440 Charge on the Ions 'produced in Air by Rontgen Rays. 



fallen in about 6 sees, in the above experiment. If we 

 suppose that two droplets, one with a positive charge and the 

 other with a negative charge, coalesced, which of course is a 

 probable thing to happen, the resulting droplet with twice 

 the mass and no charge ought to have fallen in 11*2 sees., 

 which is very nearly the mean time (10*6 sees.) taken by the 

 quickest set to fall. However, it is not easy to see how 

 droplets coalescing could produce a set of drops having a 

 sharp upper limit, for we should expect coalescence to occur 

 from time to time during the existence of the cloud. The 

 existence of a sharp upper surface to the set seems to show 

 that all the droplets forming it were formed at the moment 

 of the expansion. 



Another possible explanation of these sets seems to be that 

 when the cloud is formed some of the droplets contain more 

 than one ion. If two ions were very near together during 

 the expansion they might easily give rise to only one droplet. 

 An objection to this view is that such a droplet ought to be 

 larger than one containing a single ion. This objection, 

 however, falls to the ground when the magnitude of the 

 effect of the charge on the equilibrium size of the droplets is 

 remembered, for it is known to be very small. 



If we suppose that the droplets in the three sets are all of 

 the same size, but have different charges, then it is easy to 

 calculate these charges. The results of this calculation for 

 the observations given above are 



Principal set, charge per droplet 2*04 x 10~ 10 

 Second set, ,, 3*94 „ 



First set, „ 6*94 „ 



If, then, the principal set has one ion per droplet, the second 

 has two, and the first about three. 



It has been shown by Townsend (Phil. Trans. A. 1899, 

 p. 129) that the charge on an ion produced in air by Rontgen 

 rays or by other forms of radiation is equal to the charge on 

 the hydrogen ion or atom in solutions. According to the 

 result of the present experiments it consequently follows that 

 the charge on one hydrogen atom is 3'1 x 10~ 10 E.S. unit or 

 10 ~ 20 of an electromagnetic unit. One E.M. unit of elec- 

 tricity deposits from a solution O'OlllS gram of silver in 



electrolysis, and consequently - = 1*043 x 10 -4 gram 



of hydrogen. It follows that the mass of an atom of hydro- 

 gen is approximately 10~ 4 x 10 —20 = 10~ 2i of a gram. The 

 mass of a molecule of hydrogen is therefore 2 X 10 -24 of a 

 gram, so that since the mass of one cubic centimetre of 



