EQUATIONS OF FOG CHAMBER. 55 



millimeters of p'\ and p s . It follows, therefore, that even an approach 

 to isothermal pressure can not be discerned at the fog chamber at all, 

 to say nothing of adiabatic pressure; or that before the exhaust cock 

 can be closed again, the vacuum chamber has practically regained its 

 isothermal pressure by cooling and that the fog chamber is further 

 exhausted by a corresponding amount. The pressure p\=p" 2 observed 

 under isothermal conditions* at the fog chamber exceeds p t (computed) 

 by about 1.9 cm. on the average, which might be regarded as the average 

 vapor pressure of water at the temperature at which the observation was 

 made. Leaving this for further consideration, the final result of impor- 

 tance is the following: p 2 , the computed isothermal pressure in the 

 closed fog chamber, is from 2 to 5 cm. above the observed (nominally) 

 isothermal pressure p'i=p 2 (observed), and correspondingly more than 

 this above the common isothermal value (p 3 ) usually taken. For the 

 region in which colloidal nuclei lie the correction will be in excess of 

 the difference between the pressure regions containing Wilson's data for 

 colloidal nuclei, as reduced elsewhere,! and the region in which my own 

 data as summarized below 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 compare in efficiency if the drop of pressure taken is not the 

 apparent experimental value but that deduced for the computed isother- 

 mal pressure (p 2 ) of the fog chamber, as above explained. 



47. The same, continued. Case of air in the fog chamber saturated 

 with water vapor. It will next be necessary to compute the above 

 data with allowance for the vapor pressure of water in the fog chamber, 

 supposing the vacuum chamber to be dry, which may seem to be in a 

 measure true, since it is heated by the same transfer of air which cools 

 the fog chamber. Hence in the summary, if tz is the vapor pressure of 

 water, the equations relating to the fog chamber have to be modified to 



x _/ P~n \ 



*1 \Pi Kl/ 



(k-c) /k 



2 . 



P TC = RpT 

 Pl 7t l = Rp 1 T l 



p2~x = Rp 2 z = Rp i r 



Ps-t: =Rp 3 z p 3 =p' 3 =R P ' 3Ta pv + p l V = etc.=p 3 v + p' 3 V 



*This pressure varies but slightly. /*C\tilC 



fPresidential address, Physical Review, xxu, 1906, p. 107. /t^^^ *?* 



? 



&\ ***** J 



