736 



SCIENCE 



[N. S. Vol. XLII. No. 1090 



that of Harvey discloses that the relative pene- 

 trating power of the acids at this concentration 

 is practically identical in the two cases. Some 

 of the differences may be due to the tempera- 

 tures at which the two sets of experiments 

 were made. The figures for the Chromodoris 

 tissue represent the mean of ten concordant 

 experiments at a uniform temperature of 27°.0. 

 3. Examination of the penetration time of 

 these acids over a range of concentrations 

 (0.1 N to 0.001 N) shows, however, that the 

 series established at the single concentration 

 (0.01 N) gives an entirely misleading picture 

 of the penetrating powers of the different 

 acids, which is better judged by the nature of 

 the penetration curves as a whole. According 

 to this view the order arrived at is seen in 

 Table II. The acids studied may be arranged 



TABLE II 



Penetration Power of Acids 

 Group I: 



Ionization 

 Acid Constant C^) 



Hydrochloric (100) 



Sulphuric (100) 



Oxalic 3.8 



Nitric ■ (100) 



Group II: 



Formic 0.0214 



Salicylic 0.102 



Valeric (iso-) 0.0017 



Lactic 0.0138 



Tartaric 0.1000 



Citric 0.0870 



Butyric 0.00149 



Acetic 0.00180 



in two groups on the basis of the character of 

 their penetration-dilution curves.* The curves 

 of the second group are all more or less paral- 

 lel and uniformly concave toward the axis of 

 penetration-time, whereas the curves of acids 

 of the first group (up to 0.002 N) are from 

 the beginning concave toward the axis of dilu- 

 tion. The curves of the two sets cut across one 



8 This series has an important bearing on the 

 interpretation of sensory stimulation by acids, a 

 matter which first turned my attention to this 

 problem. 



another, as do also some of the curves within 

 each set. The acids of the first set give visible 

 evidence of i)enetration at higher dilutions 

 (n/750) than do those of the second group. 



The separation of these two groups of acids 

 is further warranted by the fact that, within 

 certain time-limits, a preliminary exposure of 

 the Chromodoris tissue to the action of acids 

 of the second group does not hasten the pene- 

 tration of acids of the first series, but does 

 that of other acids of the second set. 



4. The acids included in my group I. of 

 Table II. are all acids of strong ionization, 

 while those of the other group are of low acid 

 strength. To this extent the role of ioniza- 

 tion in determining permeability toward acids 

 is made clearer than has hitherto been the case, 

 and it seems probable that these two kinds of 

 penetration curves represent at least two differ- 

 ent and distinct methods whereby acids may 

 gain access to the interior of cells. Within 

 each of the two sets of acids the degree of 

 ionization is less important in controlling the 

 speed of penetration. Formic acid occupies 

 a somewhat peculiar position, as do also butyric 

 and valeric; the first substance shows a dilu- 

 tion curve more nearly approaching that of the 

 strong acids, agreeing with its constitution, 

 but the relative positions of butyric and valeric 

 in the series are more likely to be accounted 

 for by their rather high solubility in lipoids. 



All the evidence so far available indicates 

 that acids penetrate and combine to various 

 degrees with one or more of several constitu- 

 ents of the cell surface. It is certain, at any 

 rate, that the "lipoid theory" of permeability 

 is not even approximately complete as an ex- 

 planation. Further attempts to elucidate the 

 significance of the apparently quite general 

 uniformity in the order of cell penetrability 

 for various acids in different animals must 

 await the study of a larger series of substances, 

 especially with reference to the action of 

 acids on penetrability for other acids.^ 



W. J. Crozier 



Agae's Island, Bermuda 



9 Details, covering additional points not here con- 

 sidered, will be found in a paper to appear in the 

 Journal of Biological Chemistry. 



