Prof. Owens on Thorium Radiation. 361 



Becquerel *, and more elaborately by Rutherford f two years 

 later. In May 1898 Schmidt | announced that thorium and 

 its salts gave off a similar radiation. Briefly it was found 

 that such radiations had the power of penetrating consider- 

 able thicknesses of metals and other opaque substances, of 

 acting on a photographic plate, o£ ionizing the gas in the 

 neighbourhood of the active material, and in general possessed 

 properties similar to those of Rontgen rays. 



Rutherford investigated very fully the conductivity pro- 

 duced in different gases by uranium radiation, the absorption 

 of the radiation by different substances, the relative intensity 

 of the radiations emitted by different uranium salts, the 

 velocity and rate of recombination of the ions produced in 

 the surrounding gas, &c, and was enabled to very clearly 

 interpret his results on Thomson's ionization theory of gas 

 discharge. He also found the radiations to be complex, con- 

 sisting of at least two different types, one being readily 

 absorbed by thin sheets of metal foil or layers of gas, and the 

 other, a more penetrating kind, passing through ten to twenty 

 times the same thickness of foil with but a few per cent, 

 diminution of intensity. He also found in comparing the 

 radiations emitted by the different compounds of uranium 

 that the amount of the more penetrating kind as a per cent, 

 of the whole varied with the kind of salt used and with the 

 thickness of the radiating layer for each particular salt. 



When the same method of analysis is used, namely, the 

 screening effect of thin layers of certain metals, similar results 

 may be obtained for thorium, as will be shown later, but 

 there are indications that thorium radiation is not confined 

 to so few distinct types, if indeed the number is limited. 

 Certainly it would be difficult to formulate a theory for the 

 production of such rays which would account for only a 

 particular number of kinds being produced. If „t*-rays and 

 the radiations from uranium, thorium, polonium, &c. are dis- 

 turbances in the eether occasioned by the internal motion of 

 certain constituent parts of the atom, as has been suggested, 

 it might be expected that such disturbances would shade off 

 with some degree of regularity from a more intense to a less 

 intense kind, and such seems to be the case with thorium. 



The principal points in regard to thorium radiation treated 

 of below may be classed under the following heads : — 



1. Conditions affecting the constancy of the radiation. 



2. The relation between current and electromotive force. 



* Comptes Bendits, 1896-97. t Phil Mag., Jan. 1899. 



X Wied. Annal, May 1898. 



