108 



PROTOPLASM 



loidal" particles such as bacteria, unicellular algae, mushroom 

 spores, and blood corpuscles, and also products of living organisms 

 such as fat droplets in milk, are all negatively charged. 



We know that colloidal particles are electrically charged 

 because they move in an electrical field toward one pole or the 

 other, just as do ions. The nature of the electrical charge, of 

 what it consists, and how it is held by the particle are difficult 



problems. For the present, we 

 shall limit ourselves to suspen- 

 sions pure and simple, such as 

 metal particles dispersed in water. 

 The first constructive idea on the 

 nature of the charge was advanced 

 by Quincke and mathematically 



Fig. 78.— The Helmholtz elec 



tricai double layer on a colloidal expressed by Helmholtz, who pos- 



tulated the presence of two layers 

 of charges of opposite sign, adherent to the particle. These have 

 come to be known as the Helmholtz double layer (Fig. 78). The 

 charges could be adsorbed ions, possibly the positive hydrogen 

 ions (H+) and negative hydroxyl ions (OH") of water, but 

 Helmholtz did not have ions in mind, as they were unknown at 

 his time, nor does it now appear likely that H+ and OH- ions are 

 responsible for the charge on metal particles. 



Chiefly through the work of the Vien- 

 nese medical colloid chemist Wolfgang 

 Pauli, it seems probable that ions of salts 

 of the metal in suspension formed at the 

 time of (electrical) dispersion surround 

 the particle. 



A difficulty arose in regard to the 

 Helmholtz double layer. If an equal 

 number of charges of opposite sign sur- 

 round a particle, they will leave the particle electrically neutral, 

 but we know that colloidal particles are not neutral, for they 

 wander in an electrical field. This difficulty was obviated by 

 assuming that only the innermost layer clings to the particle and 

 slips with the particle from under the outer layer, taking on other 

 charges as the particle moves (Fig. 79). 



Stability. — The stability of colloidal particles is in a great 

 measure due to their electrical charge. As all the particles of a 



+ + 



+ + 



Fig. 79. — Manner of shed- 

 ding the outer layer in the 

 progress of a particle through 

 its medium under the influ- 

 ence of a current. 



