SOLUTION. 163 



In osmotic cells the pure solvent always passes into the solution, or the sol- 

 vent passes from a solution of less concentration into one of greater concentra- 

 tion. This flow can be accounted for on the physical principle of equilibrium, 

 namely, that in any system capable of movement or adjustment a strain in one 

 part will cause a movement tending to remove or equalize the strain. A 

 system of a membrane and a liquid on each side of it can be in equilibrium 

 only when the osmotic pressure on the two sides of the membrane is the same. 

 In the case of a semi-permeable membrane, the molecules of the dissolved sub- 

 stance cannot pass, so the only other way of equalizing osmotic pressure is by 

 the solvent passing from the exterior of the cell into the solution, until the 

 osmotic pressure is the same on each side of the membrane. If this passage is 

 obstructed the tendency still exists, which manifests itself as pressure. 



In 1887 van't Hoff deduced from theoretical considerations the laws of 

 osmotic pressure, which are verified by experiments. These laws are analogous 

 to the gas laws, and are as follows : 



The osmotic pressures of solutions of the same substance are proportional to their 

 concentrations. This is analogous to Boyle's law for gas pressures, and in gen- 

 eral is independent of the nature of the solvent. It is illustrated by the 

 following results : 



Osmotic pressure increases in proportion to the absolute temperature. This is 

 analogous to the law of Charles for gases. 



Solutions which at the same temperature have equal osmotic pressure contain 

 equal numbers of molecules of the dissolved substance in equal volumes. This is 

 analogous to Avogadro's law for gases. 



The osmotic pressure of a substance in solution is the same in value as the gas- 

 eous pressure which it would exhibit if the same weight of it were contained as a 

 gas in the same volume at the same temperature. The osmotic pressure of a solution 

 of the molecular weight in grammes of a substance in one liter of water at 

 G. is 22 to 23 atmospheres, and the same weight of the substance in gas form 

 at C., and occupying a liter volume, would have a gas pressure of 22 to 23 

 atmospheres. 



Solutions of different substances having the same osmotic pressure are said 

 to be is-osmotic or isofonic. It is not an easy matter to carry out measurements 

 of osmotic pressure except in specially equipped laboratories, but isotonic 

 solutions can be prepared, nevertheless, by taking advantage of the fact that 

 such solutions have the same freezing-points, and determination of freezing- 

 points is a routine task in laboratories. Blood-serum freezes at 0.56 C., 

 which corresponds to an osmotic pressure of about 6.6 atmospheres. A 0.95 

 per cent, aqueous solution of sodium chloride freezes at 0.56 C., and, there- 

 fore, exerts the same osmotic pressure as blood-serum. It is isotonic with 

 blood-serum. A solution of higher osmotic pressure than that of blood-serum 

 is called hypertonic, and one of lower pressure is called hypotonic. 



