PROTOPLASMIC STRUCTURE 89 



as a rule more readily adsorbed than their salts. Many 

 organic acids (the lower members of the fatty series) 

 are highly effective in lowering surface-tension, while 

 their salts have little influence; this effect, however, 

 may be in large part attributable to the undissociated 

 molecules. As a class, acids have a marked effect on 

 the surface charges of indifferent solid substances, 

 tending to make these surfaces positive; similarly bases 

 make them negative. Both effects appear in very low 

 concentrations (Perrin)^ and are undoubtedly due to 

 H and OH ions, respectively. The fact that adsorbent 

 surfaces of the most widely varying chemical composi- 

 tion (carbon, hydrocarbons, silicates, oxides, metals) 

 are thus affected indicates that adsorption rather than 

 chemical combination in stoichiometric proportions lies 

 at the basis of the effect. A slight change in H and OH 

 concentration may thus have a very marked effect 

 upon the potential difference across a surface; this is 

 well shown in the curves given by Haber and Klemensie- 

 wicz,^ and in the results of Perrin.^ The great effective- 

 ness of the H-ion as a precipitant for suspensions of 

 indifferent substances probably depends on its high 

 adsorbability as well as on its high velocity and special 

 chemical properties. 



At a certain concentration where OH and H-ions are 

 adsorbed in certain proportions the surfaces will be 

 electrically neutral; this condition defines the isoelectric 



^ Cf. Perrin, Jour. Chim. Phys., II (1904), 601. 



=^ Haber and Klemensiewicz, Z. physik. Chem., LXVII (1909), 385. 



3 See especially Perrin's curve for naphthalene, reproduced in 

 Freundlich's KapUlarchemie, p. 236. Ellis also describes this effect in 

 the cataphoresis of oil droplets, Z. physik, Chem., CXXVIII (1911), 321, 



