54 SMELL, TASTE, ALLIED SENSES 



cubic centimeters of air. Notwithstanding this infinites- 

 imally small amount of mercaptan, the quantity, just 

 designated was estimated by von Frey (1904) to contain 

 some 200,000,000,000 molecules of mercaptan. 



Passy (1892a, 1892b) has made similar minimum de- 

 terminations for a number of substances and has shown 

 that artificial musk, probably the most powerful of all 

 known odorous materials, is about a thousand times 

 stronger than natural musk. In his other determinations 

 he found that olfactory acuity ranged in thousandths of 

 a milligram per liter of air from camphor at 5 to vanillin 

 at from 0.005 to 0.0005. The last determination may be 

 expressed as equivalent to 1/2,000,000,000 of a milligram 

 of vanillin in a cubic centimeter of air, a high dilution 

 but still not so extreme as that already recorded by 

 Fischer and Penzoldt for mercaptan. 



The details of the more important of Passy 's deter- 

 minations are given in the following table in which ol- 

 factory acuity, as measured by the minimum amount of 

 substance that was stimulating to the several persons 

 tested, is expressed in thousandths of a milligram per 

 liter of air. 



Table I. 



Minimum concentrations for olfaction in thousandths of a milligram of 

 substance per liter of air (Passy, 1892b). 



Substances Thousandths of a milligram 



Camphor 5. 



Ether 1. 



Citral 0.5 to 0.1 



Heliotropin 0.1 to 0.05 



Cumarin 0.05 to OjOl 



Vanillin 0.005 to 0.0005 



Passy (1892c) has also determined the minimum con- 



