154 SCIENCE PROGRESS. 



molecules of o will pass into B than of /3 into A. Hence 

 the solutions on the two sides of the membrane will be no 

 longer equimolecular, but the total number of molecules of 

 a + (3 in B will be greater than the number of molecules of 

 a + j3 in A, and this difference will be most marked in the 

 layers of fluid nearest the membrane. The result therefore 

 of the unequal diffusion of the two substances is to upset 

 the previous equality of osmotic pressures. The layer of 

 fluid on the B side of the membrane will have an osmotic 

 pressure greater than the layer of fluid in immediate contact 

 with the A side of the membrane, and there will thus be a 

 movement of water from A to B. Hence if we have two 

 equimolecular and isotonic solutions of different substances 

 separated by a membrane permeable to the dissolved sub- 

 stances, there will be an initial movement of fluid towards 

 the side of the less diffusible substance. 



We have an exact parallel to this in Graham's familiar 

 experiment in which a porous pot filled with hydrogen is 

 connected by a vertical tube with mercury. In consequence 

 of the more rapid diffusion outwards of the hydrogen than 

 of atmospheric air inwards, the pressure within the pot sinks 

 below that of the surrounding atmosphere, and the mercury 

 rises several inches in the tube. We must therefore con- 

 clude that even when the two solutions on either side of the 

 membrane are isotonic, there may be a movement of fluid 

 from one side to the other with a performance of work in 

 the process. 



The experimental proof of the truth of this argument is 

 to be found in a recent paper by Dr. Lazarus Barlow. 

 This observer — after pointing out that the huge total 

 osmotic pressures of the salt solutions in the body can very 

 seldom come into play — insists on the fact that the most im- 

 portant point to study in this regard is the initial changes 

 that take place between dissimilar fluids separated by a 

 membrane — as he terms it — the initial rate of osmosis. For 

 this purpose he employs a funnel, the neck of which is pro- 

 longed into a capillary tube, while on the mouth is tied a 

 piece of peritoneal membrane. The funnel is filled with 

 the solution whose osmotic attraction for water it is wished 



