Chemistry and Physics. 571 



plate. The electric field is taken off before the droplet reaches 

 the upper plate and then the droplet falls again. In this manner 

 it is possible to keep the same droplet moving up and down in the 

 field of view of the telescope for many consecutive hours. By 

 observing the times taken by the same droplet to fall, and to rise 

 respectively, through the distance corresponding to the distance 

 between the horizontal cross-hairs in the telescope, it is easy to 

 deduce the speeds attained under the action of gravity alone, and 

 under the excess of the electric over the gravitational fields. This 

 operation is repeated and the speeds checked an indefinite 

 number of times, or until the droplet catches an ion either from 

 among those which exist normally in air, or which have been pro- 

 duced in the space between the plates by any of the usual ioniz- 

 ing agents like radium or X-rays. The fact that an ion has been 

 caught, and the exact instant at which the event happened, is sig- 

 nalled to the observer by the change in the speed of the droplet 

 under the influence of the field. From the sign and magnitude of 

 this change in speed, taken in connection with the constant speed 

 under gravity, the sign and the value of the charge carried by the 

 captured ion are determined. 



" The experiment is particularly striking when, as often hap- 

 pens, the droplet carries but one elementary charge and then by 

 the capture of an ion of opposite sign is completely neutralized so 

 that its speed is altogether unaffected by the field." "In this 

 case the computed charge is itself the charge on the captured ion." 



The final value for the fundamental ionic charge is given as 

 e = 4-891 X 10 -10 electrostatic units. 



The paper contains many other facts which, for lack of space, 

 must be omitted. 



The article closes with a complete list of the most important 

 of the molecular magnitudes as recalculated frome = 4*891 X lO -1 " 

 E. S. U., and with a table of " weights and diameters of mole- 

 cules " for 14 gases and vapors. — Phys. Rev., xxxii, 349. h. s. it. 



V. Homogeneous Rontgen Radiation from Vapors. — "When 

 elements having atomic weights equal to, or greater than, that of 

 chromium are exposed to Rontgen radiation, they emit chiefly 

 homogeneous, secondary X-rays which are characteristic of the 

 element used as radiator, and which are independent of the pene- 

 trating power of the exciting primary beam. The homogeneity 

 of this characteristic radiation causes it to suffer equal percentage 

 absorptions when transmitted through equal thicknesses of alumi- 

 nium. By determining these percentage absorptions the values of 



A 



— , (where 1 = I ( e and p = density of aluminium), have been 



P 



calculated for the elements of the group defined above. 



Since these elements had only been studied in the solid 



state, either pure or in the form of compounds, J. C Chapman 



has extended the investigation to the cases of the elements — 



small in number — which could also be experimented upon in the 



vapor state. 



