“Perhaps,” remarks a writer in _The Scientific American_ (February, 1922), “there will come a time when we shall use the energy in the atoms to drive our machines, cook our food and heat our rooms. Besides, already today we are actually using--even if only a very tiny part--the atomic energy. Thus, for instance, the rays emanating from radium are used for therapeutic purposes and the electrons emanating from a glowing filament can be directed so easily that they can be used in a large number of apparatus for wireless telegraphy and telephony. Most probably plants also make use of this energy in their growth because it has been demonstrated that the rays of the sun liberate electrons from the green leaves, and lastly it may also be mentioned that we humans use a little of this intra-atomic energy when seeing with our eyes, which we are enabled to do by the photoelectric action of light.”[A] During the course of the process of disintegration, atoms of uranium and thorium and their products give rise to no fewer than 36 different substances (A. S. Russell), and of these at least a dozen are “new elements.” All of the 36 radioactive elements are disintegration products of one or the other of the two parent elements, uranium and thorium. They are arranged by the chemist in three series: namely, Uranium 1, Uranium 2 (the Actinium Series), and Thorium. In the first series there are known to be 15 transmutations of matter; in the second, 11; and in the third, 10. The periods of “half change”--the period required for one-half of a given quantity of a radioactive element to decompose--of the different radioactive elements vary all the way from thousands of millions of years for the longest lived primary elements--2.6x10^{10} years for thorium, 8x10^9 for uranium 1--to .002 second for actinium A. In the case of radium itself, 1,670 years are demanded for the disintegration of half of any portion, according to the exact measurements of Profs. B. O. Boltwood and Ellen Gleditsch. The stable end product appears to be in each case an _isotope_ of lead--leads having similar chemical properties but of different _atomic weights_ (_i.e._, different atomic composition). [A] See Shipley, Maynard, “Electricity and Life,” ch. vi., Little Blue Book No. 722. Isotopes are groups of elements which cannot be distinguished (or separated from) one another by any known chemical methods, and which differ only in the atomic weights of the members of the group. In the radioactive groups, the various elements differ also in degree of stability of their atoms.