162 G. F. Earlier — Radioactivity of Thorium Minerals: 



trie quartz, one armature of wliieli was connected to the npper 

 plate. By placing weights in a scale-pan attached to the 

 quartz plate, the difference of potential produced was so regu- 

 lated as to secure compensation between the quantity of elec- 

 tricity trayersing the condenser and that of opposite sign 

 yielded by the quartz. With a giyen condenser, and a giyen 

 substance, it was obseryed that the current increased with the 

 difference of potential between the plates, and with the gas 

 pressure. It also increased as the plates were separated. 

 With high potential difference, the current tends toward a 

 constant limit, the saturation-current. This saturation-current 

 was taken as the measure of the radioactivity, the number of 

 ions produced per second being greater in proportion as the 

 radiation absorbed is greater. 



With this apparatus, the radioactivity of uranium is measur- 

 able with considerable precision ; the current having an order 

 of magnitude of 10"^' ampere. • It varied but little with tem- 

 perature, was not affected by light, and did not seem to vary 

 sensibly with time. The value of the current obtained with 

 Moissan's metallic uranium (probably containing a trace of 

 carbon) was 2-4.10"^' amperes. Tested in this way it was 

 found that thorium compounds were very active, thoria sur- 

 passing even metallic uranium. It was observed that the cur- 

 rent increased with the thickness of the material, a layer 0'25 

 millimeter thick giving a current of 2"2.10~'' ampere, and one 

 of 6 millimeters one of 5*3.10"^^ ampere. Thorium sulphate 

 gave a current of only 0*8.10"^' ampere. The phenomenon 

 was most regular with a layer only 0*25 millimeter thick, vary- 

 ing between wide limits when the thickness was 6 millimeters, 

 especially with the oxide. Thorium rays were found to be 

 more penetrating than those of uranium, those emitted by 

 thoria from thick layers being more penetrating than those 

 from thin ones.* Subsequently Owens studied the radiations 

 from thorium compounds, determining the conditions affecting 

 its constancy, the form of the saturation curve, the character 

 of the radiations from different salts, the types of radiation, 

 selectiye absorption, the effect on the rays of suspended par- 

 ticles, the variation of conductivity with air pressure and the 

 absorption of the radiations by air.f 



Inasmuch as uranium and thoriuia were the only elements 

 then known possessing radioactiye properties, it was natural 

 to examine the minerals in which these substances occur, 

 with a yiew to determine their ])roperties in this regard. 

 Accordingly Mme Curie, using the electric method and the 

 apparatus above described, tested thirteen well-known minerals, 



* Eapports au Congres iDternational de Physique, iii, 79, Paris, 1900. 

 f Phil. Mag., V, xlviii, 359, 1899. 



