Blood and most normal body fluids have an osmotic pressure of 7.7 atmospheres, while urine may have an osmotic pressure as high as 38 atmospheres. Concentrated solns. may be prepared from dilute solns. by spontaneous migration of substances through semipermeable membranes. In the model exp., a large amt. of aq. phenol soln. of conc. Co (0.1 M) is brought into contact with a smaller vol. V of cane sugar soln., conc. Co, through a rubber membrane P. P is permeable for phenol but not for H2O and cane sugar. The cane sugar soln. is also in contact with a smaller vol. Vz of cane sugar soln. of conc. Co through a membrane W1 and with a small vol. VPh of phenol soln., conc. Co, through a membrane W2. W1 and W2 are Cu ferrocyanide membranes and are permeable to H2O only. Phenol from the very large vol. passes through P until its conc, in V is Co, so that V has a total conc, of sugar and phenol of 2 Co, and its osmotic pressure is 2 CoRT. Since the osmotic pressure in Vz is only CoRT, V will withdraw H2O from Vz until the conc, of sugar in Vz is approx. 2 Co. Similarly, H2O will be withdrawn from VPh until the conc, of phenol in VPh is approx. 2 Co. By repeating the process any desired cones. of cane sugar and phenol may be obtained. The apparatus may be constructed so that the conc, process is continuous. With the membranes in the kidneys and other parts of the body, it is possible to obtain highly concentrated solns.. of substances in a few min. The free energy which arises when dilute isotonic solns. of cane sugar and phenol are mixed is used to bring a part of the sugar soln. to the higher conc.