466 Cell Penetration by Acids. IV 
the factor responsible for the rapidly increasing slope of its pene- 
tration curve. This factor is probably concerned, in part, with 
differences in the kind and quantity of intracellular materials 
with which the phosphoric acid may react to produce “‘buffer’’ 
substances. Thus in four tissues which are available for com- 
parison, the penetration relations of H;PO, appear as follows (at 
0.01 Nn). 
TABLE II. 
Transition 
Source of tissue. point of Order of apparent penetration. 
indicator. 
pH 
Chromodorts. 5.6 Oxalic, lactic > malonic > tartaric > malic > 
citric > H;PO, > acetic. 
Stichopus. * (8.0?) | Lactic, oxalic > malonic, tartaric, H;PO, > 
malic, citric’ > acetic. 
Browallia.t 6.0-5.0¢) H;PO, > oxalic > tartaric > lactic, citric > 
acetic. 
Hyacinth. t 4.0{ | Oxalic > H;PO, > tartaric > lactic > citric, 
acetic. 
* Harvey, 1914. + Haas, 1916, a. t Haas, 1916, 0. 
These differences appear in further part accounted for by the 
greater density of the Chromodoris tissue. The case of H3PQ, is 
somewhat different from that of caprylic acid, whose penetration 
cannot be observed at all by this method because of its relatively 
low solubility in the aqueous phase containing the indicator, 
although saturated solutions of this acid will produce color change 
in water solutions of the pigment. 
IV. The actual speed of protoplasmic penetration by H3PQO, is 
probably much greater than its apparent magnitude. This is 
shown by the way in which the speed of diffusion rapidly in- 
creases with increasing concentration. In solutions of 0.01 N with 
respect to Ht, Haas (1916, a) observed that this acid penetrated 
about as rapidly as other strong acids (HCl, HNOs), but less 
rapidly than acetic, formic, or lactic acids. At 0.056N total 
concentration, H3PO, solutions are about 0.01 N in hydrogen ions. 
From the curve in Fig. 1 it is seen that at this concentration the ~ 
penetration time of phosphoric acid is 6.4 minutes; under similar 
conditions the figures for several other acids are: HCl, 7.6; HNOs, 
8.4; formic < 1.0; lactic, < 2.0. The inward diffusion of H;POu, 
