160 



C. Barus — Measurement of Fog Particles. 



Art. XII. — Direct Micrometric Measurement of Fog Par- 

 ticles ; by C. Barus. 



1. Introductory. — Before using the data computed for the 

 dimensions of fog particles in the reductions of my observa- 

 tions of atmospheric nucleation,* it seemed expedient to 

 endeavor to obtain corroborative values by some straightfor- 

 ward method. Aitken's dust counter had naturally suggested 

 itself ^arly in the course of my work ; but the results so 

 obtained are essentially indirect as the fog particles are not 

 themselves observed. It was necessary, therefore, to devise 

 apparatus by which the identical fog particles of a given corona 

 could be directly entrapped and held for examination. This 

 was eventually accomplished in a way admitting both of the 

 measurement of the diameters of the particles and, probably, of 

 counting the number precipitated under known conditions. 

 Moreover, the particles caught, however fine (even less than 

 •0003 cm in diameter), can be kept in place for observation, so 

 that microscopic photography is applicable not only for the 

 purpose^ of obtaining size but (possibly) number. 



Many investigations are thus suggested, as, for instance, a 

 repetition of Thomson's method for determining the charge 

 of an ion ; an experimental test t>f the subsidence equation 

 for small spherules, etc. Again, while the corona gives merely 

 the average size of the cloud particles, the microscope is partic- 

 ularly available for indicating variations of diameter for the 

 particles of the same corona. In fact, the water particles when 

 caught are not of a size ; they are graded and hence the nuclei 

 are probably also graded in size. 



2. Apparatus. — Aitken's beautiful and highly ingenious 

 instrumentf is well adapted for the work for which it was 

 designed ; but it will not subserve the present purposes. 

 Aitken's droplets evaporate too rapidly in spite of their arti- 

 ficially increased size. The need of mixing atmospheric air 

 and dust-free air with shaking and stirring is an interference 

 with the nucleation. In fact, water nuclei may even be gen- 

 erated in this way, possibly by the friction of air passing across 

 damp surfaces. There is the tendency of the plate after long 

 use to fog permanently or to collect droplets on its own 

 account. Finally, it would be very difficult to remove the con- 

 tents of the coronal chamber to the dust counter without 

 changing the nucleation during the transfer. 



I therefore endeavored to ascertain whether the particles 



*Phys. Keview, xvii, p. 233, 1903; cf. ibid., xvi, 193, 246, 1903. 

 f My thanks are due to Mr. A. L. Rotch, who was good enough to lend me 

 his dust counter. 



