SUCCESSION OF CHANCJKS IX RADIOACTIVE BODIES. 203 



the other not. The activity of the bismuth plate did not appreciably change during 

 a month's interval.* Part of the active matter obtained from the sulphuric acid 

 solution then Iwhaves in a similar way to the radio-tellurium of MARTKWALD, 

 inasmuch as it gives out only a rays, and is readily deposited on bismuth. In order 

 to test the apparent similarity still further, an accurate comparison was made 

 tatween the penetrating power of the rays from the active bismuth and a bismuth 

 plate of radio-tellurium obtained from Dr. STHAMER, of Hamburg. The rays were 

 found initially to be about half alworbed in a thickness of '00034 centim. of 

 aluminium, and exhibited the characteristic property of the rays of rapidly increasing 

 alworption with the thickness of matter traversed. No appreciable difference in the 

 penetrating power'of the a rays from the two substances was observed, although the 

 intensity of the radiations was reduced to less than 1 per cent, of the initial value. 

 It has been found experimentally that the rays from the different radioactive 

 products differ in penetrating power, and the curve of absorption for different thick- 

 nesses of absorbing material is, in general, a characteristic of each product. 



The identity of the curves of alworption of the rays from the active bismuth 

 and the radio-tellurium, coupled with the similarity of the radiations and chemical 

 properties, is very strong evidence that the active product is in each case the same. 

 I think there can be little doubt that the active constituent of radio-tellurium of 

 MARCKWALD is a disintegration product of radium. 



The polonium (radioactive bismuth) obtained by me from Paris loses its activity 

 fairly rapidly, and the a rays from it are more readily absorbed than the a rays of 

 radio-tellurium. This greater alworption may, however, lie due in part to the fact 

 that the radiations from the polonium come from the mass of the bismuth, and in 

 consequence are made up of rays of widely different penetrating power, while the 

 rays from the radio-tellurium arise from a thin film of matter deposited on the 

 bismuth plate, in which the almorption of the issuing rays would be small. The 

 identity or otherwise of the constituent present in the polonium of Mine. CURIK and 

 the radio-tellurium of MARCKWALD has l>een a much vexed question. A definite 

 answer cannot be given until accurate observations have l>een made of the change of 

 activity with time of these products. 



25. Radioactivity observed in Ordinary Matter. A large number of experimenters 

 have observed that ordinary matter possesses the property of radioactivity to a 

 feeble degree. R. J. STRUTT found that different samples of the same metal showed 

 wide differences in radioactive power. It is a matter of great importance to settle 

 whether the weak radioactivity observed is a property of the substance under 

 examination or is due to a minute radioactive impurity. I think there is little doubt 

 that the radioactivity observed in some substances is due in part to a deposit of 

 radioactive matter on its surface from the atmosphere. It has now been conclusively 



* This bismuth plate was unfortunately mislaid during my visit to England in May. I am, in 

 consequence, unalile to give a more definite statement in regard to the change of the activity with time. 



2 D 2 



