340 C. Bar us — Drop of Pressure in Fog Chamber. 



where j?, p f ,p^ are observable with certainty. While equation 

 (5) is variously useful in checking the results, it does not 

 admit of the individual determination of p 2 and^/ 2 . For this 

 purpose, however, the equations (1) and the second and third 

 of group (2) are available, with the results (for dry air) 



p=zp(k-c)/kpc/.* p' -~p 1 {7c-c)/kpc/k (g) 



as p x c/k is given in equation (4). 



• Using these equations the data of the following table were 



computed in connection with incidental results tested for the 



purpose. 



Table. — Pressures p (cm) and temperatures t (°C.) at the fog and vacuum 

 chambers, the latter marked with an accent. 



P P' Pa p\* Pi p 2 p\ p\\ U t\ 



76 43-5 45*5 47*9 45'7 52*9 45'0 45'6 — 20'2 24'3 



76 51-5 52-5 54-3 52'4 58'3 52*1 ... — 99 215 



76 59-5 59'7 62-2 59'4 63*8 59'4 ... — '2 20* 



3. The pressures of the table are computed for dry air 

 throughout and if charted in terms of p\ graphically, are 

 found to lie very nearly on straight lines. The results of the 

 table are very important. In the first place it will be seen 

 not only that isothermal pressures or nearly isothermal pres- 

 sures are not observed, but that the effect of the vacuum 

 chamber is preponderating. Thus the pressure at the latter 

 p", read off as soon as possible and nominally adiabatic, is 

 within one millimeter of p r Similarly the computed adia- 

 batic pressure p x is within a few millimeters of p" smd p r It 

 follows, therefore, that even an approach to isothermal pres- 

 sure, to say nothing of adiabatic pressure, cannot be observed 

 at the fog chamber at all ; 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'\ observed under isothermal conditions % 

 at the fog chamber, exceeds^ (computed) by about l-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 importance is the following : p 2 the computed isother- 

 mal pressure in the closed fog chamber is from- 2 to 5 cm above 

 the (nominally) isothermal pressure p/ = p 2 observed : and 

 correspondingly more than this above the common isothermal 



* Observed as soon as possible after exhaustion at the closed fog chamber, 

 f Observed as soon as possible after exhaustion at the vacuum chamber, 

 stop cock-closed at once after exhaustion. 

 X This pressure varies but slightly. 



