of the atom may result in a considerable modification of the methods of regarding dynamical problems.
Though what we know about the atom is but a minute fraction of what there is to know, some very important conclusions about atoms have been established on what seems strong evidence in the course of the last few years. We know, for example, that there are such things as atoms, that the atoms of an element are all of one kind, that atoms of different elements contain a common constituent, the corpuscle or electron about which we know a good deal; we know, too, the number of electrons in an atom. We have strong evidence that the electrons in the atom are divided into groups, and that some properties of the atom, those which we associate with the innermost group, are connected in a very simple manner with the total number of electrons in the atom; that there are other properties, notably the chemical ones, which change in a rhythmical way with the atomic weight of the element, and which depend upon the electrons near the surface of the atom. We have evidence, too, that the atoms of the different elements are made up of simpler systems, and that considerable changes in mass have accompanied the aggregation of these systems. Lastly, we know that there are regions in the atom, probably the most interesting of all, about which we know little or nothing, whose investigation will provide intensely interesting work for many generations of physicists, who will most assuredly have no reason to be 'mournful that no new wonder may betide'. No fact discovered about the atom can be trivial, nor fail to accelerate the progress of physical science, for the greater part of natural philosophy is the outcome of the structure and mechanism of the atom.