the successive disintegrations of atoms.