ADSORPTION 295 



Evidence for the existence of some such structure in gels 

 is obtained from microscopic examination ; furthermore the 

 existence of so-called elastic gels, such as can be obtained by 

 cooling gelatine solutions containing only about 5 or 10 per 

 cent of solid and 95 or 80 per cent of water, explains to some 

 extent how it is possible to obtain a rigid structure from plants 

 such as asparagus or spinach, etc., which contain about 90 

 per cent of water. 



In such a honeycomb or sponge-like structure there is, of 

 course, a very large internal surface over all of which the 

 phenomenon of absorption can take place. In consequence 

 of this such gels will tend to retain with considerable tenacity 

 small quantities of foreign substances, which accounts for the 

 difficulty experienced in attempting to rid colloids of the last 

 traces of impurities. 



ADSORPTION. 



The phenomenon known as the occlusion of gases is an 

 example of the absorption of gaseous matter by a solid sur- 

 face ; it is exhibited to some extent by glass and platinum, 

 but far better by wood charcoal, owing to its large superficial 

 area ; on this fact depends the use of wood charcoal as a de- 

 odorant or for the absorption of the last traces of gas in the 

 production of high vacua. It is not known in what way the 

 absorption is effected, but the immediate effect is to produce a 

 concentration of gaseous molecules at the surface of contact 

 between the solid and the gas. 



Similarly, it has been shown from thermodynamical con- 

 siderations, that when a solid body is introduced into a solution, 

 the dissolved substance will tend to accumulate at the surface 

 of contact between the solution and the solid. 



To all such cases of purely surface absorption the term 

 Adsorption is generally applied. 



The concentration of a dissolved substance upon the sur- 

 face of a solid introduced into a solution may be illustrated 

 by dipping a piece of filter paper into a dilute aqueous solu- 

 tion of Congo red ; after a short time the dye will have accumu- 

 lated on the surface of the paper, leaving the solution much 

 lighter in colour. 



Moreover, since congo red is in colloidal solution and filter 



