466 Perkins — Molecular Weight of Radium Emanation. 



Methods of Determining the Diffusion Constant for Mercury. 



Two methods are available for finding the diffusion constant 

 for mercury vapor. 



Methods Employed ' : Method 1. — Introduce into the diffusion 

 chamber a quantity of mercury large enough to give a constant 

 supply of vapor as diffusion proceeds and weigh the amount 

 diffusing in a known time, when the diffusion constant is given 

 by the following relation : 



M ; — Amt. of vapor diffusing in time t. 

 M, — kM t = hpVt A 1 = Diffusion constant. 



M = Amt. of vapor in diffusion chamber. 

 p = Density at the temp. used. 

 V = Vol. of diffusion chamber. 



The use of this method of finding the diffusion constant 

 requires an accurate determination of the density. 



Method 2. — Place a very small amount of mercury in the 

 diffusion chamber and find as before the amounts diffusing in 

 .a known time. As diffusion proceeds, a condition is finally 

 reached where vapor alone remains in the diffusion chamber, 

 after which the amounts diffusing in a given time fall off very 

 rapidly according to the following exponential law, which also 

 holds for the emanation : 



M = M e _w M = Arnt. of vapor present in the diffusion 

 chamber at the beginning of collection. 

 M = Amt. present at the end " " 



' k = Diff. const, t = Time " " 



A very simple relation holds between amounts of vapor or 

 emanation collected during successive, equal intervals of time, t. 



Let M y = amount of 1st collection. 



" M,= " " 2d 



" M = " in the diffusion chamber at beginning of 

 first collection. 



Then M, = M (1 - er u ). 

 AlsoM,, =M e- u (1 - e~ kt ). 



M 



Therefore =~ = e M . 



M ,/ 



As from 5 to 8 sec. were required to withdraw one collecting 

 tube and push the other into position, 20 min. periods were 

 chosen for collection so as to introduce an error considerably 

 less than 1 per cent. 



Method used for Measuring Relative Amounts of Diffused 

 Emanation. — In order to keep the conditions for diffusion 

 similar to those for mercury vapor, it is essential that diffusion 

 of -the emanation should take place in an atmosphere of hydro- 

 gen. If the amounts of emanation diffusing through the 

 porous plug are to be measured directly, this must necessarily 



