32 THE CELL 



dissolved substance, e. g., salt, exercises a restraining influence on the freezing 

 of the water and consequently lowers the freezing point. The cause of this 

 restraining influence is that the particles of salt by means of the attraction 

 they exercise on the particles of water tend to prevent the latter from cohering 

 with one another, i. e., from passing into the solid state. The greater this 

 attraction, the more difficult is it for the water to be solidified, and the lower 

 must be the temperature before this change of state can be brought about. 



The lowering of the freezing point, which is designated with " A ," is there- 

 fore a measure of the osmotic pressure (P) of the salt, calculated in atmospheres 

 according to the formula P = 12.03 A . A one-per-cent solution of NaCl freezes 

 at -0.606 C. : the lowering of the freezing point is therefore 0.606. If one 

 finds for an unknown fluid A = 0.6$6, its osmotic pressure corresponds to 

 that of the one-per-cent NaCl solution, and amounts to 1 2.03 X 0.606 = 7.29 

 atmospheres. 



When equimolecular quantities of different substances (i. e., quantities pro- 

 portional to their molecular weights) are brought into solution in the same 

 solvent, and solutions which have the same number of molecules of the dissolved 

 substances in equal volumes are thus obtained, these solutions have at any given 

 temperature the same osmotic pressure. 



We have in the electrolytes an apparent exception to this law. The os- 

 motic pressure is higher than it ought to be according to this general state- 

 ment. This is due to the fact that the substances in question are partially 

 dissociated in water into electrically charged atoms or ions e. g., HC1 into 

 + H and - 01, KC1 into + K and - 01, NaOH into + Na and - OH, etc. 

 In this way the number of effective molecules in a solution is increased and 

 in consequence the osmotic pressure is raised in perfect agreement with the 

 general law. 



The degree of dissociation depends primarily upon the concentration and 

 upon the nature of the dissolved substance. The more dilute the solution the 

 more complete is the dissociation with one and the same electrolyte i. e., the 

 greater is the relative (not the absolute) number of free ions. In different 

 electrolytes dissociation presents certain variations into which we cannot enter 

 at this time. It will suffice here to say that the most important salts in the 

 body, those formed by the alkalies with monobasic acids, are dissociated in 

 dilute solutions of equivalent concentration to a very considerable extent, and 

 are dissociated equally. 



The combined osmotic pressure of several substances in the same solution 

 is equal to the sum of the pressures of the separate substances. 



When two solutions of different osmotic pressure are separated from each 

 other by a semipermeable membrane, water passes from the one of less pressure 

 to the one of higher pressure until the two are of equal pressure i. e., are iso tonic. 

 With reference to each other these solutions are said to be hypotonic and hyper- 

 tonic respectively. 



Dead animal and plant membranes are as a rule permeable to water and, 

 though in less degree, to substances soluble therein. When such a membrane 

 separates water from an aqueous salt solution, the former passes into the solu- 

 tion and the salt passes out until the osmotic pressure on both sides of the 

 membrane is the same. This is the case also when two isotonic solutions of 

 different salts e. g., NaCl and NaNO 3 are separated by the membrane. The 

 common salt passes from one to the other and vice versa, so that the two solu- 

 tions remain isotonic. If they are of unequal osmotic pressure i. e., anisotonic 

 to begin with, an exchange of water and salt molecules takes place until a con- 

 dition of equilibrium is established. 



