168 



C. Barus — Measurement of Fog Particles. 



upon the disc before and after the exposure, as was actually 

 observed. The difficulty may be removed by adding a capsule 

 above the plate or simply by decreasing the distance between 

 the shield and the plate to a millimeter or less. Finally if the 

 oil film is semi-fluid and not quite fixed, if there is slight 

 creeping as was often the case, the particles are redistributed 

 after falling along stream lines where they cohere in strings 

 and bunches, usually without coalescing. This was also 

 observed, and in fact the capillary forces involved are apt to be 

 strong enough to counteract viscosity. 



6. Data. — I have not thus far spent much time in correct- 

 ing these defects, chiefly because the new results for the diam- 

 eters of fog particles are more immediately interesting. Some 

 data are given in Table I. 



If the diameters measured are plotted in a chart, together 

 with the results computed from successive exhaustions and 

 coronal aperture s in the older (curve b) and in the more recent 

 (curve a) experiments, the present values again lie between the 

 two curves but now nearer the lower (recent) curve than before, 

 § 4. I shall not pause to interpret the differences which 

 remain, but only to remark that the capillary forces at the area 

 of contact of the droplet, even with the liquid oil film, may 

 transform it to an oblate spheroid and that diffraction at the 

 circular edges of the drops is not excluded. Micrometric data 

 cannot be smoother because the particles are not of a size for 

 the same corona. If the nucleation w , obtained from succes- 

 sive isothermal exhaustions and subsidence measurement, be 

 accepted as correct (lower curve a), the ratios of the nuclea- 

 tion found from the different methods tested will be 



From subsidence, a ='0029, d / d =l'0 n/n =l-0 



from lycopodium (d =-0032 cm ),a= -0034, " =1*2 n / n = -61 



from diffraction (blue), a ='0034, " =1-2 n / n a = *61 



from micrometer measurement, a =*003 7, " =1*3 n/n = *48 



old results (adiabatic conditions assumed), " =1*6 n / n Q = *24 



Since n is obtained from the cube of d, large differences of this 

 kind are as yet inevitable, particularly as the particles meas- 

 ured in these different cases are not the same. 



7. Graded nuclei. — The point of special interest which 

 comes out on using the eccentric plate to catch the subsidence 

 during 15 or 30 seconds, and at once examining the deposit, is 

 the result that particles of all sizes are present. By far the 

 greater number, however, have the maximum diameter. These 

 particles are caught from the fog without interference and it is 

 not probable that coalescence or evaporation have been appre- 

 ciably operative, so long as the corona remains the same 

 throughout the micrometer measurement. The probable 



