THE THEORY OF OSMOTIC PRESSURES. 155 



to measure, and its mouth covered with the membrane is 

 immersed in distilled water or in dilute serum. 



The experiments which are the most interesting are 

 those in which decinormal solutions of glucose, urea, 

 sodium chloride were compared as to their initial rates of 

 osmosis, the outer fluid being water. He concludes from 

 his experiments that, in the case of prepared peritoneal 

 membrane, the initial rates of osmosis of glucose, sodium 

 chloride and urea in equimolecular solutions do not corre- 

 spond to the ratio between their final osmotic pressures (as 

 estimated by the depression of freezing-point), but the 

 initial rate of osmosis of glucose {i.e., the rate with which 

 water passes into this solution) is greater than that of 

 sodium chloride, and the initial rate of osmosis of sodium 

 chloride greater than that of urea. 



In these experiments the only two solutions which 

 are strictly comparable are those of urea and glucose 

 (A = 0*189° C), since the decinormal Na CI solution had 

 nearly double the osmotic pressure of these two (A = 0*35 1). 

 In three typical experiments, each of which lasted three 

 hours, the average rates at which the fluid in the funnel 

 increased in volume during the first hour were : in the case 

 of glucose, 7! mm. in five minutes ; in the case of sodium 

 chloride, 43 mm. ; and in the case of urea, iJ T mm. 

 These figures are evidently not proportional to the differ- 

 ence of osmotic pressures between the fluid and the funnel 

 and the water in the reservoir. But we have already seen 

 that the moving force is not the total difference of pressure 

 between the fluids in the vessels on either side of the 

 membrane, but the difference of pressure between the 

 layers of fluid in immediate contact with each side of the 

 membrane. The fall of osmotic pressure across the thick- 

 ness of the membrane varies inversely as the rate of 

 diffusion of the dissolved substance. The question arises 

 therefore whether the results obtained by Lazarus Barlow 

 can be accounted for by differences in the rate of diffusion. 

 In the carefully worked-out tables by this observer we have 

 all the data necessary to decide the question. In the case 

 of glucose, the freezing-point of the solution at the begin- 



