90 The Structure of Protoplasm 



way the rate of sedimentation is correlated with the concentration 

 of the added electrolyte, it can be concluded that reversible coagu- 

 lates are formed. These are very rich in water and contain a number 

 of colloidal iron oxide particles. This number increases greatly 

 with increasing concentration of the added electrolyte. Such revers- 

 ible coagulates, "geloids," are also formed in the original dilute sol 

 on adding electrolyte, but their concentration is not high enough to 

 let them coalesce to a coherent structure and turn the whole sol 

 to a gel. If, however, the geloids are concentrated by centrifuging, 

 they coalesce to a thixotropic gel. By investigating the change of 

 so-called conservative light absorption during the process of thixo- 

 tropic gelation, i. e., the light absorption caused exclusively by the 

 scattering of light (due to the presence of the colloidal particles) , 

 it can be shown that the reversible coagulation causing thixotropy 

 is always accompanied by a certain degree of irreversible coagu- 

 lation, which increases strongly with increasing electrolyte concen- 

 tration.-- This irreversible coagulation is the chief factor in the 

 regular coagulation occurring at higher concentrations of electrolyte. 



How many colloidal particles are contained in a geloid and 

 how their number depends on the nature of the colloid is not known. 

 These differences may be marked, as can be concluded from the 

 fact that the minimum concentrations of colloid, when a thixotropic 

 gel is formed, vary distinctly: For an iron oxide sol (of the Graham 

 type) , this concentration is about 5 gm. per litre, -^ for a VoO-, sol, it 

 is only about 0.1 gm. per litre. -^ 



Thixotropy may be sensitive to very small changes in the con- 

 centration of substances contained in the sol. Thus iron oxide sols 

 are particularly sensitive to H- and OH' ions"; the time of solidifica- 

 tion is strongly increased by an increase in H* ions, decreased 

 by an increase in OH' ions, i. e., an increase in H- ions has a liquefy- 

 ing effect and vice versa.-"' A pH change from 3.9 to 3.1 caused 

 the time of solidification to rise from 82 seconds to 150 minutes. By 

 dipping a silver plate into an iron oxide gel for 18 hours, the pH 

 changed from 3.4 to 3.8, producing a decrease in the time of solidifi- 

 cation from 33 minutes to 72 seconds. It was further found that 

 amino acids had a liquefying action upon these gels, independent 

 of the change they caused in pH, i. e., they increased the time of 

 solidification, although they increased the pH.-*^ 



It is important, also, from a biological point of view, that not 

 onlv electrolytes, but also suitable organic nonelectrolytes, are able 



