velocities. The smallest of these particles, one one-thousandth the size of a hydrogen atom, bear negative charges (or, shall we say, are negative charges) of electricity, and are called electrons. They travel with about 95 per cent, of the velocity of light, penetrate "opaque" bodies, passing easily between and through their atoms, darken a photographic negative, and to a slight extent ionize a gas through which they pass. Streams of these particles constitute the ß rays of radium and other radioactive substances.
The larger particles projected from radioactive bodies are about twice the size of a hydrogen atom, or two thousand times as big as the ß particles. They move more slowly than the latter, and carry a charge of positive electricity. On account of their relatively greater size, they are much more effective ionizers, and correspondingly less penetrating to "opaque" matter than the ß particles. They were named α particles by Rutherford. Streams of them constitute α rays.
Whenever a ß particle, or electron, is started or stopped a penetrating electro-magnetic pulse in the ether (X ray) is developed. Such rays proceeding from radium are called γ rays. In addition to the emission of one or more of these three kinds of rays, radioactive materials give off a radioactive gas, called the emanation. In studying the physiological effects of radium, therefore, we have to consider these four factors — α rays, ß rays, γ rays and emanation.
Our interest in the effects of radium rays on living organisms is enhanced by the discovery that radioactivity is widely distributed in nature. It is probable that all plants and animals are adjusted to a normal degree of radioactivity in their environment, or, in other words, are in a state of radiotonus. Professor J. J. Thomson was the first to discover that air bubbled through Cambridge (England) tap-water became decidedly radioactive, and the subsequent researches of numerous other physicists have taught us that this property belongs to the waters of most deep wells, to mineral waters generally, to freshly fallen rain and snow, to the spray at the foot of waterfalls, to the water of the ocean in certain localities, and quite probably to all spring waters.
After Elster and Geitel found radioactivity a property of the "fango," or mud from the hot springs of Battaglia, in northern Italy, other investigators discovered the same property in mud from various widely separated sources, in lava from volcanoes, in the sediments of springs, the sand of the seashore, and in sedimentary rocks.
The discovery, also made by Elster and Geitel, of the presence of radioactivity in the earth's atmosphere has been abundantly confirmed. Soil-air is more strongly radioactive than air above the surface. Evidence leads to the conclusion that the radioactivity of water, air, mud, rocks, etc., is due to the presence of the emanation of radium and other radioactive substances.
Radioactivity, therefore, must be recognized as a factor of plant