SPECULATIONS as to the constitution of matter have occupied an important place in the development of scientific knowledge. The idea that all matter was composed of minute particles called atoms was put forward long ago by the Greek philosophers, and was advanced again with varying degrees of confidence by philosophic men at the dawn of the scientific age. For example, Newton suggested that matter was composed of atoms which were likened to "hard massy balls" while Robert Boyle regarded a gas to consist of atoms which were in brisk motion. The first definite formulation of the atomic theory as a scientific hypothesis was given by Dalton of Manchester in 1803 in order to explain the combination of atoms in multiple proportion. The necessity of distinguishing between the chemical atom and the chemical molecule was soon recognized, while the famous hypothesis of Avogadro that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules still further extended the usefulness of the theory. The whole superstructure of modern chemistry has been largely reared on the foundations of the atomic theory. The labors of the chemist have revealed to us the presence of more than eighty distinct types of elements, each of which has a characteristic atomic weight, and in most cases sufficiently distinct physical and chemical properties to allow of its separation from any other element by the application of suitable methods.
It has been generally assumed that all the atoms of one element are identical in shape and weight, and until a few years ago were supposed to be permanent and indestructible. The close study of the variation of chemical properties of the elements with atomic weight led Frankland and Mendelief to put forward the famous "periodic law," in which it was shown that there was a periodic variation in the chemical proper
- First course of lectures on the William Ellery Hale Foundation, National Academy of Sciences, delivered at the Washington Meeting, April, 1914.