PHYSIOLOGY OF OLFACTION 53 



very strong. By this means extremely accurate quanti- 

 tative results can be obtained. 



In testing olfactory acuity the majority of workers 

 have used the method of evaporating a known weight of 

 substance in a given volume of air. By this method Val- 

 entin (1848) found that 1/2,000,000 of a milligram of oil 

 of rose per cubic centimeter of air was odorous. Assum- 

 ing that 100 cubic centimeters of this mixture were nec- 

 essary for olf action, he concluded that the total weight of 

 oil of rose used in this operation was the very small 

 amount of 1/20,000 of a milligram. Valentin also found 

 that water containing 1/2,000,000,000 of its weight of 

 tincture of musk had a perceptible odor whereas water 

 containing only 1/3,300,000,000 of this tincture could not 

 be distinguished from ordinary water. One gram of the 

 odorous mixture called forth the characteristic smell and 

 contained only 1/2,000,000 of a milligram of tincture 

 of musk. 



More significant measurements were made by Fischer 

 and Penzoldt (1886) on chlorphenol and mercaptan. One 

 milligram of chlorphenol was evaporated in a room of 

 230 cubic meters capacity and was thoroughly mixed with 

 the air. This dilution called forth an unquestionable ol- 

 factory sensation. It contained 1/230,000,000 of a milli- 

 gram of chlorphenol per cubic centimeter of air or, if it 

 is assumed that 50 cubic centimeters of air are the mini- 

 mum needful for olfaction, the total amount of chlor- 

 phenol necessary was found to be 1/4,600,000 of a 

 milligram. By a similar method it was shown that 

 mercaptan, a liquid with a penetrating garlic odor, could 

 be recognized at a concentration of 1/23,000,000,000 of a 

 milligram per cubic centimeter, a concentration that 

 would yield 1/460,000,000 of a milligram for every 50 



