.PRESENT PROBLEMS OF RADIOACTIVITY 165 



X rays could be generated of the same penetrating power as that of 

 the f rays. 



The results so far obtained thus generally support the view that 

 the f rays are a type of penetrating X rays. This view is in agreement, 

 too, with theory, for it is to be expected that very penetrating /? rays 

 should always appear with the /? rays. 



No evidence of the emission of a type of X rays is observed from 

 active bodies which emit only a rays. If the a particles are initially 

 projected with a positive charge, such rays are to be expected. Their 

 absence supplies another piece of evidence in support of the view 

 that the a particle is projected without a charge, but acquires a 

 positive charge in its passage through matter. 



V. Emission of Energy by the Radioactive Bodies 



It was early recognized that a very active substance like radium 

 emitted energy at a rapid rate, but the amount of this energy was 

 very strikingly shown by the direct measurements of its heating 

 effect made by Curie and Laborde. They found that one gram of 

 radium in radioactive equilibrium emitted about 100 gram calories 

 of heat per hour. A gram of radium would thus emit 896,000 gram 

 calories per year, or over 200 times as much heat as is liberated by 

 the explosion of hydrogen and oxygen to form one gram of water. 

 They showed that the rate of heat emission was the same in solution 

 as in the solid state, and remained constant when once the radium 

 had reached a stage of radioactive equilibrium. Curie and Dewar 

 showed that the rate of evolution of heat from radium was unaltered 

 by plunging the radium into liquid air, or liquid hydrogen. 



It seemed probable that the evolution of heat by radium was 

 directly connected with its radioactivity, and the experiments of 

 Rutherford and Barnes proved this to be the case. The heating effect 

 of a quantity of radium bromide was first determined. The emana- 

 tion was then completely driven off by heating the radium, and con- 

 densed in a small glass tube by means of liquid air. After removal 

 of the emanation, the heat evolution of the radium in the course of 

 about three hours fell to a minimum corresponding to one quarter 

 of its original value, and then slowly increased again, reaching its 

 original value after an interval of about one month. The heat emis- 

 sion from the emanation tube at first increased with the time, rising 

 to a maximum value about three hours after its introduction. It then 

 slowly decreased according to an exponential law with the time, 

 falling to half value in about four days. If Q max is the maximum 

 heating effect of the emanation tube, the heat emission Q at any 

 time t, after the maximum is reached, is given by 



Q = Q max e^ f 

 where A is the radioactive constant of the emanation. 



