302 THE COLLOIDAL STATE 



or sponge-like structure, of concentrated solution representing 

 the continuous phase, whereas the disperse phase is represented 

 by a dilute solution filling up the interstices. 



Evidence for the existence of some such sponge-like or 

 honeycomb structure has been obtained by Hardy * in studying 

 under the microscope the formation of a gel. 



It is only by postulating some analogous structure that it 

 is possible to understand how I gram of agar can cause 99 

 grams of water to set to a stiff jelly just as the organized cell 

 structure of many plants enables them to maintain a rigid 

 form while consisting of practically 90 per cent, of water. 



ADSORPTION. 



The phenomenon known as the occlusion of gases is an 

 example of the adsorption 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 adsorption of the last traces of gas in the 

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

 adsorption 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. 



To all such cases of purely surface attachment the term 

 Adsorption is generally applied, as opposed to absorption 

 which implies something below the surface layer. 



The property of adsorption is likewise one of the most im- 

 portant characteristics of colloidal solutions resulting directly 

 from their great surface development. Wo. Ostwald has 

 calculated that if a cube of material of I cm. edge, presenting 

 a total surface of 6 sq. cms., were broken up it could yield 

 io 18 cubes of lOfji/jb edge (= -oooooi cm.), presenting a total 

 surface of 600 square metres. Such cubes would be approx- 

 imately the size of the particles of a colloidal solution, and it 

 will therefore be seen that a comparatively small mass of the 

 particles in such a colloidal solution must, in the aggregate, 

 present a very considerable surface. 



It has been calculated that the total surface presented by 



* Hardy : " Proc. Roy. Soc.," 1912, A., 87, 29, 



