52 SMELL, TASTE, ALLIED SENSES 



a rod instead of a coating to the inside of a tube. The 

 rod is made of paraffin mixed in a definite proportion with 

 the odorous substance and the extent to which the rod is 

 exposed in the olfactometer tube is a measure of the 

 concentration of the odorous particles in the air current. 

 Rods of metal, platinum, gold, or zinc, have also been 

 used ; these have been charged by immersing them in an 

 atmosphere of odorous material for a given length of 

 time and then tested. The odorous particles gather on 

 their surfaces and are subsequently freed. The success of 

 this method makes it clear that in the original evapora- 

 tion method more or less of the odorous material must 

 become ineffective in that it adheres to the walls of the 

 container in which the evaporation is carried out. 



As a means of avoiding these and other difficulties 

 Allison and Katz (1919) have recently employed in the 

 testing of stenches a type of odiometer that for accuracy 

 of work bids fair to replace most of the other devices. 

 It consists of a number of Venturi-type flow-meters so 

 arranged that a measured volume of air can be passed 

 at a uniform rate through or over the chemical, and this 

 air is then mixed with another measured volume of pure 

 air also flowing at a uniform rate. The concentration of 

 the chemical is measured by determining its loss in 

 weight after a measured volume of air has passed 

 through or over it. From this loss of weight and the to- 

 tal volume of air with which the chemical has been mixed, 

 the concentration in milligrams per liter of air is deter- 

 mined. The mixture of air and chemical passes finally 

 through a tube with a glass funnel at the open end. The 

 funnel is placed over the nose of the person who by a sin- 

 gle inhalation tests the mixture. The odors are rated ar- 

 bitrarily as detectable, faint, quite noticeable, strong or 



