468 Cell Penetration by Acids. IV 
23, cas -2 
4) 
Log. conc’ ly. 
Fic. 3. The relation between speed of tissue penetration and acid con- 
centration in solutions of HCl (light line) and H;PO, (heavy line), when 
the formol concentration and apparent time of penetration have been 
corrected for the quantity of acid necessary to induce color change in the 
indicator system studied (27.0°). 
esses. This general result is also obtained with other acids. 
Each part of these plots follows an equation of the form 
1 1 
log {| ——— }= - log (concentration’) — log k 
Poot st n 
(P. T., observed penetration time, P. T.’, corrected penetration 
time, & a constant), which is similar to the Freundlich formula 
for the adsorption isotherm and may in fact be derived from this 
formula by the aid of assumptions such as are commonly made 
in applying it. The curves shown in Fig. 3 have, however, no 
probable reference to adsorption; what they do express is the rela- 
tion between the concentration of acid external to the tissue and 
its ability to force through the tissue a quantity of acid sufficient 
to induce the intracellular color. change. With H3PO, this ability 
increases very Slowly up to a point where the osmotic pressure 
of the acid is adequate rapidly to overcome the resistance to its 
diffusion, which is produced in the tissue by the acid itself; beyond 
