Radium, 1 horium, and Actinium in Electric Fields. 367 



values for the other gases are obtained. These are given 

 in column 3. When the values given above for the effi- 

 ciency of recoil in air are divided by 0*93, the efficiency of 

 recoil is given in a form which is independent of the gas 

 iiUo which the atoms are projected and which depends only 

 on the nature and surface of the metal, the values so obtained 

 are: for the brass plates 87*8, for the steel 78*5, and for 

 zinc 79*5. 



It has been assumed in the foregoing that the total 

 amount of RaB recoiling from a plate is independent of 

 the gas in contact with it. It is difficult also to estimate 

 the reliability of the result 93 per cent, for RaB in air, 

 as it is unknown whether reflexion of RaB from the cathode 

 or from residual gas occurs to any large extent during the 

 first exposure. However, it appears from the results found 

 later that the percentage for RaB in any gas is always much 

 greater than that for RaA, and therefore that the value in 

 air for RaB is much greater than 82. 



V. The Distribution of the Active Deposits of Radium, 

 Thorium, and Actinium Radiations in Electric Fields. 



The distribution of the activity on the two electrodes is 

 the result of several distinct processes. 



1. The disintegration of the emanation gives rise to 

 A-atoms, definite fractions of which are positive and neutral 

 at the end of the recoil path. 



2. The positive A-atoms are drawn by the field towards 

 the cathode, and a fraction of them transform on the way 

 into positive or neutral B-atoms. Of the former, owing to 

 the comparatively long life of the B-atoms of the three 

 radioactive series, practically all reach the cathode as 

 B-atoms. The neutral B-atoms reach the electrodes by 

 diffusion. 



3. The neutral A-atoms produced by process (1) diffuse in 

 equal numbers towards each electrode, and a fraction of 

 them transform on the way into neutral or positive B-atoms, 

 the former going to the cathode and the latter diffusing in 

 equal numbers to each electrode. 



The A atoms which have reached the electrodes by 

 process (2) or (3) transform into B-atoms, and a portion 

 of these, depending on the efficiency of recoil from the 

 surfaces, are projected into the gas, a fraction being positive 

 and the remainder neutral at the end of the recoil path. 

 The former are drawn to the cathode, and since the recoil 

 path is a small fraction of a millimetre in these experiments, 



