PHENOMENA OF MOTION 55 



Even the age of the jelly may have an influence. Thus, F. STOF- 

 FEL * showed (from H. ZANGGER'S laboratory) that the diffusion- 

 path of crystalloids in gelatin which was rapidly solidified is greater 

 than it is in gelatin which was slowly solidified, but that this becomes 

 equalized after several days. 



The rate of diffusion may be delayed or hastened through the 

 presence of a third substance. This affects the diffusion in jellies to 

 a much greater extent than in liquids. On page 69 et seq., we shall 

 see that chlorin, iodin, nitrate, and other ions, urea, etc., favor swell- 

 ing; on the other hand, sulphate, citrate, and other ions as well as 

 alcohol, sugar, etc., as compared with pure water, diminish swelling, 

 so that to a certain extent the meshes of the colloid network may 

 be opened or closed. It is easy to understand that diffusion will 

 occur less rapidly through narrow meshes than through wide ones. 

 That such an influence on diffusion actually occurs was experimen- 

 tally shown by H. BECHHOLD and J. ZIEGLER. They showed that the 

 permeability of gelatin and agar jellies for electrolytes and non- 

 electrolytes was increased by urea, whereas it was diminished by 

 sodium sulphate, grape sugar, glycerin and alcohol. An increase is 

 also produced by sulpho-groups, according to Bom.* 



It is evident that every substance which increases the permeability 

 of other substances, paves the way for its own passage as well. If 

 a jelly has been saturated with urea, the later coming particles of 

 urea will diffuse more rapidly; conversely, sodium sulphate and 

 grape sugar particles obstruct by their influence on the gel the pas- 

 sage of the subsequent particles. 



There are other ways in which diffusion in a gel may be dis- 

 tinguished from that in aqueous solution. We know from Chapter II 

 that colloids adsorb other substances to a greater or less extent. 

 By this means diffusion may be more or less impeded and under 

 certain circumstances even entirely arrested. This may be ob- 

 served with ease in the diffusion of dyes. H. BECHHOLD and J. 

 ZiEGLER* 2 showed that gelatin was deeply stained with methylene 

 blue and thus the diffusion in the gelatin was impeded;* whereas the 

 juice of red beets is a dye which is not noticeably adsorbed. Finally, 

 if we observe that adsorption is strongly influenced by the presence 

 of salts and non-electrolytes, and that an effect on diffusion is thus 

 exerted, we shall see what great complications may appear when 

 diffusion occurs in colloid media. 



Though a diffusion of colloids in aqueous media was long doubted 

 the diffusion of colloids in jellies was positively denied. THOMAS 

 GRAHAM held it to be characteristic of colloids that they were ar- 

 rested by other colloids. H. BECHHOLD* 2 called attention to the 



* [Graham recognized the slow diffusion of colloids. Tr.] 



