12 



b'CIENCE 



[N. S. Vol. XLVII. No. 1201 



A review of the texts on colloidal chemistry 

 shows the following conceptions of this tenn. 



Cassuto^ calls a gel the gelatinous precipi- 

 tate obtained from a sol by means of an 

 electrolyte, heat or evaporation. /. e., a gel 

 is formed by coagulation of a sol. He calls 

 stiffened sols (or jellies), "gelatines." 



Bechhold^ remarks the loose use of the word 

 "gel" and states that he restricts it to the 

 description of the coagula from sols. To the 

 stiffened sols or jellies he applies the term 

 " Gallerte," which in English might be called 

 "jelly." 



Freundlich* says that systems of solid dis- 

 persion media and liquid dispersed phase are 

 gels as distinguished from the reverse which 

 are suspensions or emulsions. In other words 

 he applies the word " gel " to jellies. 



Hatsehek'i refers to the fact that Graham ■ 

 applied the name "gels" to the products ob- 

 tained by the coagulation of sols, but later 

 on in his book he calls jellies, gels also. 



Ostwald in bis " Handbook of Colloid Chem- 

 istry," considers all colloids as gels when the 

 system becomes " microscopically heteroge- 

 neous." That is to say, he applies the term 

 promiscuously. 



Taylor" uses "gel" in the same loose gen- 

 eral manner as Ostwald, Hatschek and Freund- 

 lich. 



Zsigmondy in his " Kolloidchemie," limits 

 the word " gel " to the dry residue which will 

 not redisperse in a solvent, but he applies 

 " hydrogel " to the jelly-like mass formed by 

 removal of the dispersion medium or by salt 

 coagulation. 



Hardy' recognized the difference in proper- 

 ties of substances called " gels " and he 

 qualified the term — " gels by coagulation " 

 and " gels by stiffening." 



I feel confident that Graham did not apply 

 this term as loosely as is popular at the present 



* "Die KoUoide Zustand der Materia." 

 » ' ' Kolloide in Biologie und Mediziji. ' ' 



* ' ' Kapillarchemie. ' ' 



5 "Introduction to the Physics and Chemistry 

 of Colloids." 



« ' ' Chemistry of Colloids. ' ' 



1 Z. physilc. Chem., 33, 326; 385 (1900). 



time. In his remarks on the properties of 

 colloidal tungstic acid^ he says : " It is remark- 

 able that the purified acid is not pectized by 

 acids or salts even at the boiling temperature. 

 Evaporated to drjness, it forms vitreous 

 scales, like gum or gelatine." 'Note that he 

 describes the dry residue as " scales, like gum 

 or gelatine " and not as gel. 



It is evident, then, that the original meaning 

 of the term gel has not been adhered to and in 

 fact is more often applied to the state best 

 described as jellies. Shall we adliere to 

 Graham's definition or shall we discard it, 

 restricting the term gel and its modifications 

 to jellies, as popularity favors, and do away 

 with any special terms to describe coagula 

 from sols by electrolytes, or residues formed 

 by evaporation to dryness? Special terms to 

 describe these last two cases are obviously 

 unnecessary and serve only to encumber col- 

 loid chemistry. 



Lately the word " peptization " or " peptini- 

 zation," as originated by Graham, has shown 

 tendencies of wider use than formerly. 

 Graham used this expression ■ to describe the 

 formation of a sol from a gel by the influence 

 of a small amount of foreign reagent as, for 

 example, the formation of a hydrous ferric 

 oxide sol from a coagulum of ferric hydroxide 

 by treatment with a small amount of hydro- 

 chloric acid or ferric chloride. He named it 

 "peptization" because it resembled the hy- 

 drolysis of egg white to peptone by acid. 



Cassuto, Hatschek, von Weimarn and Zsig- 

 mondy preserve the original sense of this 

 term. Bancroft^ has recently proposed, how- 

 ever, that we use peptization to describe all 

 cases of transformation of gel (using this 

 term in the present popular sense) "to sol and 

 not restrict it merely to cases where a foreign 

 electrolyte has been added to accomplish the 

 change. E. g., when dry gelatine is " dis- 

 solved " in water Bancroft would call it a 

 case of gelatine being peptized by water. This 

 usage has its merits because it eliminates the 

 word " dissolve " and the implication of " so- 

 lution." 



s L. c, p. 340. 



J. Fhys. Chem., 20, 85-117 (1916). 



