Page:Popular Science Monthly Volume 84.djvu/621

Rh rising to give a hearty vote of thanks to the lecturer.

The speaker began by enumerating some of the better known properties of radium and the radioactive products, such as the rate of decay of these substances, ranging from five thousand million years for the half period of uranium down to a few minutes for some of its descendants. He mentioned also the enormous energy of radioactive disintegration, showing that one pound of a radioactive product, if one could gather so much, has the explosive energy of ten million pounds of nitroglycerin. He dwelt also upon the experimental methods developed by Mr. P. T. R. Wilson by which the expulsion of alpha particles and beta particles can be made visible and even photographed. The first part of the path of the alpha particles is nearly straight, but as they lose energy, contact with molecules of other substances deflects them in a characteristic manner. The beta particles, on account of their greatly inferior mass, pursue very irregular courses.

Less well known are the results obtained by Moseley on the interference spectra of X-rays produced by reflection from crystals, especially that of rock salt. These spectra are capable of being photographed and are vastly more simple and more regular than those obtained from visible light. These spectra evince a regularity among the elements which does not appear in the more familiar light spectra, and these regularities tend to elucidate the nature of the atom.

Lord Kelvin conceived an atom as composed of negative electrons included within a space charged with positive electricity holding the electrons together in a single body. This, however, appears to be inconsistent with the researches of Rutherford, who has developed a theory of nuclear atoms, according to which a central nucleus of extremely high potential is surrounded by negative electrons whose motion it controls. If so, the electrons are controlled by the nucleus very much as the planets are held to the solar system by gravitation, and indeed there appears strong reason to suppose that the force involved is really inversely proportional to the square of the distance as in the case of gravitation. From this point of view, the various elements are characterized by the number of electrons in the atom. Each electron carries a single negative charge, and the nucleus carries as many positive charges as there are electrons to be controlled. This theory of the atomic constitution explains the irregularity in the movement of alpha particles through a gas. When an alpha particle approaches a nucleus carrying a charge of millions of volts, it is sharply deflected and may appear even to rebound in the direction from which it came. Sir Ernest illustrated this by a fine experiment. Similarly, if there were a small hole drilled through the center of the earth, a ball dropped from the surface would go straight down and come straight back almost as if it had been infinitely elastic and rebounded from an impenetrable surface.

It is posiblepossible [sic] to determine the number of positive charges contained in each one of the elements from hydrogen to uranium, and it seems also that if the elements are appropriately arranged the charges increase by unit steps, so that hydrogen contains a single positive charge and uranium 92. This assumption corresponds to the actual elements with a small but very important exception. In the series of 92 possible charges, there are just three gaps, corresponding, presumably, to three unknown elements, and at the same time the relationship of these unknown elements to the known elements is made clear, so that the chemists have preliminary information to guide them in the search for the missing links. This is a wonderful advance on the periodic system of Mendelèef which has itself been fruitful in the discovery of elements.

writer of the article on "The Small College and its President" which appealed in the May number of