ADSORPTION 173 



exposed per unit volume) is the same in each case. This rather 

 unexpected fact emphasizes the purely physical nature of adsorp- 

 tion and leads one to believe that a strictly chemical union does 

 not take place. That the chemical nature of the adsorbent is 

 not (usually) a factor is shown by the experiment in which the 

 order of adsorbability of a series of substances (alcohols) is pre- 

 served unchanged in substituting one of three adsorbents, viz., 

 blood charcoal, talcum, or sulphur. The order of the series of 

 substances adsorbed remains the same; therefore, the chemical 

 constitution of the adsorbent is not a factor, but the efficiency 

 of the respective adsorbents differs owing to differences in their 

 physical structure. Charcoal is fifty times as effective as talcum, 

 and talcum more effective than sulphur. Troublesome problems 

 in adsorption arise from such instances as the following: 12 grams 

 of kaolin is equivalent to 1 gram of blood charcoal in respect to 

 the adsorption of methylene blue, while 1,000 grams of kaolin is 

 insufficient to produce the effect of 1 gram of blood charcoal in 

 the adsorption of heptyl alcohol. 



While it is the physical rather than the chemical nature of the 

 adsorbent which usually determines its efficiency, this is not 

 always true. Barger has shown that the adsorption of iodine by 

 organic solids is very definitely dependent upon the chemical 

 nature of the adsorbent. Furthermore, in the commercial 

 process known as selective dyeing, each dye reacts with only 

 one type of fabric. It is thus possible to get two or more colors 

 from the same bath. A bath containing amacid milling scarlet, 

 amanil sky blue, and S. R. A. golden yellow will dye silk scarlet, 

 viscose rayon blue, and celanese yellow. Selective dyeing with 

 two colors is understandable if we interpret adsorption as a 

 phenomenon involving electric charge, but selective dyeing with 

 three colors forces one to grant that the chemical constitution 

 of the adsorbent is a factor. 



Although the chemical constitution of the adsorbent appears 

 to be unimportant in certain cases, this is seldom true of the 

 substance adsorbed, as in the case of homologous series (e.g., of 

 alcohols), each member of which is taken up (e.g., by charcoal) 

 in increasing amounts the higher they are in the series (Traube's 

 rule, which follows). 



Gibbs' Law. — Willard Gibbs developed certain physical prin- 

 ciples which have become classic. Brief reference has been made 



