1917] BROOKS—PERMEABILITY 311 
figure obtained for the salt in question by that for the control 
period of the same disks of tissue, we obtain a figure (given in the 
ratio column of table II) from which all errors due to individual 
variations of the disks of tissue are eliminated. 
6. The method for the determination of the conductance of the 
solutions was as follows. The solution was poured from the cell 
into a U tube of such dimensions as to give a conductance of the 
order of magnitude most accurately determinable, namely, about 
1500-2000 ohmsX1077. The U tube was nearly immersed in a 
constantly stirred water bath whose temperature, determined to 
o%05 C., varied less than 0°8 C. in any one set of readings. A 
temperature correction of 2 per cent per degree Centigrade was 
applied to the actual readings to reduce them to the average tem- 
perature of the set, and the results calculated from the corrected 
readings thus obtained. A slide wire bridge, a standard 1000-ohm 
bifilar resistance (supplied with current from the secondary of a 
Nernst string inductorium at about 300-500 oscillations per second), 
and a telephone as the zero instrument were used in the customary 
manner to measure the resistance between bright platinum elec- 
trodes immersed in the solution at the opposite ends of the U tube. 
The distance between the electrodes was fixed. The readings had 
an error less than 0.1 per cent. The check experiments in half- 
strength sea water usually gave an agreement of corrected readings 
within 0.05 per cent. It will be seen that this degree of accuracy 
was ample for the purpose. 
Results 
It is desirable first to find out how fast the various salts pass 
through the intercellular substance, and whether there is any 
selective permeability due to any source other than the protoplasm. 
The data presented in table I show that the cell walls intercell 
substance of Laminaria are permeable to the salts used, and that 
the passage through the walls is nearly independent of the nature 
of the diffusing salt. In dead material the change of concentration 
is so rapid that owing to the decrease in the concentration gradient 
the rate of passage of salts through the tissue decreased, as is shown 
by the lower rate for the longer periods in both sea water and sodium 
