454 W. J. CROZIER 



II. Practically the whole of the difficulty is contained in the 

 often considered case of acetic and hydrochloric acids. A compar- 

 ative study of the reactions to these and other acids as given by 

 the earthworm (Hurwitz, '10; Crozier, '16), leads to a preliminary 

 simplification of the matter, namely to the formulation of two 

 separate problems: these acids stimulate, they also result in 

 a sour taste. These two problems must be considered inde- 

 pendently; we must distinguish between (a) that property of 

 an acid which causes it to be efficient in stimulation and (b) 

 that which determines the sourness of an acid solution. 



The relative efficiency of an acid as an agent of stimulation 

 may be measured by the liminal effective dilution. The point 

 I wish to make is that the property of hydrochloric acid which 

 causes it to be tasted in solutions about five times as dilute as 

 that limiting the tastability of acetic acid may have no immedi- 

 ate connection with the common cause of the sourness perceived 

 when these acids act upon the tongue. 



This point of view contains the possibility of an explanation 

 for (and is, reciprocally, in part justified by) the fact that acids 

 exhibit a characteristic astringency in solutions so dilute that 

 they are no longer sour. There is some evidence, also, that the 

 sourness and astringency may be separated experimentally, 

 as by the use of cocaine. There is therefore ground for the 

 opinion that in taste excitation by acids two processes occur, 

 and that the production of a sour taste is the secondary one. 



The points which require solution with respect to human 

 taste excitation by acetic and hydrochloric acids are: 



(a) Both these substances stimulate. 



(b) They stimulate in different degrees, in dilute solutions 

 the stimulus from hydrochloric acid being the more intense. 



(c) They both result in a sour taste. 



(d) They fail to stimulate when too greatly diluted. 



(e) The dilutions which limit the capacity of the two acids 

 to stimulate are very different (HCl n/900 ± ; acetic, n/200 ±). 



Each of these points — except (c) — is closely paralleled by the 

 details of earthworm reactions to acids (Crozier, '16), and also 



