456 Prof. Rutherford and Mr. Soddy on the 



consequence about 20 per cent, in three days, and remained 

 constant at the higher value. 



In the converse experiment the thorium hydroxide was 

 kept for three days in liquid air, i. e. under conditions where 

 its emanation is condensed and produces the excited activity 

 in the compound itself. It was spread on a plate and its 

 activity found to decrease about 12 per cent, after a few days. 

 These results are to be expected if the rate of production of 

 emanation is constant and independent of chemical or physical 

 conditions. When the emanation is prevented from escaping, 

 its activity, and also the excited activity produced by it, 

 cause an increase in the intensity of the radiations emitted. 

 In the second case, the activity decreased, since some of the 

 emanation escaped from the compound at ordinary tempera- 

 tures, and, in consequence, some of the excited activity 

 deposited in the compound gradually decayed. 



§ 5. The Radiations of Badium. 



Radium, like thorium and uranium, emits two types of 

 radiation, the a, or easily absorbed rays (deflectible in very 

 intense magnetic fields), and the /3, or penetrating rays, 

 readily deviated in a magnetic field. It also emits some very 

 penetrating rays, which, however, have not yet been fully 

 investigated. The non-separable activity of radium, which 

 remains after the emanation and excited activity have been 

 removed, consists only of a rays, the /3 radiation being less 

 than 1/200 of the amount normally present. In this respect 

 the three radio-elements are analogous. 



The radiation from the radium emanation was tested by 

 introducing it in a cylinder made of copper-sheet 'C05 cm. 

 thick, which absorbed all the a rays and allowed the (3 rays 

 to pass through with but little loss. The external radiation 

 from this cylinder was determined at intervals commencing 

 about 2 minutes after the introduction of the emanation. 

 The amount at first observed was extremely small, but in- 

 creased rapidly and reached a practical maximum in 3 or 4 

 hours. Thus the radium emanation also only gives a. rays, 

 the /3 rays appearing after the latter has changed into the 

 excited activity. On sweeping out the emanation from the 

 cylinder by a current of air there was no appreciable decrease 

 of the radiation immediately, but the radiation commenced 

 to decay rapidly with the time, falling to half value in about 

 30 minutes. A similar result has been obtained by P. Curie. 



Attention has been called (Rutherford, Phil. Mag. Jan. 

 1903) to the irregular character of the curves of decay of 

 both thorium- and radium-excited activity, as measured by 



