ISOLATION OF AN ION MILLIKAN. 255 



This behavior was shown consistently by all the drops experi- 

 mented upon (six or eight in number) throughout a period of two 

 days. Imagining that the vapor from the more volatile machine 

 oil upon the plates was condensing into the less volatile but similar 

 oil of the drop I took down the apparatus, cleaned the plates^ care- 

 fully, and oiled them again, this time with the gas-engine oil. Every 

 gas-engine oil drop tried thereafter showed the sort of constancy 

 which is seen in Tables III to XII. Series of observations similar 

 to that made upon gas-engine oil and tabulated in Tables XIII and 

 XIV M'ill ultimately be made upon other substances. Thus far the 

 aim has been to take enough observations upon other substances to 

 make sure that the results obtained from these substances are sub- 

 stantially in agreement with those obtained from gas-engine oil and 

 to concentrate attention upon an accurate series of observations upon 

 one substance. As a matter of fact, we have a fairly complete series 

 upon machine oil and a number of observations upon watch oil, 

 castor oil, and glycerine, all of which are in agreement within the 

 limits of observational error, in some cases as much as 2 or 3 per 

 cent, with the observations upon gas-engine oil. 



The conclusion to be drawn from all of the work thus far done 

 on substances other than oil is merely that there is nothing in it to 

 cast a doubt upon the correctness of the value of e obtained from the 

 much more extended and much more accurate work upon gas-engine 

 oil. 



COMPARISONS WITH OTHER DETERMINATIONS. 



The value of e herewith obtained is in perfect agreement with the 

 result reached by Regener ^ in his remarkably careful and consistent 

 work in the counting of the number of scintillations produced by 

 the particles emitted by a known amount of polonium and measur- 

 ing the total charge carried by these same particles. His final value 

 of this charge is 9,58 X 10"^", and upon the assumption that this is 

 twice the elementary charge — an assumption which seems to be jus- 

 tified by Eutherford's experiments ^ — ^he finds for e 4.79 X lO'^**, 

 with a probable error of 3 per cent. Since the difference between 

 this value and 4.89 X 10~^° is but 2 per cent the two results obviously 

 agree within the limits of observational error. * * * 



[The author then discusses several other determinations of c, and 

 explains some discrepancies which appear.] 



In conclusion there is presented a summary of the most important 

 of the molecular magnitudes, accurate values of which are made 



1 E. Regener, Sitz. Ber. d. k. Preuss. Acad. d. Wiss., 37, p. 948, 1909. 



2 Rutherford, Phil. Mag., 17, p. 281, 1909. 



