138 SMELL, TASTE, ALLIED SENSES 



hydrochloric acid one- third as concentrated. Nevertheless 

 the acetic acid is dissociated only about one-fourteenth as 

 much as the hydrochloric. Hence ion for ion the acetic 

 acid solution is the more sour of the two. This result 

 was also arrived at by Kahlenberg (1898) who estimated 

 the sourness of acetic acid at a concentration of 0.005 

 molar to be about four times what should be expected from 

 its hydrogen ion content. These differences were sub- 

 sequently reaffirmed by Becker and Hertzog (1907). 



It is by no means easy to explain the excess of sour- 

 ness on the part of acetic and other like acids. Richards 

 has suggested, without putting great stress on the idea, 

 that the additional sourness of acetic acid may be due to 

 the undissociated molecules, which, serving as a reserve, 

 producing additional hydrogen ions as those present are 

 used up in the reaction between the acid solution and the 

 surface of the receptor, an opinion supported by the 

 recent work of Harvey (1920). Crozier (1916, 1918a, 

 1918b), on the other hand, has pointed out the probability 

 that the question is double, one part having to do with 

 penetration and the other with the production of the 

 sour taste. By taking advantage of natural indica- 

 tors, such as the blue pigment in the integument of 

 Chrompdoris, it can be easily shown that acids pene- 

 trate living cells. This may be assumed to be the 

 first step in sour gustation. But penetration observed 

 in this way is a much slower process than gustation, 

 hence the penetration concerned with taste can have 

 to do only with the most superficial layer of the 

 taste cells. It is the ease of combination with this layer 

 that may make the difference between acetic acid and 

 other acids. Different acids having penetrated the sur- 



