EFFECT OF DISSOLVED SALTS ON WATER MOVEMENT 6i 

 The matric potential difference across the block is 



/i3-/j2 = (iA3-^2)-A'"o{expUl^j-exp( -1-jj (8) 



and, anticipating that ql/Dg will be small, this reduces to 



^3- K = (!A3-</'2)-2Amo^//D« (9) 



In the steady state 



q=k{h^-h^l2l (10) 



where k cm sec~^ is the true capillary conductivity of the porous system. 

 In the experiment the total potential difference is measured, from which an 

 apparent conductivity, k' is calculated 



q=k'{^s-^,)l2l (II) 



Combining equations 9,10 and 1 1 



I I Xnin , » 



At a given moisture content -and so far there has been no assumption 

 about the degree of saturation of the porous system-a plot oiijk' against 

 niQ should yield a hne of slope A/Dg, intercepting the axis at ijk. 



The most convenient check of the analysis is at saturation, when it is to 

 be expected that the rate of diffusion of ions through the pore space, 

 expressed as a fraction of the rate in free water [DslDso say) will be the 

 same as the corresponding ratio {D/Dq) for gaseous diffusion through the 

 same system when completely dry. 



EXPERIMENTAL 



Measurements were made in sealed brass diffusion cells. Fig. ib. The porous 

 sample BCD is held between an evaporating source A and a condensing 

 sink E, separated from them by fixed air gaps maintained by spacing rings 

 F of piano wire encased in polyvinylchloride insulation sleeving. The sam- 

 ples used were circular disks, 5 cm diameter, approximately i cm thick, of 

 porous building stones, kindly supplied by the Building Research Station. 

 The source and sink are pads of filter paper saturated with NaCl solutions 

 at concentrations chosen to match the water potential in the sample so that 

 equal fluxes of water vapour distil from the source to the upper surface of 

 the sample and from the lower surface of the sample to the sink, the moisture 



