the a. Particles of Radium. 339 



through an atom is proportional to the square root of the 

 atomic weight. 



It should, perhaps, be pointed out that this does not mean 

 that the « particle makes the same number of ions out of 

 each atom of the gas through which it is passing ; but merely 

 compares the average per atom. It asserts, for example, 

 that in going through oxygen the a particle makes a number 

 of ions which is four times as many as the number made by 

 the particle moving with the same speed in passing through 

 hydrogen at the same pressure and temperature. 



It is possible that this rule implies that a particle must 

 make more than one pair of ions in passing through any one 

 atom, unless ionization is only rarely the consequence of the 

 encounter. For if only one pair could be made in each 

 atom, and if the a. particle made one pair in crossing each 

 atom of hydrogen, then in oxygen at the same pressure and 

 temperature as the hydrogen, only the same number of ions 

 would be made per centimetre, which is not the case. Nor 

 does it appear possible to suppose that ionization does not 

 occur at a large proportion of encounters ; the number of 

 ions made is of the same order as the number of atoms 

 traversed, according to Rutherford. 



The most reasonable interpretation of these results seems 

 therefore to be (1) that the a particle makes the same number 

 of ions during its course no matter what the gas which it 

 traverses ; ^2) that the energy required to make a pair of 

 ions is always the same ; and (3) that the observed variations 

 in the conductivities in some cases are due to the failure of 

 ions to get free from the molecule in which they are made. 



Under these circumstances we must look for the origin of 

 the square-root law in some disposition which limits the 

 number of ions that can be made in an atom. It is not to 

 depend on any variation of the energy required to make a 

 pair of ions ; but must be due to some condition which only 

 gives the a particle such an opportunity of making ions as is 

 to be measured by the square-root of the atomic weight; 

 and, where it forbids the act of ionization, forbids also the 

 corresponding expenditure of energy. 



We can only make guesses as to what this condition may 

 be. It has been maintained, on good evidence (see Meyer, 

 'Kinetic Theory of Gases/ § 112), that the atom has a 

 disk-like form, and that the various atoms and molecules 

 differ in two dimensions only. It is possible that an ex- 

 planation of the square-root law may be found in the 

 hypothesis that ions can only be formed on the circumference 

 of the atom's disk, for the amount of that portion of the atom 



