136 C. Bancs — Nuclei and Ions in Dust-free Air. 



Art. XI Y. — Colloidal Nuclei and Ions in Dust-free Air 

 saturated with Alcohol Vapor / by C. Barus. 



1. Introductory. — In my report* on the solutional nucleus 

 and elsewheref I came to the conclusion that the differences 

 in promoting condensation exhibited by positive and negative 

 ions were more probably to be ascribed to the difference in 

 chemical structure or composition involving a difference of 

 size, than to the electrical differences as such. Experiments 

 made in Wilson's apparatus by Dr. Donnanj: with vapors of 

 methyl and ethyl alcohol, carbon tetra-chloride, carbon-disul- 

 phide, benzol, chloro-benzol, show that the super saturation 

 needed to produce condensation was not necessarily greater in 

 ionizing than in non-ionizing solvents. With similar appa- 

 ratus Dr. K. Przibram§ recently examined a series of alcohols 

 and other bodies ionized by the X-rays, obtaining among a 

 variety of data a noteworthy result with a direct bearing on 

 the question here at issue. It appears that whereas in the case 

 of water-vapor the negative ions are more efficient condensa- 

 tion nuclei than the positive ions, the reverse holds for the 

 alcoholic vapors. In cases of methyl, ethyl, amyl and heptyl 

 alcohols (including some other bodies like chloroform) the 

 positive ions invariably require less supersaturation to precipi- 

 tate condensation than the negative ions of the same body. 



Interesting differences are therefore manifest in the behavior 

 of vapors, and it seemed desirable to test the nucleation of a 

 dust-free medium of ethyl alcohol and air in comparison with 

 the media of water-air and water-carbon-dioxide hitherto exam- 

 ined. The former behaves in fact as if the nuclei were through- 

 out larger than in the latter cases. Hence the colloidal nuclei 

 of dust-free wet air should be associated rather with the satu- 

 rated vapor than with the gas. 



2. Apparatus. Method. — The experiments were conducted 

 with an apparatus in which the connecting pipes between the 

 fog chamber (18 inches long, 5 in. in diameter) and the 

 vacuum chamber (5 feet long, 1 foot in diameter) were 4 

 inches in diameter containing a 4-inch counterpoised, plug 

 stop-cock. The whole connecting system was about 22 in. 

 long, one-half of it belonging to the fog chamber. Experiments 

 made with water-vapor, however, did not show any further 

 marked advantage arising from the use of the large passage 

 way specified, over the former apparatus, in which the corre- 



* "Structure of the Nucleus": Smiths. Contrib., No. 1373, 1903, p. 161. 



f " Ions and Nuclei " : Nature, lxix, 1903, p. 103. 



% F. G. Donnan : Phil. Mag. (6), iii, p. 305 to 310, 1902. 



^ K. Przibram : Wien. Sitzungsber., cxv, pt. Ha, p. 1 to 6, 190fi. 



