1917] BROOKS—PERM EABILITY 243 
some substance. These charged atoms, known as ions, are not 
created by the electrical conditions imposed, but already exist in 
all solutions capable of conducting a current. The rate at which © 
the current will be conducted by the ions of a given salt will depend 
upon two factors, the potential gradient and the frictional or other 
resistance to the migration of the ions. If the potential gradient be 
kept constant, we may follow fluctuations in the last factor by a 
measurement of the current, or by a direct measurement of the 
electrical resistance. If, therefore, we force the current to pass 
through living protoplasm in a solution, the resistance offered by the 
protoplasm to the passage of the ions will measure its permeability 
to the ions in question (the permeability may be regarded as vary- . 
ing inversely as the resistance). By the use of alternating currents 
of rather high frequency we avoid large effects due to accumulation 
of ions at surfaces impermeable to them.” 
A method of this type was independently employed at about 
the same time by R6rH (54), BuGARsKY and TANGt (2), and 
STEWART (58), who found that the conductivity of blood serum was 
greater than that of blood itself, and that the resistance rose rapidly 
with increase of the proportion of corpuscles to serum. The blood 
corpuscles seemed to be slightly or not at all permeable to the 
electrolytes of blood. The conductivity of the suspension of 
corpuscles was shown to be increased by haemolytic agents, the 
corpuscles then being permeable to salts (cf. WOELFEL 70, also 
STEWART 58, 59). McCieNpon has also attempted to study the 
changes in permeability of sea urchin eggs during fertilization (32) _ 
and of muscles in tetanus (33). The evidence from his —— 
ments on sea urchin eggs agrees with that of Harvey (12), previ- 
ously mentioned, but difficulties in technique which McCLEeNnpon — | 
found it impossible to avoid make the data of these experimer 
exceedingly unreliable. 
be experiments of OstERHOUT (42-47) 0 on the conductivity a : : : 
. tissue of ene alga I Lami: tinct 
