368 Mr. Briggs on Distribution of Active Deposits of 



it is held that all the neutral B-atoms return by diffusion to 

 the surface from which they are projected. 



Direct recoil on to the electrodes is negligible in the 

 experiments of Wellish, Henderson, and the author. The 

 magnitude of the effect produced by processes (2) and (3) 

 depends on the transformation constants of the A-atoms of 

 the three radioactive series of elements. ISTo transformations 

 after that of A- into B-atoms need be considered, since /3-recoil 

 of C-atoms is entirely negligible in these experiments, and 

 the products of the (J-atoms contribute a neuligible amount 

 to the ionization by which the activity of the electrodes is 

 measured. 



In a theory which will be given later, it is held that the 

 last few collisions at the end of the recoil path determine 

 whether the recoil atom will be positive or neutral. If this 

 is so, the percentage initially positively charged in a gas 

 will be the same for RaA, ThA, and AcA, since they are 

 isotopes, although their initial velocities are different. In 

 the same way, the percentages for RaB, ThB, and AcB will 

 be identical and independent of whether the transformation 

 takes place in the body of the gas or if the atom recoils into 

 the gas from an electrode. 



The fraction of radium and thorium positive A-atoms 

 which transform into B-atoms before reaching the cathode 

 is negligible in the experiments described here. The fraction 

 of the neutral A-atoms which so transform in a parallel plate 

 vessel may be found as follows : — In section III. it was 

 proved that the diffusion of neutral atoms between the 

 central disks is very approximately linear along the £-axis. 



Let D be the coefficient of diffusion of the A atoms, and 

 X their transformation constant, q and z have the same 

 meanings as before. 



Let n be the number of neutral A-atoms per c.c. at a 

 distance z along the ^-axis. 



We have 7Q 



dz 2 = Xll ~ < i' 



Writing y = \n — q, the equation becomes 



dz< 

 the solution of w T hich is 



i>53 = * 



l \ 2 ^ 



COsl1 1 Di (* +a J j = ^' 



where a x and a 2 are constants. 



