C. Barus — Nuclei and Ions in Dust-free Air. 137 



sponding tube was 2 inches in diameter. It is therefore super- 

 fluous to adduce for comparison the new data for water-vapor. 

 The general method of work was that frequently described in 

 connection with these investigations. With the exceptions 

 stated all data, to be at once comparable, must be obtained 

 with a given pair of fog and vacuum chambers. 



3. Properties of alcohol fog. — While the experiments of my 

 last paper with the medium of water- vapor and carbon-dioxide 

 gas showed unusually high values of the exhaustions needed to 

 produce coronal condensation, the case of alcohol air shows 

 correspondingly low values of exhaustion, as compared with 

 those for water-air. The number of colloidal nuclei entrapped 

 by alcohol vapor are about 3*5 times larger than is the case for 

 water-vapor nnder like conditions. Hence the coronas for 

 alcohol are exceedingly dense by contrast. They are also 

 much less regular in color and, particularly at high exhaustions, 

 become fog-like. The phenomenon is coarsened and measure- 

 ment less satisfactory. 



As the alcohol fog particles are larger in size, they subside 

 more rapidly at the same exhaustion than water particles : but 

 the occurrences are in the former case far from simple. While 

 in the earlier experiments the corona (if not too large) remained 

 nearly the same throughout the slow subsidence of water parti- 

 cles, the coronas for water and for alcohol particles in the 

 present work decreased one-half or more in size during this 

 period. In other words, the fog particles now experience very 

 rapid growth* during subsidence, from which it follows many 

 of them must evaporate to compensate in part for the eight- 

 fold or more enlargement in bulk of the survivers, or further 

 vapor may condense. The same fact may account in alcohol, 

 where the phenomenon is more rapid, for the blurred coronas ; 

 for the true initial corona, being very evanescent, is probably 

 not seen. Conformably with this view, it is impossible to 

 exceed large white reddish forms in the present apparatus and 

 to reach the high greens observed with water-vapor. 



4. Nnmoer of particles. — In order to determine the Hum- 

 bert of particles corresponding to a given corona, it is first 

 necessary to compute the amount of alcohol precipitated per 

 cubic centimeter of the exhausted vessel, by the sudden cool- 

 ing incident upon exhaustion. This may be done by a straight- 

 forward approximation^: with results shown in the following 

 table, where t 1 is the initial temperature of the saturated air 

 within the fog chamber, l 2 the temperature after sudden 

 exhaustion and before condensation, and t the temperature 



* The precise reasons for this growth may be of some importance in rela- 

 tion to rain. 



fC. T. E. Wilson: Phil. Trans., London, vol. clxxxix, 1897, p. 300. 



% The size of fog particles in terms of the apertures of the coronas is found 

 as shown in my earlier papers (Smithson. Contrib., No. 1373, Chap. viii). 



