Page:Popular Science Monthly Volume 84.djvu/619

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most notable features of the annual meeting of the National Academy of Sciences held, in Washington in the third week of April were two lectures by Sir Ernest Rutherford, the distinguished physicist of the University of Manchester. These are the first of a series of lectures, established by the children of William Ellery Hale, which are to treat problems of evolution from the atom to man. At the autumn meeting of the academy, stellar evolution will be reviewed by Dr. W. W. Campbell, director of the Lick Observatory. Sir Ernest's lectures, which were reported stenographically, will be printed in Science and ultimately in a book with the other lectures of the series. It is almost impossible to represent their contents by an abstract, but in view of the great interest and importance of the subject and the originality of some of the experiments and theories, it may be well to attempt to give an outline.

Sir Ernest began by reminding his audience that the idea that matter is composed of very minute discrete particles incapable of subdivision and therefore called atoms was familiar to the Greeks, atom in their language being equivalent to the indivisible. This idea was little developed until the beginning of the last century when Dalton first applied it to the chemical constitution of compounds showing that each separate element such as oxygen, iron, nitrogen, etc., always combines in a certain definite equivalent proportion. This is in fact the basal conception of modern chemistry and renders it possible to derive the composition of a substance from a chemical analysis. Chemistry thus lent exceedingly strong support to the hypothesis of the atomic constitution of matter, but no further advance in the subject was made till about the middle of the century when the so-called kinetic theory of gases was developed by Clausius and Maxwell. This theory accounts for the pressure and other properties of gases by supposing them constituted of single atoms, or of small groups of atoms united into molecules, moving with an amount of energy which is proportional to the temperature. The mathematical developments of this theory and the conclusions to be drawn from the formulas have been verified in cases so very numerous that no one now or for a long time has doubted the essential correctness of the underlying hypothesis. In spite of the conviction that the kinetic theory was true, it was for long supposed that no actual demonstration of atomic or molecular structure could ever be reached. Of late years, however, the study of an almost forgotten phenomenon called the Brownian movement has led to actual demonstrations. Brown as long ago as 1827 observed that microscopic spores of plants suspended in a liquid were in constant motion. The smaller the particles the more actively they were displaced while their movements were thoroughly irregular, the spores passing one another in opposite directions or intersecting one another's paths without any general drift such as would have resulted from ordinary currents in the fluids due, for example, to differences of temperature. Of late years this Brownian movement has been studied with great precision by M. Perrin and others. It has been established that particles sufficiently small, say one ten thousandth of an inch in diameter, are really displaced in a random manner by the vastly smaller invisible molecules of the fluid in which they are suspended, and that the movements of the