52 THE ATTRACTION OF LIQUIDS FOR MEMBRANES 



Experi- 

 ment to 

 shew that 

 an external 

 pressure 

 prevents 

 change of 

 volume. 



rig.9. 



If the tube (Fig. 9), which is 

 closed with bladder at its wide 

 opening, be filled with brine to 

 the mark a, if so much mercury 

 be then poured into the narrow 

 vertical part as by its pressure 

 to cause brine to begin to flow 

 out in fine drops from the pores 

 of the bladder, and if now, after 

 removing so much of the mer- 

 cury that the efflux is no longer 

 visible, we place the apparatus 

 in a vessel with pure water, co- 

 loured blue, as in the figure, the mercury does not 

 change its level ; and when, after one or two hours, 

 we carefully remove the tube from the water, we 

 find that in the upper part of the wide end of the 

 tube, which contained colourless brine, a dark blue 

 stratum has been formed, which floats on a colour- 

 less liquid. After a longer time, the blue colour 

 spreads gradually downwards, till at last the brine 

 acquires a uniform blue tint. 



It will readily be perceived, that the two liquids 

 here mix, as if no pressure had been applied to the 

 brine, for a mechanical pressure exerts no influence 

 on the mixture ; but, in consequence of the pres- 

 sure, the mixture takes place without change of 

 volume. The mechanical pressure which the water, 

 in virtue of its stronger affinity for the bladder, 

 exerts on the brine in the pores of the bladder, is 



