GELS 405 



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 

 deodorant 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 

 10^8 cubes of lOja/x edge (=-000001 cm.), presenting a total 

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

 mately 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 

 the particles of a red colloidal gold solution containing 0-5 

 grams of gold per litre amounts to about 8 square metres. It 

 is, therefore, easy to understand that with such an enormous 

 development of surface there is the possibiHty for a marked 

 manifestation of adsorption by suspensoids. 



In order to appreciate the effect of such surface develop- 

 ment it is necessary to realize that all liquids tend to reduce 

 their surface energy to a minimum ; in the case of a solution 

 this end may be assisted by increasing the concentration at 

 the surface of any substance which lowers the surface tension. 

 The most active substances in producing this effect are the 

 fatty acids, soaps, albumen, enzymes, etc., and it follows, 



