172 PROTOPLASM 



said to be selective because charcoal adsorbs acetone more 

 readily than it does water. 



A striking demonstration of so-called selective adsorption is 

 that of the dyeing of silk or cotton in an ammonia solution of 

 fuchsin. The dye, fuchsin, is red, but the ammonia solution of 

 it is colorless. If a piece of silk is put into such a colorless solu- 

 tion of fuchsin, the silk is stained red through (selective) adsorp- 

 tion by the silk of the fuchsin molecules in the solution. 



Selective adsorption is better illustrated in the process known 

 as capillary analysis in which the selective adsorptive powers of 

 cellulose (filter paper) are used to separate substances; it is a 

 routine laboratory procedure. If a piece of filter paper is placed 

 with one edge in a solution of two dyes which are not adsorbed 

 with equal readiness, then the more strongly adsorbable one will 

 not ascend so high; it is taken up more rapidly. If the mixture 

 is of methylene blue and eosin, there occurs first a blue zone 

 containing a mixture of both dyes, and above a red zone of eosin 

 alone. (Capillary analysis is probably not a very exact method. 

 More than simple adsorption is involved. The calcium content 

 of the paper is a variable that will affect the result.) 



Possibly the best illustration of selective adsorption is that of 

 ions which can as a result be arranged in a so-called Hofmeister 

 series. 



Hofmeister Series. — Preliminary reference is here made to the 

 Hofmeister series (page 445), as it is in respect to adsorption 

 that such series particularly appear. Rona and Michaelis found 

 that salts of the same cation are adsorbed from water by charcoal 

 in the following order: 



SO4 < HPO4 < Cl< Br < NO3 < I < SCN < OH, 



which is the same order in which Hofmeister found them to affect 

 the swelling of protein gels, and Trondle observed their penetra- 

 tion into living cells. 



The Chemical and Physical Constitution of the Adsorbent. — It 

 has long been customary to say that the adsorption constant of an 

 adsorbent; i.e., its efficiency or capacity to adsorb gases or sub- 

 stances in solution, is not influenced by its chemical nature but 

 only by its physical constitution; in other words, it makes no 

 difference whether the adsorbent is charcoal, meerschaum, or 

 silica gel, if the structure (determined by porosity or surface 



