1026 ABSORPTION 



electrical series has been established by the alternate position of plus 

 and minus elements. Water, on the other hand, is not easily dissoci- 

 ated and hence, cannot serve as a good conductor of electricity. The 

 same is true of sugar. Upon these differences is based the division 

 of substances into electrolytes and non-electrolytes. Now, since an 

 ion plays the same part in the production of osmotic pressure as a 

 molecule, it will be seen that a solution of an electrolyte must exert 

 a proportionally greater osmotic pressure, because it contains a greater 

 number of particles consisting, on the one hand, of molecules and, on 

 the other, of ions. 



The Diffusion of the Proteins. — Conditions in our body are 

 complicated still further by the fact that its different fluids do not 

 contain solely crystalloids, but also other substances, such as proteins. 

 The latter are practically indiffusible through animal membranes, 

 although most of them are soluble in water, weak salt solutions, and 

 dilute acids and alkalies. Moreover, they form compounds with 

 metallic salts, acids or alkalies and, when in solution or pseudo-solution, 

 can be converted into an insoluble form by various simple means, 

 such as changes in the reaction and temperature, shaking, and the 

 addition of neutral salts. By reason of their indiffusibility, they may 

 be separated from the diffusible crystalloid substances by dialyzers, 

 such as vegetable parchment. This separation, however, cannot be 

 accomplished without difficulty. 



Considerable progress has been made in this direction more recently by the 

 work of J. Loeb.i It has been shown that while non-ionized gelatin may exist in 

 gelatin solutions on both sides of the isoelectric point (which equals an hydrogen 

 ion concentration of CH = 2.10"^ or pH = 4.7), gelatin when it ionizes, can only 

 exist as an anion on the less acid side of its isoelectric point (pPH>4.7) and as a 

 cation only on the more acid side of its isoelectric point (pH>4.7). At the iso- 

 electric point gelatin can dissociate practically neither as anion nor cation. 



On the acid side of the isoelectric point amphoteric electrolytes can only com- 

 bine with the anions of neutral salts, on the less acid side of their isoelectric point 

 with cations; and at the isoelectric point neither with the anion nor cation of a 

 neutrat salt. It has also been shown that the isoelectric point of an amphoteric 

 electrolyte is not only a point where the physical properties of an ampholyte ex- 

 perience a sharp drop and become a minimum, but that it is also a turning point 

 for the mode of chemical reactions of the ampholyte. It is suggested by Loeb 

 that this chemical influence of the isoelectric point upon life phenomena over- 

 shadows its physical influence. 



Surface-tension. — Another factor which no doubt plays a part 

 in absorption is surface-tension. Its action may be illustrated by 

 placing a drop of water upon an oily surface or by suspending a globule 

 of oil in a fluid with which it does not readily mix. In either case, 

 there is a tendency on the part of the drop to assume a spherical out- 

 line. This is brought about by the fact that its surface-layer is under 

 a certain tension which tends to give to the whole as small a surface as 

 possible, and naturally, the force here at work is cohesion, i.e., a 

 mutual attraction between its constituent molecules. Supposing 

 that we single out a molecule in its interior, it will be found that this 

 1 Jour, of Gen. Physiol., i, 1918, 39. 



