136 SMELL, TASTE, ALLIED SENSES 



disappears on the addition of some of this gas to insipid 

 water. Henle (1880) showed that insipidity was char- 

 acteristic of fluids that contained less salt than the saliva. 

 Insipidity is probably a deficiency phenomenon and may 

 be produced by the absence of several classes of sub- 

 stances. Nevertheless it must not be forgotten that a 

 condition of staleness or flatness in water, practically 

 indistinguishable from insipidity, can be produced by 

 introducing into the water very small amounts of caustic 

 alkali whereby hydroxyl ions are liberated (Kiesow, 

 1894-1896). 



4. The Sour Taste. Sour taste has long been asso- 

 ciated with acid substances. In fact it seems very prob- 

 able that the sour taste is excited only by acids, acid 

 salts, or materials that produce acids. All these sub- 

 stances on going into aqueous solution give rise to hydro- 

 gen ions by the dissociation of acid molecules. If the 

 solutions are strong they will also contain a certain 

 number of undissociated acid molecules. It was pointed 

 out by Richards (1898) that, since all such solutions have 

 the sour taste and since the one component that they all 

 have in common is the hydrogen ion, this ion must be the 

 occasion of their common taste. This conclusion was 

 independently arrived at in another way by Kahlenberg 

 (1898). A 0.0025 molar solution of hydrochloric acid has 

 a pronounced sour taste and its dissociation into hydro- 

 gen and chlorine ions is practically complete. A corres- 

 ponding solution of sodium chloride is also about 

 completely dissociated into sodium and chlorine ions but 

 is without taste. It follows, therefore, since there are 

 as many chlorine ions in the salt solution as in the acid 

 solution per unit volume and the salt solution is without 



