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



ThX was divided into two equal parts ; one was exposed for 

 20 hours to the heat of a Bun sen burner in a platinum 

 crucible, and then compared with the other. No difference 

 in the activities was observed. In a second experiment, one 

 half was ignited for 20 minutes on the blast, and then com- 

 pared with the other with the same result. The difference of 

 temperature and the conversion of thorium hydroxide into 

 oxide thus exercised no influence on the activity. 



Some experiments that were designed to test in as drastic 

 a manner as possible the effect of the chemical condition of 

 the molecule on the rate of production of ThX brought to 

 light small differences, but these are almost certainly to be 

 accounted for in another way. It will be shown later 

 (section JX.) that about 21 per cent, of the normal radio- 

 activity of thorium oxide under ordinary conditions consists 

 of a secondary activity excited on the mass of the material. 

 This portion is of course a variable, and since it is divided 

 among the total amount of matter present, the conditions of 

 aggregation, &c, will affect the value of this part. This 

 effect of excited radioactivity in thorium makes a certain 

 answer to the question difficult, and on this account the 

 conclusion that the rate of production of ThX is independent 

 of the molecular conditions is not final. The following ex- 

 periment, however, makes it extremely probable. 



A quantity of thorium nitrate as obtained from the maker 

 was converted into oxide in a platinum crucible by treatment 

 with sulphuric acid and ignition to a white heat. The de- 

 emanated oxide so obtained was spread on a plate, and any 

 change in radioactivity with time, which under these circum- 

 stances could certainly be detected, was looked for during 

 the first week from preparation. None whatever was 

 observed, whereas if the rate of production of ThX in thorium 

 nitrate is different from that in the oxide, the equilibrium 

 point, at which the decay and increase of activity balance 

 each other, will be altered in consequence. There should 

 have therefore occurred a logarithmic rise or fall from the 

 old to the new value. As, however, the radioactivity 

 remained constant, it appears very probable that the changes 

 involved are independent of the molecular condition. 



It will be seen that the assumption is here made that the 

 proportion of excited radioactivity in the two compounds is 

 the same, and for this reason compounds were chosen which 

 possess but low emanating power. (Compare section IX. last 

 paragraph.) 



Uranium is a far simpler example of a radioactive element 

 than thorium, as the phenomena of excited radioactivity and 



