Permeability 25 
the two halves of a partially split rubber stopper and the strips held 
hori zontally so that the free ends could move in a horizontal plane. The 
strips so held were immersed in 20 c.c. of a solution of the substance 
to be examined in which the strips underwent no appreciable change 
in curvature. The concentration of the solution was then increased 
by the addition of a known small volume of a molecular solution of 
the salt. As a result the strip of tissue underwent a decrease in 
curvature, which soon ceased and was followed by a slow increase in 
curvature. The time that elapsed between the increase in concentra¬ 
tion and the instant that the strip regained its original curvature was 
called the time of recovery. Immediately on recovery the concentra¬ 
tion of the solution was again raised and the time of recovery again 
noted. The process was repeated several times and in this way a series 
of times of recovery was obtained. An empirical value for the rate 
of penetration was obtained by dividing the change in concentration 
by the time of recovery. Curves were then plotted between these 
rates of penetration and the times that had elapsed between the first 
immersion of the tissue in the solution and the middle of each recovery 
time. These curves are supposed to represent the rate of penetration 
of the salt into the tissue. 
Brooks appears to regard the values he obtains for rate of pene¬ 
tration as measures of permeability. He has good grounds for doing 
this, as the difference in concentration between external solution and 
cell sap with regard to the substance under investigation is the same at 
the moment at which each recovery period commences. But the 
effects of exosmosis are not considered, nor are the possibilities of 
any reactions between the penetrating substance and the contents 
of the cell sap. Also the results obtained during the first 15 or 20 
minutes were very irregular, and were therefore disregarded. The 
method in its present form is thus not free from objection. 
13. Diffusion Method 
This method has also been described by Brooks (1916 a , 1917 b). 
Disks cut from the thallus of Laminaria Agardhii were placed between 
two short pieces of glass tube applied to the two sides of the tissue 
so as to make a watertight join. The lower piece of tube was closed 
below by a rubber tube and clip. The cell thus formed was filled with 
a salt solution having the same conductivity as sea water, or with 
sea water itself, and the upper tube with a known quantity of a solu¬ 
tion of the same composition but of one-half the concentration. The 
