The loss of a beta particle causes no change in the atomic weight but does cause a shift for each beta particle of one group toward the right or negative side of the System. Two such losses, then, will counterbalance the loss of an alpha particle and bring the new element back to the group originally occupied by its progenitor. Thus uranium in the sixth group loses an alpha particle and the product UX_{1} falls in the fourth group. One beta particle is then lost and UX_{2} belonging to the fifth group is formed. With the loss of one more beta particle the new element returns to the sixth group from which the transformation began. The table on page 48, as adapted from Soddy, affords a general view of these changes. Isotopes An examination of the table will show a number of different elements falling in the same position in a group of the Periodic System irrespective of their atomic weights. These are chemically inseparable so far as the present limitations of chemical analysis are concerned. Even the spectra of these elements seem to be identical so far as known. This identity extends to most of the physical properties, but this demands much further investigation. For this new phenomenon Soddy has suggested the word isotope for the element and isotopic for the property, and these names have come into general use. [Illustration: RADIO-ACTIVE ELEMENTS FROM URANIUM AND THORIUM PLACED IN THE PERIODIC SYSTEMS Adapted from Soddy] Manifestly, we have come across a phenomenon here which quite eliminates the atomic weight as a determining factor as to position in the Periodic or Natural System or of the elemental properties in general. All of the properties of the bodies which we call elements, and consequently of their compounds and hence of matter in general, seem to depend upon the balance maintained between the charges of negative and positive electricity which, according to Rutherford's theory, go to make up the atom. It is evident that any study of chemical phenomena and chemical theory is quite incomplete without a study of radio-activity and the transformations which it produces. Radio-activity in Nature In concluding this outline of the main facts of radio-activity, it is of interest to discuss briefly the presence of radio-active material on this planet and in the stars. Facts enough have been gathered to show the probable universality of this phenomenon of radio-activity. Whether this means solely the disintegration of the uranium and thorium atoms, or whether other elements are also transformed under the intensity of the agencies at work in the universe, is of course a question as yet unsolved. Radio-active Products in the Earth's Crust