April 7, 1916] 



SCIENCE 



503 



activities of cells, in so far as they involve an 

 interchange of material through the surface 

 layer, depends upon the shifting of a surface- 

 solution equilibrium. Since an interchange 

 of material eventually becomes essential for 

 the continuation of any living system, we 

 have called this solution equilibrium the 

 " Vital Equilibrium." 



If we examine a series of two-phase sys- 

 tems beginning with a coarse suspension and 

 extending through fine suspensions, colloidal 

 suspensions, colloidal solutions, hydrophilous 

 colloidal the "molecular disperse" systems of 

 Ostwald and ionically disperse systems such 

 as dilute solutions of electrolytes like ITaCl, 

 we observe two striking changes : first, an in- 

 creased subdivision of the disperse phase and, 

 second, an increased intimacy of relation be- 

 tween disperse phase and solvent, a necessary 

 result of the enormously developed surface in 

 the former.^ We find, furthermore, that in 

 any of these systems there always exists an 

 equilibrium between disperse phase and sol- 

 vent. The opposing forces are in the direc- 

 tion of an increased aggregation and disper- 

 sion, respectively; we may therefore speak of 

 an aggregation equilibrium. This equilibrium 

 is shifted by the addition of solutions of any 

 substance, organic or inorganic, by heat, ultra- 

 violet light, etc. For example, if we add CaCl, 

 to the negative suspension colloid As^Sj, a 

 precipitation occurs, i. e., there is an increased 

 aggregation of the disperse phase. Recipro- 

 cally, small quantities of 0.1 N" OaCl, will 

 clear an opaque colloidal solution of egg- 

 white. There is an increased dispersion and 

 the system becomes more like a true solu- 

 tion. I>row the limits of the above series are 

 total insolubility and complete solubility. 

 Any change in the direction of increased dis- 

 persion means a change in the direction of 

 a true solution, i. e., an increased solubility. 

 Wo sharp limits occur between true solutions 

 and colloidal solutions. A solution of cane 

 sugar, for example, though a molecular dis- 

 perse system, certainly represents a lesser 



3 Heber, E., '14, Physikal. Chemie d. Zelle u. d. 

 Gewebe. 4 Aufl., Kap. 7, p. 305 fif. 



degree of dispersion than any solution of 

 an electrolyte. Again, when two salt so- 

 lutions having a common ion are com- 

 bined, there appears the familiar phenomenon 

 of association or decreased dispersion, an equi- 

 librium shift in the direction of greater aggre- 

 gation, in this ease from ionic to molecular 

 dispersion. We may therefore legitimately 

 dispense with the term " aggregation equi- 

 librium " and, even though we are dealing 

 with colloidal systems, substitute the more fa- 

 miliar " solution equilibrium." 



The hydrophilous colloids which are of par- 

 ticular interest to physiologists are peculiarly 

 susceptible to slight changes in hydrogen ion 

 concentration. Here the changes in aggrega- 

 tion are reversible to a far greater degree than 

 in the colloids lower in the scale. The limits 

 of reversibility of the solution equilibrium 

 may be said to include a far greater range of 

 aggregation states than in the colloids of the 

 lower classes. 



An examination of the experimental data* 

 shows that for a number of different hydrophi- 

 lous colloids the following anion order of dis- 

 persion obtains: 



SOi < Tartrate < Citrate < Acetate < CI <C103 < 

 NO3 < Br < I <SON. 



The most indifferent region lies between ace- 

 tate and chloride; SO, has the greatest tend- 

 ency towards aggregation, while SON" produces 

 maximum dispersion.^ In many cases, espe- 

 cially in precipitation experiments, the addi- 

 tion of the electrolyte may have no visible 

 effect upon the colloid. When this occurs the 

 changed equilibrium may be detected by a vis- 



4 Hofmeister, F., '91, Arch, exper. Pathol, u. 

 Pharmakol., 28, 210; Hober, E., '07, Sofmeisters 

 Beitr., 11, 3.5; Porges u. Neubauer, '07, Biochem. 

 Zeitschr., 7, 152; Hardy, '05, Jour, of Physiol., 

 33, 251. 



5 By a sufSeient increase in the hydrogen ion 

 concentration the anion order may be completely 

 inverted. Thus the effect of an alkali salt upon 

 the state of aggregation of any hydrophilous col- 

 loid depends directly upon the hydrogen ion con- 

 centration. Posternak, '01, Ann. Inst. Pasteur, 

 15, 85; Pauli, W., '03, Sofmeisters Beitr., 5, 27; 

 Hober, B., '07, iUd., 11, 35. 



