January 4, 1918] 



SCIENCE 



11 



" Colloidal solutions " 



are Colloidal dispersions, 



and 

 "True solutions" are Molecular dispersions. 



The particles, or " internal phase " of these 

 systems are known as the " dispersed phase," 

 and the liquid in which they are dispersed, 

 the external phase, is known as the " disper- 

 sion medimn." 



The present usage of the word " sol " and 

 of its modifications — " hydrosol," "alcosol," 

 etc., to describe a colloidal dispersion, is the 

 same as that of Graham, their originator. I 

 have noticed, however, that the word " sol " 

 has been used by a few writers in the sense of 

 the dispersed phase, i. e., the particles in col- 

 loidal dispersion. Is this use of the term to 

 be permitted? Graham invented it as a short 

 and convenient substitute for colloidal " solu- 

 tion " and a perusal of the worlts by Bechhold, 

 Cassuto, Freundlich, Hatchek, Ostwald, 

 Taylor and Zsigmondy shows that the orig- 

 inal sense of the expression has been retained 

 by these writers. 



The use of the term "gel," however, and of 

 its modifications, " hydrogel," " alcogel," etc., 

 is deplorably loose and confusing. This term 

 was coined by Graham' and first used by him 

 in discussing the " pectization " or coagula- 

 tion of the hydrosol of silicic acid. There- 

 fore, if we are to limit the use of " gel " to its 

 original sense, we should use it only when 

 speaking of the definite coagula of sols. This 

 is not the case, however. Any substance 

 which resembles a jelly in appearance is 

 called " gel," although in chemical and phys- 

 ical properties it may be entirely different 

 from the kind of matter which Graham had 

 in mind 'when he invented the term. This 

 use of the expression is exceedingly popular 

 and would be very diflScult to overcome. 



Let us take up a case or two to show how 

 unscientific this latter usage of the term is. 

 For example, consider the dissolution or dis- 

 persion of gelatine in hot water. It forms a 

 very mobile " solution " or rather dispersion, 

 and in this form is called a hydrosol or sol. 

 If this hydrosol be allowed to cool, it becomes 



iProc. Boy. Soc, 13, 337 (1864). 



very viscous and " sets " to a jelly-like mass. 

 In this stiffened form it is popularly known as 

 a " hydrogel " or " gel." If it be warmed 

 again, its viscosity decreases, it becomes 

 mobile and is called a " sol." Now if this 

 use of the term " gel " (which is not aa 

 Graham intended) is to be permitted, then 

 what is the line of demarcation between the 

 sol and gel states ? A change in state has oc- 

 curred, it is true, for in the sol condition the 

 water was the external phase whereas in the 

 so-called gel state the gelatine became the 

 external phase and the water internal. The 

 change is very gradual, however, and our 

 change of terms to suit the change in appear- 

 ance of the system is exceedingly arbitrary 

 and unscientific. Furthermore no chemical 

 change in the nature of the gelatine has taken 

 place. 



If some alum or a trace of mercuric chloride 

 be added to the gelatine hydrosol, a coagulum 

 is obtained which is correctly termed a gel 

 according to Graham. This is a coagulated 

 compound, however, a chemical reaction has 

 taken place. It will not redissolve in water 

 and is, therefore, entirely different from the 

 case discussed above, which is also commonly 

 called gel. 



To further complicate matters, the dry 

 pieces of gelatine (and in fact any other col- 

 loid which will " dissolve " readily) which 

 were used to make the original sol are also 

 quite commonly called " gel." Lottermoser 

 called attention to this several years ago and 

 suggested that such pieces of gelatine, or of 

 any other dry " soluble " colloid be called 

 " solid sol." This suggestion is obviously bad. 

 Why apply any special term at all? 



T\Tien a hydrous ferric oxide sol is allowed 

 to evaporate spontaneously, it will go through 

 a jelly-like stage and finally become a hard 

 scaly residue when all or nearly all of the 

 water has left it. The jelly-like form will 

 redisperse when warmed with an excess of 

 water, while the amorphous form will not. 

 Yet both forms are called " gel " ! Zsigmondy 

 distingishes between these two different forms 

 by calling the first one " hydrogel " and the 

 second, " gel." 



