C. Barus — Drop of Pressure in Fog Chamber. 341 



value <p 3 usually taken. For the region in winch colloidal 

 nuclei lie the correction will be 6 to 8 cm . Now this is in 

 excess of the difference between the pressure regions in which 

 Wilson's data for colloidal nuclei, as reduced elsewhere,* and 

 the region in which my own data as summarised heretofore, 

 would lie. 



In other words, the data in my large coronal apparatus lie in 

 regions of exhaustion at least as moderate as those observed 

 in Wilson's small apparatus ; or the two types of apparatus 

 comparing efficiency if the drop of pressure taken is in my 

 case not the (apparent) experimental value, but that deduced 

 for the computed isothermal pressure^ of the fog chamber as 

 above explained. 



4. For the case of air saturated with water vapor in both 

 chambers, all pressures must be reduced by the corresponding 

 vapor pressure, ir of water, except p\, when the vapor is 

 slightly superheated. Apart from this, the equations take the 

 above form, though special computation is needed, since a dif- 

 ferent initial pressure (p~7r) enters. So computed, the rela- 

 tion between the observed drop of pressure jp— p 3 and the com- 

 puted drop j?— p 2 was found to be 



(P-P,)/(P—P,) = ' 7 ' 75 

 and very nearly constant with the pressure interval involved. 

 The conclusions as to efficiency are like the above. 



The fact that a limit has been reached for condensations, 

 within the given type of fog chamber, may be considered as 

 proved, apart from comparison with Wilson's results, since for 

 a successively increasing drop of pressure {p—jp 3 ), no matter 

 whether the nuclei are relatively large like the ions or rela- 

 tively small like the colloidal nuclei, the same terminal corona 

 is eventually reached in both instances. Higher exhaustions 

 are thereafter powerless. Finally, since the colloidal nuclei in 

 case of dust-free air saturated with alcohol vaporf are larger 

 than in case of water vapor (caet. par.), these nuclei must 

 probably be associated with the saturated vapor, the gas being 

 but secondarily in question. 



Brown University, Providence, E. I. 



* Presidential address ; Physical Eeview, xxii, 1606, p. 107. 

 f This Journal, August. 



