Septekbeb 30, 1910] 



SCIENCE 



441 



mous speed to the plates as soon as they are 

 formed, their velocities in the fields here used 

 being not less than 10,000 cm. per sec. Hence 

 an ion can not be caught when the field is on 

 unless the molecule which is broken up into 

 ions happens to be on the line of force run- 

 ning from the plates through the drop. With 

 minute drops and relatively small ionization 

 this condition is very unlikely to occur. When 

 the field is off, however, the ions are retained 

 in the space between the plates and sooner or 

 later, one or more of them, by virtue of its 

 energy of agitation, makes impact upon the 

 drop and sticks to it. 



These considerations lead up to assertion 4 

 in the introduction. It will be seen from the 

 readings in the first half of the table that even 

 when the drop had a negative charge of from 

 12 to 17 units it was not only able to catch 

 more negative ions, but it apparently had an 

 even larger tendency to catch the negatives 

 than the positives. Whence then does a nega- 

 tive ion obtain an amount of energy which 

 enables it to push itself up against the exist- 

 ing electrostatic repulsion and to attach itself 

 to a drop already strongly negatively charged? 

 It can not obtain it from the field, since the 

 phenomenon occurs when the field is not on. 

 It can not obtain it from any explosive proc- 

 ess which frees the ion from the molecule at 

 the instant of ionization, since again in this 

 case, too, ions would be caught as well, or 

 nearly as well, when the field is on as when it 

 is off. Here then is an absolutely direct proof 

 that the ion must he endowed with a Mnetic 

 energy of agitation, which is sufficient to push 

 it up to the surface of the drop against the 

 electrostatic repulsion of the charge already 

 on the drop. 



This energy may easily be computed as fol- 

 lows: As wiU appear later the radius of the 

 drop was in this case .000197 cm.; further- 

 more, the value of the elementary electrical 

 charge obtained as a mean of all of our ob- 

 servations, is 4.902 X 10"^". Hence, the energy 

 required to drive an ion carrying a unit 

 charge up to the surface of a charged sphere 

 of radius r, carrying sixteen elementary 

 charges, is 



16e'_16X (4.901X10-'°)' 

 r "' .000197 



= 1.95X10-" ergs. 



Now the kinetic energy of agitation of a 

 molecule as deduced from the value of e here- 

 with obtained, and the kinetic theory equa- 

 tion, p = imnu', is 5.7.56 X 10"^* ergs. Ac- 

 cording to the Maxwell-Boltzmann law, which 

 doubtless holds in gases, this should also be the 

 kinetic energy of agitation of an ion. It will 

 be seen that the value of this energy is ap- 

 proximately three times that required to push 

 a single ion up to the surface of the drop in 

 question. If, then, it were possible to load up 

 a drop with negative electricity until the po- 

 tential energy of its charge were about three 

 times as great as that computed above for this 

 drop, then the phenomenon here observed, of 

 the catching of new negative ions by such a 

 negatively charged drop, should not take place, 

 save in the exceptional case in which an ion 

 might acquire an energy of agitation consid- 

 erably larger than the mean value. Now, as 

 a matter of fact, it was regularly observed that 

 the heavily charged drops had a very much 

 smaller tendency to pick up new negative ions 

 than the more lightly charged drops. And in 

 one instance Mr. Fletcher and myself watched 

 for four hours a negatively charged drop of 

 radius .000658 cm., which carried charges 

 varying from 126 to 150 elementary units, 

 and which therefore had a potential energy of 

 charge (computed as above on the assump- 

 tion of uniform distribution) varying from 

 4.6X10"" to 5.47X10"" ergs, and in all 

 that time this drop picTced up hut one single 

 negative ion, and that despite the fact that the 

 ionization was several times more intense than 

 in the case of the drop in table I. This is 

 direct proof, independent of all theory, that the 

 order of magnitude of the kinetic energy of 

 agitation of a molecule is 3 X 10"^*, as the 

 kinetic theory demands. 



The first portion of assertion 5 is directly 

 proven by the readings contained in the table, 

 since the great majority of the changes re- 

 corded in column 4 corresponds to the addition 

 or subtraction of one single elementary charge. 

 The second portion of the assertion seems at 

 first sight to be proven by the remaining 



