Bar us — Diffusion of Vapor into Nucleated Air. 



Art. XLYIII. — The Diffusion of Vapor into Nucleated Air ;* 



by C. Barus. 



1. The apparatus with which experiments of the present 

 kind are made is conveniently described by aid of the accom- 

 panying diagram. The appurtenances necessary in practice are 

 given in my report on the structure of the nucleus (Smithson- 

 ian Contributions, No. 1373, 1892), to which reference will 

 frequently be made. A is a tall glass vessel about one meter 

 high, either cylindrical or rectangular in section, in the latter 

 case with opposed plate glass sides. The liquid, Z, whose 

 vapors are to be tested, is placed in the bottom. The wide tube, 

 c, is used for sudden exhaustion, while a vacuum gauge, g, regis- 

 ters the pressure differences. The tubes' a and b to the top and 

 the bottom of A serve for the admission either of filtered air 

 or of nucleated air. They are used together, one for influx and 

 the other for efflux, in connection with the suction of an 

 aspirator. 



When the diffusion of the necessarily heavy vapors from L 

 is to be measured, the air in A is first cleansed of vapor by a 

 current of nucleated air from a to b. Thereafter the stopcocks 

 are closed at a stated time. If now at a subsequent time a 

 sudden exhaustion is made in A through c, for a stated pressure 

 difference Bp\ shown at g, the progress of the diffusion may be 

 computed from the height of the fog-bank after an allowance 

 is made for the rise due to the exhaustion. 



On the other hand, if the aspirating current is of filtered air 

 and moves in the direction from b to «, over the surface of the 

 volatile liquid, the receiver, A, should become uniformly satu- 

 rated to a high degree throughout. If nuclei are added at a 

 stated time below, near the surface of the liquid, the corre- 

 sponding height of the fog-bank seen on exhaustion at a later 

 time, should indicate the rate at which the nuclei diffuse, if 

 they diffuse more slowly than the residual concentration of 

 vapor. This method for nuclei, which I pursued with entire 

 confidence, leads however to erroneous results, as the present 

 paper will show : for the diffusion of the nuclei is a much more 

 rapid process than the accompanying complications of vapor 

 diffusion. 



2. To state the case specifically, let p be the vapor pressure 

 relative to the saturation pressure at the temperature # , at a 

 time t after diffusion of vapor commences and at a height x 



* Paper contributed to the meeting of the National Academy, April, 1903. 



