916 APPENDIX. 



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 carry an electrical charge 

 and when an electrical current is led into the solution it is conducted in a 

 definite direction by the movements or migration of the charged 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 number of its molecules become dis- 

 sociated into a Na ion charged positively with electricity and a ( '1 ion charged 

 negatively, and the solution becomes a conductor of the electrical current. 

 Substances that exhibit this property of dissociation into electrically-charged 

 ions are known as electrolytes, to distinguish them from other soluble sub- 

 stances, 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 ob- 

 viously 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 mole- 

 cule. 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 neces- 

 sary, 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; Cl, 35.5) of the salt to 

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

 (C^HggOn) 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 has 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 ~ of 22.32 atmospheres, or 0.65 of an atmosphere, which in terms of 



