DIFFUSION AND OSMOSIS. 67 



trating the meaning of the term. Before stating these met Ik ids it hecomes necessary to 

 define two terms, namely, electrolytes and gram-molecular solutions, that are much used 

 in this connection. 



Electrolysis. — The molecules of many substances when brought into a state of solution 

 are believed to be dissociated into two or more parts, known as ions. The complete- 

 ness of the dissociation varies with the substance used, and for any one substance with 

 the degree of dilution. Roughly speaking, the greater the dilution the more nearly 

 complete is the dissociation. The ions liberated by this act of dissociation are 

 charged with electricity, ar.d when an electrical current is led into such a solution it is 

 conducted through the solution by the movements of the ions. The molecules of per- 

 fectly pure water undergo practically no dissociation, and water therefore does not appre- 

 ciably 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 con- 

 ception 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, there- 

 fore, that the osmotic pressure of any given electrolyte in solution will be 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 press- 

 ure a more convenient method is the use of the unit known as a gram-molecular 

 solution. A gram-molecule of any substance 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 chloride is one containing 58.5 grams (Na 23, CI 35.5) of the salt to a liter, while a 

 gram-molecular solution of cane-sugar contains 342.1 grains (C 12 H 22 O n ) to a liter. Sim- 

 ilarly a gram-molecule of H is 2 grains by weight of this gas, and if this weight of II were 

 compressed to the volume of a liter it would be comparable to a gram-molecular solution. 

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

 to 342.1, it follows that a liter containing 2 grams of II contains the same number of 

 molecules of H in it as a liter of solution containing 342.1 grains of sugar has of sugar 

 molecules. Since it is known 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 ^ram-molecular solution of cane-sugar, or of any other substance that is not an 

 electrolyte, is equal to the gas-pressure of 2 grams of II 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 flu' Osmotic I'rcssurr of Solution.-!. -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 gram of II at atmospheric pressure occupies a volume of 

 11.16 liters ; 2 grains of II, therefore, under the same conditions will occupy a volume of 

 22.32 liters. A gram-molecule of H— that is, 2 grams of II — when broughl to the volume 

 ofl liter will exert a gas-pressure equal to that of 22.32 liters compressed to I liter — that 

 is, a pressure of 22.32 atmospheres. A gram-molecular solution of cane-sugar, since it con- 

 tains the same number of molecules in a liter, must therefore exert an osmotic pressure 



