Dr. R. D. Kleeoian on 8 Rays. 199 



certain number of free ions and clusters of various com- 

 plexities in the gas. Suppose the gas now subjected to an 

 electric field just sufficiently strong to produce ionizations 

 by collision of the negative ions. The field will seize upon 

 each free electron and give to it as it passes over its mean 

 free path a velocity sufficient to produce ionization by col- 

 lision. But the velocity given to the clusters in the gas 

 would not be sufficiently large to produce ionization by 

 collision. When, however, the electron forming the nuclei 

 of a cluster becomes free, which occurs at the end of its 

 period of life, it is at once seized upon by the electric field, and 

 ionization by collision is produced in the same way as before. 



Since the period of life of a cluster depends on the 

 nature &c. of its collision with the molecules of the gas, it 

 will be affected by the electric field when the velocity given 

 to the cluster by the field becomes comparable to its velocity 

 of translation. An increase in the field would then produce 

 a decrease in its period of life. 



Let us now return to the discussion of the experiments 

 mentioned. Suppose the electric field in a gas is parallel to 

 the direction of propagation of the stream of ionizino- 

 a particles, and that each electron ejected by an a particle 

 has a component of motion in the same direction as the 

 ol particle is moving. Firstly, let us suppose that the direc- 

 tion of the electric field is such as to give a motion to the 

 electron in the same direction as its initial motion. The 

 field will seize upon the electron, and it may have a sufficient 

 velocity given to it before it strikes another molecule to be 

 able to produce ionization by collision. Suppose now that 

 the field is reversed. The field will then have to reduce the 

 electron to rest first before a velocity in the direction of the 

 field can be given to it. But when the electron has little or 

 no velocity it is very liable to attach itself to a neutral 

 molecule. The cluster produced would then have to run a 

 distance A (which depends on its velocity and period of life) 

 before the electron becomes free, after which it is seized 

 upon by the electric field, and possibly ionization by collision 

 produced. The conditions for the production of ionization 

 by collision are therefore less favourable in the latter case 

 than in the former, owing to the ejected electron forming a 

 cluster more readily in the latter case. The effect produced 

 is roughly as if we were to suppose the electron ejected in 

 all directions by the a. particle, but the distance between the 

 electrodes decreased by A in the latter case. 



It should also be pointed out that when the electric field is 

 in such a direction as to give to the electron a motion in the 



