254 PHYSIOLOGICAL PHYSICS. [Chap. xxn. 



causes it to pass through the membrane in fine drops. 

 When this is seen some of the mercury is removed, 

 till no more drops are seen to ooze through the mem- 

 brane. The wide tube is then immersed in a vessel 

 of water, the water being tinged blue. After the 

 lapse of some hours a blue stratum will be found 

 inside the wide tube, though no change has taken 

 place in the level of the mercury. After a long enough 

 time the brine and water will mix so that the quantity 

 of salt is uniform, and still the level of mercury will 

 indicate no change of volume. The pressure of the 

 mercury has prevented [change of volume, though it 

 has been unable to prevent mixture of the liquids. 



A large share in the production of osmosis is 

 ascribed to capillarity. The septum being a porous 

 body may be considered as containing a large number 

 of capillary tubes. Some liquids are capable of wetting 

 certain capillary tubes, others are not. Those that 

 wet the tube can ascend in it, the others cannot. Of 

 those liquids that wet the same kind of tubes, one can 

 ascend the tube higher than others. The tube, that 

 is, has . a greater attractive force for one liquid than 

 another ; or one liquid is able to resist the attractive 

 force better than another. Thus, of two liquids on 

 opposite sides of a porous partition, one is capable of 

 permeating the membrane to a greater extent than the 

 other. The liquid that has the greater affinity for the 

 tube meets, in its course through it, the other liquid 

 advancing in the opposite direction, but advancing 

 with less force because of its less degree of affinity. 

 The greater force overcomes the less, and the one 

 liquid, consequently, occupies the pores of the mem- 

 brane, having ejected, so to speak, the other. But 

 now the element of diffusion enters into the question. 

 The force of capillarity can cause one liquid to enter 

 and occupy the pores from one side, but it cannot 

 cause that same liquid to flow out of the membrane 



