Helium, on account of its chemical inactivity and physical properties, is classed along with argon, neon, krypton, and xenon in the zero group of the Periodic System, and forms with them the monatomic, inert gases. In this class are now placed also the three radio-active gases, emanating respectively from radium, thorium, and actinium. These are generally known as radium emanation, thorium emanation, and actinium emanation. The first mentioned was once called niton. Emanium was the name originally proposed by Giesel for the body now known as actinium. The calculated rate of production of helium in the series in equilibrium with one gram of radium is 158 cubic millimeters per year. This corresponds quite well with the experimental results. Table of Constants Some of the more important atomic and radio-active constants are given in the following table. They are recorded here to show how helpful the study of radio-activity has been in working out the composition of matter, and to give some idea of the magnitude of the numbers and the minuteness of the quantities dealt with. Electric charge carried by each H atom in electrolysis 4.65 × 10^{-10} e.s.[1] Electric charge carried by each [alpha] particle 9.3 × 10^{-10} e.s. Number of atoms in 1 gram of H 6.2 × 10^{23} Mass of 1 atom of H 1.6 × 10^{-24} gram Number of molecules per cc. of any gas at standard pressure and temperature 2.72 × 10^{19} Number of [alpha] particles expelled per second per gram of radium itself 3.6 × 10^{10} Number of [alpha] particles expelled per second per gram of radium in equilibrium with its products 14.3 × 10^{10} [1] The expression 10^{-10} means multiplying by .000,000,000,1; 10^{10} means multiplying by 10,000,000,000. CHAPTER V THE STRUCTURE OF THE ATOM Properties of Radium A study of the properties of radium will aid in throwing light upon the question as to the building up of the atom. First to be considered are the usual properties which distinguish an elementary body. Metallic radium has been prepared by a method similar to that used in the preparation of barium. It is a pure white metal, melting at 700°, and far more volatile than barium. It rapidly alters on exposure to the air, probably forming a nitride. It energetically decomposes water and the product dissolves in the water. Its atomic weight is 226.