the Radiations from Radioactive Substances. 7 



results so as to include a variety of substances in the solid and 

 liquid state. 



Absorption of the Rays by Solids and Gases. 



The rays not acted on by a magnetic field can be dis- 

 tinguished from each other by their power of penetrating 

 through thin layers of metal, and their absorption in gases. 

 If, on examination, the penetrating power of two types of 

 radiation proves to be the same in each case for all sub- 

 stances, it is extremely probable that the two radiations are 

 identical. By examining the diminution of intensity of the 

 radiation when sheets of metal of the same thickness are 

 placed over the radioactive substance, the homogeneity or 

 complexity of the radiations can be tested. If the intensity 

 I of the radiation after passing through a distance of metal 

 is given by l e~ xd , where I is the original intensity and X, the 

 coefficient of absorption, we can conclude that the radiation 

 is homogeneous in character. If this condition is not fulfilled 

 the radiation is complex. 



One of us * has at different times given results for the 

 absorption of some of the different radiations in solids and 

 gases. In this paper we have extended the results and 

 compared the different types of radiation under, as far as 

 possible, the same conditions. 



In the case of both uranium * and thorium f it has been 

 shown that the absorption of the radiation is the same for all 

 the different compounds of each element examined. When 

 the types of radiation are complex, the relative amount of 

 rays of different types may vary for different compounds, 

 but so far there is no evidence that the actual radiations 

 themselves are altered. It is only necessary therefore to 

 examine one compound of each element for the purpose of 

 comparison of the types of rays emitted. 



The following substances have been employed in the 

 experiments : — 



Uranium Oxide & Thorium Oxide. — Two different samples 

 of each obtained from Schuchart of Germany and Eimer 

 and Amend of New York gave similar results. 

 Polonium. — This substance was kindly prepared for me 

 by Dr. Walker of McGill University from pitchblende, 

 after the method described in Curie's first paper. Since 

 that time the intensity of the radiation given off has 

 steadily diminished ; but the type of radiation has been 

 unaltered. 



* E. Rutherford, Phil. Mag. Jan. 1899, Feb. and March 1900. 

 t Owens, Phil. Mag. Oct. 1900. 



