504 



SCIENCE 



[N. S. Vol. XLIII. No. 1110 



cosity determination, one of the most acces- 

 sible methods for tracing changes in the state 

 of colloidal aggregation. 



The viscosity of colloids or its reciprocal, 

 fluidity, shows peculiar variations with differ- 

 ent degrees of dispersion. When the disper- 

 sion is greatest, i. e., when the disperse phase 

 is in " solution," we iind that the fluidity is 

 also at a maximum. An increased aggrega- 

 tion means a decreased fluidity which, how- 

 ever, continues only to the point at which the 

 disperse phase begins to separate out from the 

 dispersion medium as a suspension colloid or 

 suspension. When this point is attained, the 

 fluidity is suddenly reversed and approaches 

 more and more that of the pure dispersion 

 medium. Ifow whether the precipitation of 

 the disperse phase is brought about by the ac- 

 tion of electrolytes or, for example, by elevated 

 temperature (heat coagulation) the physical- 

 chemical effect upon the fluidity is the same 

 (Fig. 1,B). 



Since all substances have an influence one 

 way or the other upon the solution equilibrium 

 of a colloidal system, we may, theoretically, di- 

 vide them into two groups; (1) those favoring 

 solubility of the disperse phase (increased dis- 

 persion, increased fluidity) and (2) those 

 favoring insolubility (aggregation, precipita- 

 tion, coagulation, initial increased viscosity). 

 Turning now to the conditions of the col- 

 loids especially at the surfaces of cells, we 

 find, in some cases, sharply differentiated mem- 

 branes. In many animal cells such mem- 

 branes are not demonstrable, but for our pres- 

 ent discussion this is of little moment, since 

 we are concerned with a colloidal boundary 

 which must exist at the surface of every cell. 

 In experimental studies upon single cells as, 

 for example, animal eggs, variations in the con- 

 stitution of the environmental medium pro- 

 duce profound changes in the cell. Whatever 

 changes may occur within the cell as a result 

 of such variations, it is certain that these 

 changes are secondary to an initial or primary 

 effect at the cell surface. Liquefying agents 

 are believed to produce an increase in cell per- 

 meability.^ Arbacia eggs, for example, when 



6 Lillie, R. S., '13, Jour, of Morphol., 22, 695. 



treated with solutions of sodium or potassium 

 thiocyanate, begin to lose their pigment after 

 two or three minutes. This is to be regarded 

 as an expression of an increase in the normal 

 permeability of the cell surface. 



It has been shown for a nmnber of physio- 

 logical objects' that the deleterious action 

 (liquefying action) of neutral alkalin salts 

 decreases from SON to CI in an order corre- 

 sponding satisfactorily with the Hofmeister 

 series. In these experiments, when the solu- 

 tions of the salts are brought into contact with 

 the cell surface, the degree of dispersion of the 

 surface colloids must be increased. The dis- 

 persion is greatest in solutions of thiocyanates 

 and least in chlorides. A physico-chemical 

 expression of this increased dispersion is the 

 increase in the fluidity of the cell surface. 

 Now, since the speed of diffusion of ions and 

 molecules through any fluid medium depends 

 upon the viscosity of that medium,* it is clear 

 that an increased fluidity of the cell surface 

 involves a facilitation of diffusion of soluble 

 substances from either side of the cell surface. 

 In other words, by a liquefying action at the 

 surface, the permeability of the cell is in- 

 creased and diffusion in both directions across 

 the surface is facilitated. 



Since there is this very definite correlation 

 between liquefaction (dispersion) and in- 

 creased permeability, it is obvious that, in the 

 normal condition of the cell, we must have a 

 greater aggregation of the surface colloids 

 than during liquefaction. Bearing in mind 

 the fact that pure solutions of all substances 

 affect the solution equilibrium of colloids one 

 way or the other, we should expect, a priori, 

 to find certain agents producing an increased 

 aggregation of the surface colloids of the cell; 

 we should expect to find true solutions of 



'Sehwarz, C, Pfliigers Arch., 117, 161; Lillie, 

 E. S., '10, Amer. Jour, of Physiol., 26, 106; 

 Spaeth, R. A., '13, Jour, of Exper. Zool., 15, 527. 



8 In the ease of water -swollen gels, the speed of 

 diffusion of crystalloids is approximately the same 

 as in pure water, but it diminishes rapidly when 

 the water content falls below a certain value. 

 Bechhold u. Ziegler, '06, Zeitschr. f. physik. 

 Chemie, 56, 105. 



