C. Barus — Measurement of Fog Particles. 169 



explanation is this : while the pressure decrement is growing 

 from zero to the maximum Bp, condensation is taking place on 

 the greater number of particles throughout the whole of this 

 interval. In other w T ords, although the nuclei are graded in 

 size, the greater number exceed a certain dimension and require 

 almost no pressure decrement to induce condensation. These 

 are the particles (diameter exceeding a certain inferior limit) 

 which give character to the persistent corona. A minority of 

 the graded particles are below the dimension in question and 

 upon these condensation does not take place until the higher 

 values of the pressure difference are reached, some may even 

 require the full pressure decrement, Sp. Thus it is that in the 

 deposit of fog particles, one finds those of diameter 'OOl 0111 

 intermixed with others of smaller diameter, even as far as 

 •0002 cm or less. When fresh phosphorus nuclei are first intro- 

 duced into the condensation chamber the result is a grey fog, 

 but a relatively small white reddish corona is nevertheless dis- 

 eernable. Accordingly the crop of droplets seen under the 

 microscope contains not only surprisingly small but also rela- 

 tively large droplets, with all intermediate diameters. Hence 

 the indefinite fog and the small corona. The large olive (g b p) 

 corona and other of the early coronas are very apt to fade into 

 a Coarse white reddish corona. This is the evaporation of the 

 smaller particles into the larger, which accounts moreover, for 

 the loss of nuclei during the first precipitation, to be caught in 

 subsequent exhaustions. The successive coronas in a series 

 gradually become sharper and the larger particles more uni- 

 form, but extremely fine particles are still present even when 

 one approaches the normal coronas. The fine particles, how- 

 ever, belong to coronas so large and diffuse that their coronal 

 effect scarcely modifies the strong coronas of the large particles, 

 even before the former vanish by evaporation. 



When I first observed these different sizes of drops caught 

 on a single plate, it seemed not improbable that a difference of 

 the condensational effect of the negative and the positive ions 

 might here be actually in evidence ; but as all intermediate 

 sizes are present at the outset, and particularly as large and 

 small droplets still appear together long after all electrification 

 has certainly vanished, this conclusion is not warranted. What 

 continually favors uniformity is subsidence of fog. As the 

 phosphorus nuclei are graded, it is probable that the very fine 

 droplets are due to the initial or primitive nuclei from which 

 the larger nuclei have grown by cohesion ; or the fine droplets 

 may be due to air nuclei associated with the phosphorus nuclei. 

 All this will appear in a more minute photographic study of 

 the subject with which I am now engaged, and it will then 

 be further interesting to decide whether the nuclei generated 



