372 Prof. E. Rutherford and Mr. F. Soddy on 



one minute, and for the excited activity a period of eleven 

 hours, causes the activity to fall to half its value. 



These actions — (1) the production of radioactive material, 

 and (2) the dissipation of its available energy by radiation — 

 which are exhibited by thorium compounds in the secondary 

 effects of emanating power and excited radioactivity, are in 

 reality taking place in all manifestations of radioactivity. 

 The constant radioactivity of the radioactive elements is 

 the result of an equilibrium between these two opposing 

 processes. 



II. The Experimental Methods of investigating Radioactivity. 



Two methods are used for the measurement of radioactivity, 

 the electrical and the photographic. The photographic 

 method is of a qualitative rather than a quantitative character; 

 its effects are cumulative with time, and as a rule long- 

 exposures are necessary when the radioactivity of a feeble 

 agent like thoria is to be demonstrated. In addition, Russell 

 has shown that the darkening of a photographic plate is 

 brought about also by agents of a totally different character 

 from those under consideration, and, moreover, under very 

 general conditions. Sir William Crookes (Proc. Roy. Soc. 

 (1900) lxvi. p. 409) has sounded a timely note of warning 

 against putting too much confidence in the indications of the 

 photographic method of measuring radioactivity. The un- 

 certainty of an effect produced by cumulative action over 

 long periods of time quite precludes its use for work of 

 anything but a qualitative character. 



But the most important objection to the photographic 

 method is that certain types of rays from radioactive sub- 

 stances, which ionize gases strongly, produce little if any 

 effect on the sensitive film. In the case of uranium, these 

 protographically inactive rays form by far the greatest part 

 of the total radiation, and much of the previous work on 

 uranium by the photographic method must be interpreted 

 differently (Soddy, Proc. Chem. Soc. 1902, p. 121). 



On the other hand, it is possible to compare intensities of 

 radiation by the electrical method with greater rapidity and 

 with an error not exceeding 1 or 2 per cent. These methods 

 are based on the property generally possessed by all radiations 

 of the kind in question, of rendering a gas capable of dis- 

 charging both positive and negative electricity. These, as 

 will be shown, are capable of great refinement and certainty. 

 An ordinary quadrant electrometer is capable of detecting 

 and measuring a difference of potential of at least 10 -2 volts. 

 With special instruments, this sensitiveness may be increased 



