Efficiency of the Plug- Cock Fog- Chamber. 357 



to the relative absorption o£ metal and air. But this again 

 is opposed to the experiment of Levin (Phys. Zeit. Aug. 

 1906), who found that equal layers of foil dropped the 

 ionization cuiwe by equal amounts ; and to a similar experi- 

 ment described by Kucera and Masek themselves (Phys. Zeit. 

 xviii. p. 630) . 



The following figures are taken from measurements by 

 Kleeman and myself : — 



Drop of curve when sheet of metal is placed 1*5 cm. 

 above Ra. 





Drop for 

 EaC. 



Drop for 

 EaA. 



Drop for 

 Emanation. 



Drop for 

 Ea. 



1. Al 



231 

 T92 

 1-91 



•86 

 1-63 



2-25 

 1-77 

 1-81 



•81 

 1-46 







TG7 



•50 

 1-36 



•73 



2. Sn 



3. Sn 



4. Ag 



5. An 



XXYI. The .Efficiency of ths Plug-Cock Fog- Chamber. 

 By Carl Barus *. 



1. TNTRODUCTION.—Iv the last few years I have 

 had occasion to use the fog-chamber extensively for 

 the estimation of the number of colloidal f nuclei and of 

 ions in dust-free air under a great variety of conditions. 

 These data were computed from the angular diameter of the 

 coronas of cloudy condensation ; and it is therefore necessary 

 to reduce all manipulations to the greatest simplicity and to 

 precipitate the fog in a capacious vessel, at least 18 inches 

 long and 6 inches in diameter. To obtain sufficiently rapid 

 exhaustions it is thus advisable to employ a large vacuum- 

 chamber, and the one used was about 5 feet high and 1 foot 

 in diameter. The two vessels were connected by 18 inches 

 of brass piping, the bore of which in successive experiments 

 was increased as far as 4 inches ; but 2-inch piping, pro- 

 vided with a 2J-inch plug stop-cock, sufficed to produce all 



* Communicated by the Author. 



t See Smithsonian Contributions, No. 1309, 1901; No. 1373.1903; 

 No. 1651, 1906. Carnegie Publications, No. 40, 1906; No. 62, 1907 

 (forthcoming). 



