HYDROPHILIC SOLS AND GELS 139 



termed a sol unless it flows freely. More important is it to 

 realize that one of the distinguishing features of organic (lyophilic) 

 colloidal systems is their capacity to exist in a state that is not 

 truly solid or truly liquid and to carry some of the characteristic 

 properties of the one state over to the other. 



Methods of Preparation. — A gelatin gel is easily prepared by 

 allowing dry gelatin to swell in water or by dissolving gelatin in 

 hot water (3 grams or more in 100 cc. of water) and allowing the 

 solution to cool. Dry sheet gelatin swells to many times its 

 original volume when put in cold water, but it does not go into 

 solution, i.e., dissolve, until heated above 30°C. A poor grade of 

 glue goes into solution in cold water. Similar gels may be 

 prepared from agar (agar-agar) which is v^ery like gelatin in its 

 behavior, though quite a different substance chemically. Gel- 

 atin is a protein from animals, and agar a carbohydrate from 

 plants. It is agar that causes some seaweeds (dulse and "Irish 

 moss") to set to jellies when prepared for desserts. A 1 per cent 

 solution of agar in hot water sets to a firm jelly when cold. 

 Other naturally occurring jellies are egg white and vegetable gums 

 (the former is a fluid gel, which means that it possesses the 

 capacity to flow yet has other gel qualities, such as elasticity). 



The most familiar example of a coagulum, the irreversible type 

 of gel, is the clot formed by blood. Differing from it in some 

 respects, yet a typical coagulum, is the silicic acid gel. It is 

 made by adding moderately strong hydrochloric acid to an equal 

 volume of sodium silicate (waterglass) of about 1.15 sp. gr. 

 Free silicic acid, H2Si03, is formed, which, being insoluble, is 

 precipitated as a gelatinous coagulum; this sets to a firm gel of a 

 shght bluish opalescence. The time of setting may be instan- 

 taneous or prolonged to a day or more, depending on the concen- 

 tration of acid and silicate used. When freshly made, the gel is 

 of sihcic acid; but when dehydrated, it becomes pure sihca (sand). 

 The chemistry of the process is NasSiOg + 2HC1 = NaCl -|- 

 HaSiOa -^ Si02 + H2O. The hydrated gel is springy but not 

 extensible. Dry silica gel is as hard as glass. It takes up water 

 and other substances but is nonturgescible. Among the extra- 

 ordinary properties of some gels is one especially well exhibited 

 by the hydrated silica gel. When given a sharp blow, the gel 

 will ring. If the concentration and the form of the gel are 

 suitable, the sound is musically pleasant. 



