152 SCIENCE PROGRESS. 



correct. The other three are only correct under certain 

 defined conditions which are rarely fulfilled in the body. 

 There are factors at work which have been practically dis- 

 regarded by most of the recent workers on the subject, and 

 which may tend to produce movement of fluid in apparent 

 opposition to the difference of osmotic pressure. Instances 

 of such cases are afforded in a paper by Lazarus Barlow, to 

 a consideration of whose work we shall shortly return. 



There can be no doubt that in the phenomena of trans- 

 ference of fluid or dissolved substances across a membrane 

 the nature of the membrane itself is all-important. I will, 

 therefore, shortly run through the various modes in which 

 interchanges may take place across membranes of varying 

 permeability. We shall see that the close analogy which 

 exists between substances in solution and gases, when 

 dealing with "semi-permeable" membranes, is also borne 

 out by experiment when used to predict the behaviour of 

 solutions separated by such permeable membranes as occur 

 in the body. 



The simplest case is that in which two fluids are sepa- 

 rated by a perfect semi-permeable membrane that permits 

 the passage of water but is absolutely impermeable to dis- 

 solved substances. In this case the transference of water 

 from one side to the other depends entirely on the difference 

 of osmotic pressure between the two sides. 



m. 



If we suppose two vessels, A and B, separated by such 

 a membrane, A containing a solution of a and B a solution 

 of (5, water will pass from A to B so long as the osmotic 

 pressure of /3 is greater than the osmotic pressure of the 

 solution of a. If B be subjected to a hydrostatic pressure 

 greater than the osmotic difference between the two fluids, 

 water will pass from B to A until the force causing filtration 

 or transudation (the hydrostatic pressure) is equal to the 



