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 inclilTerent 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 adsor]:)tion 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 dej)ends 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. Pcrrin, Jour. Chini. Phys., II (1904), 6or. 



* Haber and Klcmcnsicwicz, Z. phys'ik. Cheni., LX\ II UiJOq), i^S- 



3 See especially Pcrrin's curve for naphthalene, rcprtxluccd in 

 Freundlich's KapiUarchcmic, p. 236. Ellis also describes this ctTccl in 

 the cataphoresis of oil droplets, Z. physik. CJtcm., CXWIII (191 1), 321. 



