112 Prof. E. Rutherford on Uranium Radiation and 



Becquerel* examined the opacity of glass for uranium 

 radiation in the solid and also in a finely-powdered state by 

 the method of electric leakage, and found that, if anything, 

 the transparency of the glass for the radiation was greater in 

 the finely-divided than in the solid state. 1 have repeated 

 this experiment and obtained the same result. As Becquerel 

 stated, it is difficult to reconcile this result with the presence 

 of refraction. 



Polarization. — An arrangement very similar to that used 

 by Becquerel was employed. A deep hole was cut in a thick 

 lead plate and partly filled with uranium oxide. A small 

 tourmaline covered the opening. Another small tourmaline 

 was cut in two and placed on top of the first, so that in 

 one half of the opening the tourmalines were crossed and in 

 the other half uncrossed. The tourmalines were very good 

 optically. The photographic plate was supported 1 to 3 mm. 

 above the tourmalines. The plate was exposed four days, and 

 on developing a black circle showed up on the plate, but in 

 not one of the photographs could the slightest difference in 

 the intensity be observed. Becquerel f stated that in his 

 experiment the two halves were unequally darkened, and 

 concluded from this result that the radiation was doubly 

 refracted by tourmaline, and that the two rays were unequally 

 absorbed. 



§3. Theory of Ionization. 



To explain the conductivity of a gas exposed to Rontgen 

 radiation, the theory % has been put forward that the rays in 

 passing through the gas produce positively and negatively 

 charged particles in the gas, and that the number produced 

 per second depends on the intensity of the radiation and the 

 pressure. 



These carriers are assumed to be so small that they will 

 move with a uniform velocity through a gas under a constant 

 potential gradient. The term ion was given to them from 

 analogy with electrolytic conduction, but in using the term 

 it is not assumed that the ion is necessarily of atomic dimen- 

 sions ; it may be a multiple or submultiple of the atom. 



Suppose we have a gas between two plates exposed to the 

 radiation and that the plates are kept at a constant difference 

 of potential. A certain number of ions will be produced 

 per second by the radiation and the number produced will in 

 general depend on the pressure of the gas. Under the electric 



* C. It. 1896, p. 559. t C. R. 1896, p. 559. 



% J. J. Thomson and E. Rutherford, Phil. Mag, Nov. 1896. 



