CONSTITUTION OF MATTER RUTHERFORD. 187 



rate of 23,000 horsepower. 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 corre- 

 sponds to an engine working at 128,000 horsepower 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 Aveight of the most powerful known 

 explosive. 



The emission of energy from radioactive substances does not con- 

 trovert 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 ordi- 

 narily 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 radio- 

 active 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 dis- 

 cussed 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 of millions of years. The possibility, however, of 

 altering the rate of transformation of radioactive matter or of induc- 

 ing similar effects in ordinaiy matter does not at present seem at all 

 promising. 



STRUCTURE OF THE ATOM. 



We have seen that in recent years a number of methods have been 

 devised for determining with precision the actual weight of any 

 atom of matter. If it be assumed that in the solid state the atoms, 



