146 PROTOPLASM 



is determined to a great extent by what it has previously 

 experienced. 



The setting temperature of a gel is always a few degrees lower 

 than the point of liquefaction. As a result, there exists a hystere- 

 sis range within which the colloidal system may exist either as a 

 sol or as a gel. This range is much greater with agar than with 

 gelatin. Firm, hydrated agar does not liquefy until heated to 

 95 to 100°C. but does not set to a gel until cooled below 35°C., 

 and this gel must again be heated to about 95° to be converted 

 into a sol. There is thus a range of 60° within which the system 

 may exist as either sol or gel. For gelatin, the hysteresis range 

 is but 5 to 10°C. 



Syneresis. — Syneresis, a word meaning a "drawing together," 

 is the contraction of gels which results in the giving off of water. 

 The gel "sweats." The process is best observed in a mixture of 

 2 per cent rubber and 2 per cent sulphuric chloride in benzol; 

 equal volumes of the two solutions rapidly mixed will soon set 

 to a gel which shows syneresis in about twenty minutes. 



The separations of serum from clotted blood and of sour milk 

 into curd and whey are syneresis phenomena. The housewife 

 has to deal with syneresis in the making of foods that involve 

 the setting of albumin, gelatin, or other protein into a jelly, 

 which later, contrary to the housewife's plans, gives off some of 

 the water it holds, i.e., separates, sweats, or leaks by syneresis. 



The exudation of water and other secretions from protoplasm 

 and tissues (glands) may, in certain instances, be syneresis. 



Stability.— The stability of lyophilic and lyophobic colloidal 

 systems (gelatin and gold suspensions) is, in both instances, due 

 to the immediate environment of the particle, but the nature of 

 the stabilizing envelope may differ in the two cases. Aqueous 

 suspensions of gel-forming substances such as gelatin, casein, 

 gum, and soap probably stay up because of adsorbed water rather 

 than adsorbed ions, though the distinction may not be a very 

 great one, in that we may have to do with electrical forces in 

 both cases. The water molecule is polar (Fig. 138), which means 

 that it is electrically unlike at its two ends. It will, consequently, 

 be attracted (adsorbed) and held by a charged protein molecule 

 or colloidal particle just as in the case of an ion. Water dipoles 

 will orient themselves around protein molecules like a lot of 

 magnets, with one end or the other facing inward, depending on 



