170 PROTOPLASM 



A precise definition of adsorption is difficult to give and is 

 usually unsatisfactory. It is best to define and use the term 

 simply to mean the concentration of one substance at the surface 

 of another, or, as Freundlich says, the loose fixation of particles at 

 surfaces. 



The origin of the word "adsorption" is somewhat obscure. 

 Probably it arose from "absorption." Adsorption is commonly 

 distinguished from a&sorption by the fact that the former has to 

 do with purely surface forces, while the latter is concerned with 

 capillary forces; for example, a dye is adsorbed to the surface of 

 charcoal, while water is absorbed by (in the capillaries of) a sponge 

 or blotter. This distinction is, however, weak and arbi- 

 trary, for the ultimate forces in both are similar. Furthermore, 

 the chemist technically uses absorption in quite another sense, 

 viz., to indicate the taking up of gases by liquids, which involves 

 essentially a going into solution. Yet it seems permissible to 

 distinguish between the adsorption of a gas by charcoal, through 

 condensation of it on the surface of the charcoal where it is held 

 by adhesion; and, on the other hand, the absorption of water by 

 capillaries which hold the water in pores by capillary action or 

 cohesion. The term sorption has been introduced to avoid con- 

 fusion. It implies the taking on of a substance without indi- 

 cating the mechanism (though usually denoting that no new 

 compound has been formed). It is a question whether another 

 term for what is ordinarily called adsorption is a help, 



A few examples of adsorption will give a better understanding 

 of the phenomenon in a general way and of the mechanism 

 involved than do terms and definitions. A neat experiment 

 demonstrating that adsorption is a surface phenomenon is the 

 following : Very dilute soapy water is colored with methyl violet, 

 thoroughly shaken, and the foam pipetted off. Shaking is 

 repeated, and the foam again collected. After some dozen repe- 

 titions of this process, the color of the liquid which has settled 

 from the foam collected will be considerably darker than that 

 of the original solution. The dye has each time been concen- 

 trated on the surface of the membranes of the soap bubbles. 

 The experiment may be done in another way, by shaking a 

 dilute solution of albumin and drawing off the foam. After 

 repeating the operation a number of times, that portion drawn 

 off will be found to be richer in albumin than the watery solution 



