212 PROFESSOR E. RUTHKRFORD ON THE 



with helium. These compounds break up spontaneously and at a very slow rate even 

 in the case of radium. The disintegration takes place in successive stages, and at 

 most of the stages a helium atom is projected with great velocity. This disintegration 

 is accompanied hy an enormous emission of energy. The liberation of such a large 

 amount of energy in the radioactive changes at once explains the independence of the 

 rate of change on the physical and chemical agencies at our command. On this view, 

 uranium, thorium and radium are in reality compounds of helium. The helium, 

 however, is held in such strong combination that the compound cannot be broken up 

 by chemical or physical forces and, in consequence, these bodies behave as chemical 

 elements in the ordinary accepted chemical sense. 



It appears not unlikely that many of the so-called chemical elements may prove to 

 be compounds of helium, or, in other words, that the helium atom is probably one of 

 the secondary units with which the heavier atoms are built up. 



30. The Charge Carried by the a. Rays. It is of great importance to determine as 

 directly as possible the total number of a particles expelled from a known weight of 

 radium in order to deduce the number of atoms which break up per second. The 

 most direct method of determining this number is to measure the positive charge 

 carried off by the a rays. Assuming that the charge of the particle is equal in 

 magnitude to that carried by the ions in gases, the number of a particles expelled pel- 

 second can at once be determined. 



A thin film of radium was obtained on a plate by evaporation of a radium bromide 

 solution of known strength. Some hours after evaporation, the activity of radium 

 measured by the a rays is about 25 per cent, of its maximum value, and the /8 rays 

 are almost completely* absent. The arrangement of the experiment is shown in 

 fig. 16. 



The active plate A was insulated in a metal vessel, C, and was connected to one 

 pole of the battery, the other pole being earthed. The upper plate B was insulated 

 and connected to a Dolezalek electrometer. The outside vessel C could be connected 

 to either A or B, or to earth. By means of a mercury pump, the vessel C was 

 exhausted to a very low pressure. If the a rays carried a positive charge, the current 

 between the two plates measured by the electrometer should be greater when A is 

 charged positive than when. A is negative. No appreciable difference, however, 

 between the currents in the two directions was observed even when a very good 

 vacuum was obtained. In addition, it was found that the current between the plates 

 at first diminished rapidly with the pressure, but reached a limiting value which was 

 not altered by further lowering the pressure. These results are very difficult of 

 interpretation. There is no doubt that the a particle behaves as a positively charged 

 body, inasmuch as it is deviated in a magnetic and electric field in the opposite 

 direction to the ft particle. The failure to detect the charge carried by the rays is 

 probably due in part to a strong secondary ionization set up by the a rays at the 

 surface of the plates A and B. The current due to this effect does not diminish with 



