4i2 



SCIENCE 



[N. S. Vol. XXXII. No. 822 



changes whicli correspond to the addition or 

 subtraction of 2 or 3 times this amount. The 

 conclusion, however, that valency is exliibited 

 in gaseous ionization is not to be so easily 

 drawn. The arguments for it which are fur- 

 nished by our experiments will be presented 

 fully elsewhere. Space here only permits the 

 statement that the only strong argument fur- 

 nished by table I. is found near the end of the 

 table where, when the field was on, the drop 

 caught a douhle negative ion, while I was 

 loohing at it. 



Some idea of the intensity of ionization 

 used in these experiments may be gained from 

 the statement that during the observations 

 recorded in the first half of the table, a closed 

 tube of radium, containing 500 mg. of radium 

 bromide of activity 3,000, stood about five feet 

 away from the testing chamber, so that its 

 y rays could enter this chamber. At the end 

 of the observations in the group in which 

 G = 23.14, this radium was brought up to 

 within a few inches of the testing chamber, 

 and six elementary charges were forced upon 

 the drop in a manner which will presently be 

 explained. The radium was then taken en- 

 tirely out of the room, so that the changes re- 

 corded in the last haK of the table are entirely 

 due to such ionization as exists in air under 

 normal atmospheric conditions. 



There is but one more comment to be made 

 upon table I. At a point indicated in the 

 table by the remark " change forced with 

 radium," it will be noticed that the charge 

 was suddenly changed from eleven negative 

 units to five negative units, i. e,, that six posi- 

 tive units were forced upon the drop. This 

 sort of a change was one which, after the phe- 

 nomenon had once been got under control, we 

 could make at will in either direction; i. e., 

 we could force charges of either sign or in any 

 desired number, witliin limits, upon a given 

 drop. We did this as follows: when it was 

 desired to load the drop up negatively, for 

 example, we held it with the aid of the field 

 fairly close to the positive plate, and placed 

 the radium so that it would produce uniform 

 ionization throughout the chamber. Under 

 these conditions, if the positive and negative 



ions were alike in number and mobility, the 

 chance that the drop would catch a negative 

 ion would be as many times its chance of 

 catching a positive ion as the distance from 

 the drop to the negative plate was times the 

 distance from the drop to the positive plate. 

 Similarly, if we wished to load the drop posi- 

 tively it was held by the field close to the 

 negative plate. On account of the slightly 

 greater mobility of the negative ions and also 

 on account of the somewhat greater numbers 

 in which they occur, we found, in general, a 

 slightly greater tendency of the drops to take 

 up negative than positive charges. In view, 

 therefore, of the greater ease with which nega- 

 tive drops could be held for long intervals 

 without being lost to the plates most of the 

 drops studied have been of negative sign. 



§ 5. The Failure of Stohes's Law.- — When 

 the values of e^ were computed, as above, for 

 different drops, although each individual drop 

 showed the same sort of consistency which was 

 exhibited by the drop of table I., the values of 

 e^ at first came out difl^erently even for drops 

 showing the same value of the velocity under 

 gravity. This last irregularity was practically 

 completely eliminated by blowing the drops 

 into air which was strictly dust-free, but even 

 then drops of diiierent sizes as determined by 

 Vj always gave consistently different values of 



tabm; n 



Negative drop No. 5 



Distance between cross hairs = 1.303 cm. 



Temperature =:: 24.6° C. 



Density of oil at 25° C. = .9041 



.01085 



Mean &, (weighted) =5.490 



