84 PROTOPLASMIC ACTION AND NERVOUS ACTION 



by hydrogen in the presence of platinum, and the oxida- 

 tion of compounds by blood-charcoal (oxalic acid, etc.). 

 The combination of tannin with leather is a good illus- 

 tration of the determination of a chemical reaction by 

 a previous adsorption; the tannin is first adsorbed, then 

 it combines. The same condition is shown in the union 

 of dyes with heat-denatured egg-white; the process 

 is at first readily reversible (by acid), but not later, 

 indicating that the first stage of the process is a close 

 contact or adhesion, which is then followed by chemical 

 combination.^ The toxin-antitoxin reactions and the 

 opsonin reaction with leucocytes are further biological 

 instances of a similar kind. According to Morgenroth, 

 tetanus toxin is taken up or attached by living cells 

 at 8°, but does not become active until 20°. In the action 

 of enzymes adsorption processes play an important part, 

 as already indicated.^ 



INFLUENCE OF ELECTRICAL STATE OF SURFACE 



ON ADSORPTION 



The relations of electrostatic attraction or repulsion 

 between the charged surface of the adsorbent and the 

 charge on the particles of the dissolved substance consti- 

 tute a factor of decisive importance in many adsorption 

 processes. For example, acid dyes (whose colloidal 

 particles are negatively charged) are as a class more 

 readily adsorbed by suspended alumina (with positive 

 particles) than by kaolin, a silicate with negative 

 particles, and vice versa. Color bases show the reverse 

 behavior, being adsorbed by substances which form 



^ Unpublished observations of my own. 



' Cf. the data and discussion in Bayliss' textbook, p. 324. 



