170 PROFESSOR E. RUTHERFORD ON THE 



when most of the emanation present would be carried off. On the other hand, the 

 solution containing the ThX gives off a large quantity of emanation, showing that 

 the power of giving an emanation belongs to the product ThX. Now it is found 

 that the quantity of emanation given off by the separated ThX decreases according 

 to an exponential law with the time, falling to half value in four days. The rate of 

 production of emanation thus falls off according to the same law and at the same rate 

 as the activity of the ThX measured in the ordinary manner by the rays. Now 

 this is exactly the result to be expected if the ThX is the parent of the emanation, 

 for the activity of ThX at any time is proportional to its rate of change, i.n., to the 

 rate of production of the secondary type of matter by the emanation in consequence 

 of a change in it. Since the rate of change of the emanation (half transformed in 

 1 minute) is very rapid compared with the rate of change of ThX, the amount of 

 emanation present will be practically proportional to the activity of the ThX at any 

 instant, i.e., to the amount of unchanged ThX present. The observed fact that the 

 hydroxide regains its power of emanating in the course of time is due to the 

 production of fresh ThX by the thorium, which in turn produces the emanation. 



In a similar way, excited activity is produced on bodies over which the emanation 

 is passed, and in amount proportional to the activity of the emanation, i.e., to the 

 amount of the emanation present. This shows that the active deposit, which gives 

 rise to the phenomenon of excited activity, is itself a product of the emanation. The 

 evidence thus seems to be conclusive that ThX is the parent of the emanation and 

 that the emanation is the parent of the deposited matter. 



3. Each of these radioactive products is marked by some distinctive chemical and 

 physical properties which differentiates it from the preceding and succeeding products. 

 For example, ThX behaves as a solid and is soluble in ammonia, while thorium is not. 

 The thorium emanation behaves as a chemically inert gas and condenses at a 

 temperature of 120 C. The active deposit from the emanation behaves as a solid 

 and is readily soluble in sulphuric and hydrochloric acids and is only slightly soluble 

 in ammonia. The two emanations and the products, UrX and ThX, lose their 

 activity according to an exponential law with the time, and at a rate that is 

 independent as far as observation has gone of the chemical and physical agents at 

 our disposal. The time taken for the radioactivity of each of these products to fall 

 to half its value is thus a definite physical constant, which serves to distinguish it 

 from all other products. 



On the other hand, the ' excited activity ' produced in bodies by exposure in the 

 presence of the thorium and radium and actinium emanations does not decay according 

 to a simple exponential law. The variation of the excited activity with time is very 

 complicated, especially in the case of radium, and is dependent on the time of exposure 

 of the body in the presence of the emanation. 



It will be shown in the paper that the complicated rate of decay of the excited 

 activity of the thorium, radium, and actinium is due to the fact that the deposited 



