HYSTERESIS 



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facts regarding the very slight effect of gels upon diffusion, conductivity, and 

 the velocity of chemical reactions. Ultramicroscopic examination of certain 

 gels has also yielded evidence which appears to support this theory. 



Since elastic gels can be readily transformed into hydrophilic sols, and 

 hydrophilic sols into gels, there is good reason for believing that hydrophdic 

 sols may also possess a fibrillar structure. Such a structure is also postulated 

 by many authorities for protoplasm, as later discussion will show. 



Hysteresis. — The statement is sometimes encountered that gels possess 

 the faculty of "memory." This statement is not, of course, to be accepted 

 literally. It is merely a way of saying that the previous treatments to which 

 a gel has been subjected have an influence, often 

 marked, upon its behavior, so that in an allegori- 

 cal sense the gel may be said to "remember" those 

 treatments. This phenomenon of the influence 

 of the previous treatment of a gel upon its be- 

 havior is known as hysteresis^ It is well illus- 

 trated by the following experiment of Gortner 

 and Hoffman (1927). Three gelatin gels were 

 prepared containing respectively 10, 20, and 40 g. 

 of gelatin per lOO cc. of water. Strips of these |2 

 gels of equal rectilinear dimensions and thickness 

 were then dried in a current of warm air until all 

 of them were reduced to a moisture content of 

 about 3.5 per cent. In other words the three gels 

 were all brought into what would superficially 

 seem to be identical physical conditions. The 

 dried sections were then placed in distilled water 



and allowed to imbibe water, weighings being made from time to time 

 results are shown in Fig. 9. 



In spite of the fact that all three of these gels possessed the same water 

 content when immersed in water their swelling behavior was quite different, 

 depending in each sample on the previous history of the gel. The gel which 

 was originally prepared in the proportion of 10 parts of gelatin to lOO of 

 water swelled the most, followed in order by the one originally prepared in a 

 proportion of 20 parts of gelatin to lOO of water, and finally by the gel 

 originally prepared in a proportion of 40 parts of gelatin to lOO parts of water. 



The example cited is just one of the many hysteresis effects which have 

 been recognized in gels. All sorts of factors — mechanical, thermal, electrical, 



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HOURS 



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Fig. 9. Curves illustrat- 

 ing hysteresis in gelatin gels. 

 Data of Gortner and Hoff- 

 man (1927). 



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* This use of the term hysteresis should not be confused with its common 

 use in another sense in physics and engineering. 



