APPENDIX 



587 



face tension of a medium is measured against air. In the colloidal 

 state each colloidal particle is exposed at its surface to the disper- 

 sion medium and there exists at the surface a tangential force 

 which maintains the integrity of the surface and attempts to re- 

 duce the surface area to a minimum and this is the interfaeial 

 tension. 



Electrical Phenomena and Surface Potentials. — The terms 

 ''membrane potential" and "critical potential" have, during re- 

 cent years, crept into immunological papers having to do with ag- 

 glutination, precipitation and complement fixation. It is hoped 

 that the following simple explanations may liel]) the student in 

 his reading of current literature. 



+ 



Fig. 24. 



Fig. 24. — Negatively charged particle surrounded by single layer of positive 

 charges, theory of Helmholtz. 



Fig. 25. — Negatively charged particle surrounded by diffuse atmosphere of 

 positively charged particles, theoiT suggested by Gouy. 



Electrical Double Layer, Helmholtz. — In colloidal solutions 

 the dispersed particles are conceived of as being somewhat like a 

 condenser in that at the surface there is a double layer of opposite 

 electrical charges. One layer is within the wall of the particle and 

 the layer of opposite charges is at a molecular distance away but 

 in the dispersion medium. This donl)le layer conception was origi- 

 nated by Helmlioltz. More recently the outer layer is conceived of 

 as forming a diffuse atmosphere rather than a single layer. These 

 two concepts are illustrated in Figs. 24 and 25 which show the 

 particle to be negatively cliarged and surrounded with a simple or 

 diffuse atmosphere of positive charges. 



