ADSORPTION 171 



remaining. Not only do these experiments illustrate that adsorp- 

 tion is a surface phenomenon, but they prove the rule of Willard 

 Gibbs that substances which lower surface tension accumulate at 

 the surface. 



The use of charcoal for removing moisture and odors from the 

 air and color from liquids (brown sugar is made white in this way) 

 is a well-known commercial practice. Charcoal as a gas filter, 

 for purifjdng air for breathing, played an important part in the 

 World War. It filled the gas masks of soldiers, and the more 

 compact the wood (coconut shells and peach pits) the greater 

 the number of pores and adsorbing surface per volume. Silica 

 gel, which, when wholly dry, is a hard and very finely porous 

 aggregation of sand particles (Fig. 90), is one of the most 

 efficient of commercial adsorbents (for removing odors in refriger- 

 ation, for drying air, etc.). Air blown into a steel blast furnace 

 must be dry. To accomplish this, it is passed through charcoal 

 or silica gel which adsorbs the moisture suspended in the air. 

 The adsorbent may be used again after reactivation by drying 

 at high temperature. 



Negative and Selective Adsorption. — The terms negative and 

 selective adsorption are useful at times but are inexact, as no 

 adsorption is negative, and all is selective. 



Negative adsorption is said to take place when a solution 

 becomes more concentrated (in regard to the solute) after being 

 shaken with an adsorbent. Negative implies that the solute 

 has been pushed away by the adsorbent, but even this unlikely 

 event would not add more of it to the solution. It is quite evident 

 that what has actually happened is (selective) adsorption of 

 the solvent (water), leaving more solute in proportion to the 

 solvent than there was at first. Negative adsorption exists 

 solely by definition. It is similar to negative pressure (suction) 

 and negative surface tension. A negative force exists because 

 there is another positive force working in the opposite direction. 

 So it is with negative adsorption. One talks of adsorption and 

 measures concentration. With respect to that something meas- 

 ured which is a ratio and not a thing or event, the result is nega- 

 tive; but with respect to that something which is a process — a 

 physical happening — the action is positive. 



Reduction in the concentration of an acetone solution when 

 shaken with charcoal is a classical example of adsorption; it is 



