C. Barus — Changes of Colloidal Nucleation. 203 



ate the present phenomena, let the pressure drop from a given 

 upper limit to varying lower limits as well as from varying 

 upper limits to a given lower limit. The results so obtained 

 are enormously different for the same drop of pressure. Much 

 of this would he anticipated ; but the question nevertheless 

 arises whether the colloidal nucleation of the gas is actually 

 dependent in so marked a degree on its pressure, or whether 

 this dependence can be quite explained away. 



Later in the course of the work, I made additional compari- 

 sons with the cotemporaneous ionizations of the air determined 

 by Miss L. B. Joslin, and with the temperature of the fog 

 chamber as distinguished from the temperature of the atmos- 

 phere. These results, as a whole, finally showed that a direct 

 dependence of the colloidal nucleation of the dust-free air in 

 the fog chamber on the barometer, on the ionization of the 

 air, on any form of external radiation, or on the temperature 

 of the atmosphere, cannot be detected. 



2. Data. — The results* refer to a fog and vacuum chamber 

 the volume ratio of which is about v/V='06, combined with 

 sufficiently wide piping (2 inch bore) and an interposed (2^ 

 inch) plug stopcock. The former communicates with the filter, 

 the latter with the air pump. At the same temperature the 

 fog and vacuum chambers are initially (before exhaustion) at 

 pressures p and p', finally at pressure p 3 , when in isothermal 

 communication after exhaustion ; p 2 and p\, respectively, 

 would be the pressures at the given temperature if the cham- 

 bers could be isolated immediately after exhaustion and before 

 the precipitation of fog. P denotes the barometric pressure, 

 and^? m the initial gauge reading within the fog chamber before 

 exhaustion, so that the drop of pressure is (apart from the 

 moisture contents, which will be treated in turn below) Sp = 

 P—J>va—Pv an d the drop of pressure takes place irova p> — P — 

 p m adiabatically to j?,, isothermally to p t if the fog chamber 

 were isolated as specified, or isothermally to p 3 when fog and 

 vacuum chambers are left in communication. 



For a given value of P the same drop of pressure, 8p, may 

 thus be obtained in two ways : either by giving a suitable value 

 toj^mj i- e - by starting with a partially exhausted fog chamber 

 and a vacuum chamber at fixed exhaustion p', which implies 

 a nearly fixed p 3 \ or (keeping p m constant and nearly zero), by 

 starting with the fog chamber at (nearly) atmospheric pressure, 

 and varying^/ of the vacuum chamber and therefore p> % . 



Briefly then the condensational effects of a given drop, 

 8p, when lying between different pressures'^? and p a , is to be 

 tested, and this is best accomplished by constructing separate 



*The tables will be published by the Carnegie Institution. They are 

 sufficiently reproduced for the present purpose by the graphs of this paper. 



