850 Dr. A. F. Kovarik on the Absorption and 



velocity represented by Hp of about 1720, where H is strength 

 of field and p radius of curvature of rays. On account of 

 the long period of RaD this source is constant, and hence 

 very convenient. 



Actinium C, which emits rays whose coefficient of absorp- 

 tion by aluminium is 28*5, corresponding to a velocity repre- 

 sented by Up of about 2150. This was obtained by the recoil 

 method for some experiments and by the use of actinium 

 active deposit for others. 



Radium B, which emits rays whose coefficient of absorption 

 by aluminium is 75"0, corresponding to a velocity represented 

 by Hp of about 1200. This was obtained by the recoil method 

 from radium A. 



Thorium A-t-B + C + D, the thorium active deposit, which 

 emits rays of at least two velocities whose coefficients of 

 absorption by aluminium are 110 and 16*3, which would be 

 represented by Up of about 900 and 2650, respectively. 



Radium active deposit whose rays have a very wide range 

 of velocities. 



With all the sources used except Ra D + E, the work is 

 quite laborious on account of the corrections for the decay 

 since the periods of all of the others are quite short. 



Generally, when the coefficients of absorption are obtained, 

 the material is deposited on metals of considerable thickness. 

 It will be shown in this paper that under such conditions 

 the reflected rays play an important part in the coefficients 

 of absorption. When thick layers of radioactive material 

 are used, the /3-particles from the various depths emerge with 

 various velocities. In order to avoid these complications, the 

 active material used was always in the form of a very thin 

 layer deposited on a very thin aluminium leaf, whose absorp- 

 tion or scattering effects were negligible. 



Scattering, 

 The /3-particles from a uniform thin layer of radioactive 

 material radiate equally in all directions. The measurement 

 of the absorption by thin sheets is generally carried out by 

 placing the radioactive matter some distance below an 

 ordinary /3-ray electroscope, and by placing the absorbing 

 sheets at some distance above the active matter, so that only 

 the more or less parallel rays normal to the sheets are con- 

 sidered. If the absorbing matter is some distance below the 

 opening in the electroscope, the scattering observed by 

 Crowther * produces a steeper incline in the initial portion 

 of the absorption curve than would be expected from the 

 latter portion of the curve. If, however, the thin absorbing 

 * J. A. Crowther, Proc. Roy. Soc, A. vol. lxxx. p. 186 (1908). 



