1018 APPENDIX. 



molecules of gas equal to that of the crystalloid in solution, if confined within 

 the same space and kept at the same temperature.* A perfectly satisfactory 

 explanation of the nature of osmotic pressure has not been furnished. Wo 

 must be content to use the term to express the fact described. It is a matter 

 of great importance to measure the osmotic pressures of various solutions. 

 As was stated above, this measurement can be made for any solution pro- 

 vided a really semipermeable membrane is constructed. As a matter of 

 fact, however, the use of such membranes has not been general. In actual 

 experiments other methods have been employed, and a brief statement 

 of a theoretical and a practical method of arriving at the value of osmotic 

 pressures may be of service in further illustrating the meaning of the term. 

 Before stating these methods it becomes necessary to define two terms 

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

 in this connection. 



Electrolytes. 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 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 Cl 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 gms. 

 (C 12 H22O n ) 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 litei 

 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 



* The interesting researches of Morse and Frazer (" The American Chemi- 

 cal Journal," 34, 1, 1905), who have succeeded in making semipermeabte 

 membranes in such a form as may be used for determining directly the os- 

 motic pressures of concentrated (normal) solutions, have shown that this 

 law is not accurately stated. The actual pressure is that which would be 

 exerted if the particles in solution were gasified at the same temperature and 

 kept to the volume of the pure solvent used (water), instead of the volume 

 of the entire solution. 



