the Electrical Conduction produced by it. 123 



The curve showing the relation between the rate of leak 

 and the thickness of the metal traversed is shown in fig. 2 

 (p. 118), together with the results for uranium. 



It will be seen that thorium radiation is different in pene- 

 trative power from the a radiation of uranium. The radiation 

 will pass through between three and four thicknesses of alu- 

 minium foil before the intensity is reduced to one-half, while 

 with uranium radiation the intensity is reduced to less than a 

 half after passing through one thickness of foil. 



With a thick layer of thorium nitrate it was found that the 

 radiation was not homogeneous, but rays of a more penetrative 

 kind were present. On account of the inconstancy of thorium 

 nitrate as a source of radiation, no accurate experiments have 

 been made on this point. 



The radiations from thorium and uranium are thus both 

 complex, and as regards the a type of radiation are different 

 in penetrating power from each other. 



In all the experiments on uranium and thorium, care was 

 taken that no stray radiation was present which would obscure 

 the results. Such precautions are very necessary when the 

 rate of leak, due to the radiation transmitted through a con- 

 siderable thickness of metal, is only a small percentage of the 

 total. The method generally employed was to cover the layer 

 of active salt with the metal screen, and then place in position 

 over it a large sheet of lead with a rectangular hole cut in it 

 of smaller area than that of the layer of salt. The lead was 

 pressed tightly down, and the only radiation between the 

 parallel plates had to pass through the metal screen, as the 

 lead was too thick to allow any to go through. 



§8. Absorption of Uranium Radiation by Gases. 



The a. radiation from uranium and its compounds is rapidly 

 absorbed in its passage through gases. The absorption for 

 hydrogen, air, and carbonic acid was determined, and was 

 found to be least in hydrogen and greatest in carbonic acid. 

 To show the presence of absorption, the following arrange- 

 ment (fig. 3) was used : — 



A layer of uranium-potassium sulphate or uranium oxide 

 was spread uniformly over a metal plate P, forming a lamella 

 of 11 cm. diameter. A glass vessel Gr, 12 cm. in diameter, was 

 placed over the layer. Two parallel metal plates A and B, 

 1*5 cm. apart, were insulated from each other by ebonite 

 rods. A circular opening 7 cm. in diameter was cut in the 

 plate A, and the opening covered by a sheet of aluminium 

 foil O0005 cm. thick. The plate B was connected through a 



