THE TASTE OF ACIDS 457 



equal, the speed of this penetration varies with the acid and its 

 concentration; 3) the curves relating penetration time to con- 

 centration are different for the two acids, dilute solutions of hydro- 

 chloric penetrating much more easily than acetic. Points of re- 

 semblance, in fact of identity, are found among the characteris- 

 tics of stimulation bj' these acids, and have been enumerated as 

 a, b, e, in section 11. The closeness of the parallehsm indicates 

 that the stimulus is due to the penetration of the surface layer 

 of the cell. In stimulation by these acids the essential step is 

 the union of the stimulating agent with some constituent of 

 the ceptor surface. 



According to Becker und Ilcrzog ('07), the decreasing order 

 of intensity of taste sensation due to the acids they tested was 

 HCl, HNO3, trichloroacetic, formic, lactic, acetic, butyric. These 

 acids penetrate the indicator-containing cells of Chromodoris 

 zebra in the order HCl, HXO3, formic, monochloroacetic, lactic, 

 butyric, acetic (consiilering a range of concentrations; C'rozier, 

 '16). There is reason to believe that trichloroacetic penetrates 

 more speedily than monochloroacetic; and with this in mind, 

 the parallelism between these two series becomes surprisingly 

 close. This particular case is quoted with the purpose of point- 

 ing out that a small series of acids, such as that just cited, may 

 to all ai^pearances support the view that the intensity of taste 

 excitation is proportional to the ionization strength of the acids; 

 similarly, a penetration series may be chosen which seems to 

 prove a corresponding relation. Yet, further examination of a 

 large number of acids shows that ionization is but one of the 

 factors determining cell penetrating power, and the existence 

 of influences in addition to ionization has also been suspected 

 with reference to acid stimulation; direct evidence upon this 

 point has hitherto been lacking. 



IV. Analysis of penetration data obtained with eighteen acids 

 has shown (Crozier, '16) that in any given case abihty to pene- 

 trate the cell appears to depend upon two (or more) factors: 

 one is the ionization strength of the acid; the second depends upon 

 some other aspect of the acid molecule. To account for these 

 results it is further necessary to assume that the cell surface is 



