Actinium in an Electric Field. 555 



We may also assume that the recoil atoms effect the process of 

 ionization by some sort of collision with or bombardment of 

 the gas molecules, expelling negative electrons from the mole- 

 cules with much greater initial velocity than that imparted to 

 the main bulk which forms the positive ion. We may picture 

 to ourselves, therefore, the recoil deposit atom moving all the 

 time in and at the head of a column the core of which con- 

 sists of an overwhelming majority of positive gas ions and the 

 outer layer is constituted of negative electrons. Now, what- 

 ever the charges acquired by the deposit particles at their 

 formation, it is only reasonable to expect that travelling in 

 this core, lined on all sides almost exclusively with positively 

 charged gas ions, they should possess a greater probability of 

 emerging from the recoil columns with a positive charge than 

 otherwise. If the positive charge is acquired by the combina- 

 tion of the deposit atoms with the positive ions of the core, 

 then the resulting positively charged deposit particle assumes 

 molecular dimensions and becomes a cluster under favorable 

 circumstances; this agrees very well with the conclusions 

 arrived at from the discussion of the curves of fig. 3. Further, 

 the more complete the initial exclusion of the negative electrons 

 from the columns, the less will be the number of deposit atoms 

 emerging from the columns as neutral particles and hence the 

 higher the limiting value of the percentage of the cathode 

 deposit. It is also evident that no negatively charged deposit 

 particles, even if they existed initially in great numbers, could 

 emerge as such from the recoil columns. 



The supposition here involved is that recoil atoms of differ- 

 ent initial velocity impart different amounts of energy to the 

 electrons which they expel in the act of ionization, and as a 

 consequence, for a very short time, during the motion of recoil, 

 a separation of positive and negative columns of ions actually 

 occurs. The fact that the limiting value of the cathode 

 deposit for actinium is higher than that for radium is 

 exactly what would be expected when we consider that 

 the recoil atom from actinium A is expelled with a greater 

 velocity than that from radium A. Moreover, since the recoil 

 deposit atom of thorium has a velocity intermediate between 

 that of radium and actinium, we would expect to find the 

 limiting value for the cathode percentage of the thorium 

 active deposit to lie between 88*2 and 94*9. Experiments on 

 the distribution of the active deposit of thorium are now in 

 progress and will soon be published. 



In conclusion I wish to express my gratitude to Prof. E. M. 

 Wellisch for suggesting this problem and for his continual 

 interest and advice throughout the course of the investigation. 

 I am also indebted to Prof. Boltwood for his helpful sugges- 

 tions. 



Sloane Physical Laboratory, Yale University, New Haven, Conn., 

 May 18, 1914. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 228. —December, 1914. 

 38 



