170 SMELL, TASTE, ALLIED SENSES 



known of the stimuli for the vomero-nasal organ, and 

 very little has been done on those for the common chemi- 

 cal sense. The senses of smell and of taste are naturally 

 much better known. When their stimuli are compared 

 they are found in general to belong to different categories 

 of material ; what is smelled is generally not tasted and 

 what is tasted is not smelled. 



These two categories of substances afford an impor- 

 tant basis for comparing taste and smell. This can be done 

 from the standpoint of the minimum concentrations of 

 materials that serve as stimuli for the two sets of recep- 

 tors. Bitter substances are apparently the most effec- 

 tive stimuli for the sense of taste. Quinine hydrochloride 

 can be tasted in a solution as weak as 0.00004 molar, 

 but this threshold is exceeded by that of what is probably 

 the most bitter of all substances strychnine. According 

 to Grley and Richet (1885) the weakest solution in which 

 the bitter taste of strychnine hydrochloride can be distin- 

 guished contains only 0.0004 gram of this substance in 

 one liter of water. This is approximately equivalent to 

 one and a half millionths of a molar solution (1.48xlO~ 6 

 molar), and much exceeds in this respect the efficiency 

 of quinine. One of the strongest odors known is that of 

 mercaptan of which according to Fischer and Penzoldt 

 (1886), 0.01 milligram evaporated in 230 cubic meters of 

 air gives a perceptible smell. Assuming the substance 

 used by these investigators to have been methyl mercap- 

 tan, such a dilution would be represented by about a 

 million-millionths molar solution (9xlO' 13 ) or approxi- 

 mately one and a half million times more dilute than the 

 weakest solution of strychnine that can be tasted. Thus 

 the olfactory receptor is open to stimulation by a very 



