1921] MEIER—ROOT TIP 119g 
the surface of contact with the clay. The constriction became 
hard to the touch, the remainder of the cylinder swelling somewhat. 
When the current was reversed, the constricted end became soft 
and enlarged and the opposite end became constricted. 
In repeating a part of JURGENSEN’s work, QUINCKE discovered 
that not under all conditions do the particles in suspension move 
toward the positive pole. Starch grains in water in a glass tube, 
as well as particles of silk, cotton, and paper migrated toward the 
positive pole when suspended in water, and toward the negative 
pole when suspended in oil of turpentine. The theory in explana- 
tion of these phenomena of poorly conducting liquids migrating in 
one direction and suspended particles in the opposite direction under 
the influence of the electric current, was propounded by HELMHOLTZ 
(10). The fundamental assumption of this theory is that at the 
surface of contact of any suspended particle there exists a double 
electric layer. If the particle bears a negative charge, ‘the layer of 
medium immediately surrounding it bears a positive charge. On 
passing the current a displacement of one system against the other 
takes place, the liquid particles migrating toward one pole, the 
suspended particles toward the opposite pole. How the charge 
originates is not explained. 
An explanation of the movement in opposite directions of oil of 
turpentine and water, and substances suspended in them, was first 
suggested by CorHN (5). He showed that with reference to the 
sign of the charge of a solid in contact with a liquid, the substance 
with the greater dielectric constant is positive to the other sub- 
stance. The dielectric constant of oil of turpentine is 2.23, that of 
glass 4~7 (according to composition), and that of water is 81. In 
agreement with this explanation, glass is positive in oil of turpen- 
tine and negative in water. Water has a much higher dielectric 
constant than most other substances, and, as we have seen, most 
substances are negatively charged in water. 
Observations on electrophoresis in colloidal ‘‘solutions” or sols 
were published by Picton and LInDER (18) in 1892. Such colloidal 
sols consist essentially of very finely divided ultra-microscopic 
Particles suspended in a liquid. Since LrnpER and Picron’s publi- 
cation the work has been much extended, and the conclusions seem 
