10 ROYAL SOCIETY OF CANADA 



gas, i.e., to the rate of movement of the electrometer needle when a 

 large E.M.F. acts between the plates. 



It was found that layers of this aluminum foil over the active sub- 

 stance cut off the intensity of the radiation gradually, but not 

 so rapidly as in the case of uranium. Tlie decrease of the intensity for 

 the first few layers of aluminum foil follows a regular absorption law. 

 After a certain thickness of foil has been added the intensity of the 

 ^transmitted radiation diminishes very slowly and a much larger pro- 

 portional thickness of metal is required to reduce the intensity still 

 further. These results show that the radiation emitted from thorium 

 is complex, and that different typos of radiation are emitted, some of 

 which are more readily absorbed than others. 



Using aluminum foil, the amount of the more penetrating type of 

 radiation depends largely on the thickness of the layer of the active 

 substance. With thin layers of the radio-active material, the rate of 

 leak due to the more penetrating rays is hardly measurable compared 

 with that due to the more absorbable rays. 



The action of paper in cutting down the intensity of radiation is 

 worthy of remark. The first thickness of ordinary foolscap paper cuts 

 down the intensity of the radiation to about two-thirds of its value. 

 The addition of successive layers changes the intensity but little; an 

 extra layer, several millimetres thick, is quite transparent to the rays. 

 Uranium radiation is in this respect different from thorium radiation, 

 for a few thicknesses of paper are almost opaque to the uranium rays. 



It was found that all the compounds of thorium examined, viz., 

 thorium oxide, thorium sulphate, and thorium nitrate gave out the 

 same kinds of radiation as measured by the transparency of aluminum 

 foil for the rays. The intensities of the radiation differed largely ; for 

 equal weights of active substance being greatest for thorium oxide. 

 The nature of the radiations is thus independent of the particular state 

 of chemical combination of the compound, but depends only on the 

 presence of thorium in the material. 



Thorium radiation is absorbed in its passage through the air, but 

 not so rapidly as the uranium rays. Before an infinite plane of thorium 

 oxide the intensity of the radiation at a distance of 10 mms. from the 

 surface would only be one-half of that at the surface. For uranium 

 there would be the same diminution in about 4 mms. 



By means of a s^pecially constructed apparatus, the absorption of 

 thorium radiation at different pressures was examined. From a pres- 

 sure of one-half to four atmospheres the absorption was found to be 

 directly proportional to the pressure of the gas. The method of 

 measurement was similar to that described in a previous paper. 



