62 



D. A.ROSE 



content of the sample thus remaining constant. All measurements were 

 made in a thermally insulated box in a constant-temperature room, and 

 possible temperature gradients across a cell were minimised by maintaining 

 metal-to-metal contact around a cell by inserting each into a G-clamp. 

 Each experiment lasted for 24 hours during which temperatures were kept 

 constant to within o-i°C at 25°C. Total vapour transfer was about 100 mg 

 day"\ i.e. q is of the order of 5 x 10 "^ cm sec~^. 



A 



B 



D 

 E 



Fig. lb. The diffusion cell. 



Knowing the potentials ^^ and ifj^ at the source and sink respectively, and 

 measuring the mass of vapour transferred across each air gap, the potentials 

 02 and i/fg can be inferred from the physical properties of water vapour and 

 the dimensions of the diffusion cell. Hence the conductivity of the porous 

 medium can be found. 



Each experiment was conducted with five repHcates of each of two stones, 

 Portland hmestone (a consohdated bed of oolitic aggregates), and Green- 

 brae sandstone (a consohdated bed of sand grains). Vapour transfer was 

 measured with the pore space saturated with NaCl solutions of molal 

 concentrations varying between 0-15 and o-8o. On Fig. 2 are results of 

 ijk' plotted as a function of Wq. Clearly relations of the form of eq. 12 hold, 

 for which least-squares analysis gives : 



Portland ^ = (3-53 + 7-50 Wq) x io1« 



Greenbrae ~ = (2-97+ 4-8i Wq) x io^o 



From eq. 12, using the values of A= 4-73 x 10*, and Dso= 1-475 >< 10 "^ 

 cm2 sec-i given by Robinson and Stokes (i955. pp- 46i, 495) the diffusion 

 ratios are 



Portland DsjDso = 0-0427 



Greenbrae DsjDsn = 0-0667 



