INTERRELATION OF THE CHEMICAL SENSES 171 



much weaker concentration than the gustatory one is. 

 It might be maintained, however, that the line of ar- 

 gument used in the last paragraph is invalid because it 

 is based upon measurements of one substance for taste 

 and another for smell, and that, therefore, the two sets 

 of figures are not fairly comparable. But the conclusion 

 just reached is also supported by determinations made 

 with a single substance. Ethyl alcohol is soluble in both 

 water and oil and is one of the relatively few substances 

 that has at once both taste and smell. As a matter of 

 fact it is also a stimulus for the common chemical sense. 

 Hence it may be conveniently employed for comparing all 

 three classes of receptors. When such a test is made, 

 it is found that the weakest concentration of alcohol vapor 

 that can be smelled is about 0.000125 molar and that the 

 weakest aqueous solution of this substance that can be 

 tasted is 3 molar. To stimulate the common chemical 

 sense with ethyl alcohol requires an aqueous solution of 

 strength 5 to 10 molar. Hence so far as ethyl alcohol 

 is concerned smell may be said to be about 24,000 times 

 more delicate than taste and about 60,000 times more 

 delicate than the common chemical sense. From the 

 standpoint of a single substance then, smell must be ad- 

 mitted to be vastly more efficient than either taste or 

 the common chemical sense both of which lie in this re- 

 spect close together (Parker and Stabler, 1913). Unfor- 

 tunately the stimulation of the vomero-nasal organ has 

 not yet been studied so that its capability from this 

 standpoint is not known, but, judged from its structure, it 

 probably has a receptive efficiency not far from that of 

 the olfactory organ. In that case the chemoreceptors of 

 vertebrates would fall into two groups, the olfactory and 



