motion of the swift alpha and beta particles into heat, and is thus in a sense a secondary effect of the radiations. The enormous emission of energy is most simply illustrated by considering the case of the radium emanation together with its swiftly changing products, radium , radium and radium . The heating effect of a given volume or weight of this gas has been accurately determined. From the data, it can be calculated that one pound weight of the emanation would emit heat energy initially at the rate of 23,000 horse power. The rate of emission decreases with the time, falling successively to half value after intervals of 3.85 days. During the life of the emanation the total energy emitted corresponds to an engine working at 128,000 horse power for one day. Such a quantity of emanation would be an enormously concentrated source of power, for the total energy emitted is many million times greater than for an equal weight of the most powerful known explosive.
The emission of energy from radioactive substances does not controvert the law of the conservation of energy; for the energy is derived from the atom itself where it exists in kinetic or potential form. We shall see later that the atom is believed to consist of a large number of positively and negatively charged particles which are collected in a very small volume and held together by intense electrical forces. Such an idea of atomic structure involves the necessity of a large store of energy resident in the individual atom. The great emission of energy from a radioactive substance like the emanation illustrates in a striking way the enormous reservoir of energy that must exist in the atoms themselves; for there is every reason to believe that an equivalent amount of energy is present in the atoms of the common heavy elements. This store of energy ordinarily does not manifest itself and is not available for use. It is only when there is a drastic rearrangement of the atom resulting from an atomic explosion that part of this store of energy is liberated.
It must be borne in mind that the processes occurring in radioactive matter are spontaneous and uncontrollable. There is at present no evidence to indicate that we shall be able in any way to influence radioactive changes. We are at present only able to watch and investigate this remarkable phenomenon of nature without any power of controlling it. In a recent book, H. G. Wells has discussed in an interesting way some of the future possibilities if this great reservoir of energy resident in the atoms were made available for the use of man. This will only be possible on a large scale if we are able in some way to alter the rate of radioactive change and to cause a substance like uranium, or thorium, to give out its energy in the course of a few hours or days instead of over a period of many thousands or millions of years. The possibility, however, of altering the rate of transformation of
