DIFFUSION AND OSMOSIS. 883 



a state of solution are believed to be dissociated into two or more parts, 

 known as ions. The completeness of the dissociation varies with the sub- 

 stance used, and for any one substance with the degree of dilution. Roughly 

 speaking, the greater the dilution, the more nearly complete is the dissocia- 

 tion. The ions liberated by this act of dissociation are charged with elec- 

 tricity, and when an electrical current is led into the solution it is conducted 

 by the movements of the ions. The molecules of perfectly pure water undergo 

 almost no dissociation, and water therefore does not appreciably conduct 

 the electrical current. If some NaCl is dissolved in water, a certain num- 

 ber of its molecules become dissociated into a Na ion charged positively 

 with electricity and a CI ion charged negatively, and the solution becomes 

 a conductor of the electrical current. Substances that exhibit this property 

 of dissociation are known as electrolytes, to distinguish them from other 

 soluble substances, such as sugar, that do not dissociate in solution and 

 therefore do not conduct the electrical current. Speaking generally, it may 

 be said that all salts, bases, and acids belong to the group of electrolytes. 

 The conception of electrolytes is very important for the reason that the act 

 of dissociation obviously increases the number of particles moving in the 

 solution and thereby increases the osmotic pressure, since it has been found 

 experimentally that, so far as osmotic pressures are concerned, an ion plays 

 the same part as a molecule. It follows, therefore, that the osmotic pressure 

 of any given electrolyte in solution is increased in proportion to the degree 

 to which it is dissociated. As the liquids of the body contain electrolytes 

 in solution it becomes necessary, in estimating their osmotic pressure, to 

 take this fact into consideration. 



Gram-molecular Solutions. The concentration of a given substance 

 in solution may be stated by the usual method of percentages, but from the 

 standpoint of osmotic pressure a more convenient method is the use of the 

 unit known as a gram-molecular solution. A gram-molecule of any sub- 

 stance is a quantity in grams of the substance equal to its molecular weight, 

 while a gram-molecular solution is one containing a gram-molecule of the 

 substance to a liter of the solution. Thus, a gram-molecular solution of 

 sodium chlorid is one containing 58.5 gms. (Na, 23; CI, 35.5) of the salt to 

 a liter, while a gram-molecular solution of cane-sugar contains 342 gms. 

 (Cj-jH^Oh) to a liter. Similarly a gram-molecule of H is 2 gms. by weight 

 of this gas, and if this weight of H were compressed to the volume of a liter 

 it would be comparable to a gram-molecular solution. Since the weight 

 of a molecule of H is to the weight of a molecule of cane-sugar as 2 is to 

 342, it follows that a liter containing 2 gms. of H contains the same number 

 of molecules of H in it as a liter of solution containing 342 gms. of sugar 

 has of sugar molecules. On the assumption that a molecule in solution exerts 

 an osmotic pressure that is exactly equal to the gas-pressure exerted by a 

 gas molecule moving in the same space and at the same temperature, we 

 are justified in saying that the osmotic pressure of a gram-molecular solu- 

 tion of cane-sugar, or of any other substance that is not an electrolyte, is 

 equal to the gas-pressure of 2 gms. of H when compressed to the volume 

 of 1 liter. This fact gives a means of calculating the osmotic pressure of 

 solutions in certain cases according to the following method: 



Calculation of the Osmotic Pressure of Solutions. To illustrate this 

 method we may take a simple problem such as the determination of the 

 osmotic pressure of a 1 per cent, solution of cane-sugar. One gm. of H at 

 atmospheric pressure occupies a volume of 11.16 liters; 2 gms. of H, there- 

 fore, under the same conditions will occupy a volume of 22.32 liters. A 

 gram-molecule of H that is, 2 gms. of H when brought to the volume 

 of 1 liter will exert a gas-pressure equal to that of 22.32 liters compressed 

 to 1 liter that is, a pressure of 22.32 atmospheres. A gram-molecular solu- 

 tion of cane-sugar, since it contains the same number of molecules in a liter, 

 must therefore exert an osmotic pressure equal to 22.32 atmospheres. A 

 1 per cent, solution of cane-sugar contains, however, only 10 gms. of sugar 

 to a liter; hence the osmotic pressure of the sugar in such a solution will 

 be 34 ^ of 22.32 atmospheres, or 0.65 of an atmosphere, which in terms of 



