SENSATIONS OF SMELL 487 



fore, analogous to the organ of hearing. The chief support of this view was 

 that with certain strongly odorous substances no loss in weight could be de- 

 tected with the balance, whereas, if it were true that the olfactory organ was 

 excited by material particles set free from them there should be such a loss. 

 But Berthollet demonstrated that material particles are given off from odorous 

 substances. He placed a piece of camphor in a vacuum of a barometer; the 

 mercury of the barometer gradually fell, thus showing that small particles 

 of camphor were given off, that they collected in the empty space and exerted 

 pressure on the mercury. 



Another proof which we owe to Tyndall is the following: Radiant heat 

 passes through an absolutely empty space without being absorbed; if, how- 

 ever, a gas is placed in the path of the heat rays, a greater or less amount 

 of heat, according to the nature of the gas, is held back by it. Now Tyndall 

 showed that an atmosphere which had been in contact with odorous substances 

 and had taken up its vapor, absorbs radiant heat to a much greater extent 

 than pure atmospheric air. Thus the 

 vapor of patchouli absorbed thirty-two 

 times as much as air, oil of rose thirty- 

 six times, oil of anise three hundred 

 and seventy-two times as much. 



Odors are carried in a quiet atmos- 

 phere by diffusion. Of course air cur- 

 rents and the like also aid much in this 

 distribution. The carrying power i. e., 

 the power of diffusion varies with dif- 

 ferent odors. 



Johannes Miiller and several other 



authors assumed that the particles of FIG. 192. Olfactometer, after Zwaarde- 



odoriferous substance were first dis- maker, 



solved in the mucus covering the olfac- 

 tory region and then stimulated the olfactory epithelium. Since, however, very 

 many odorous substances are very slightly, if at all, soluble in water, Zwaarde- 

 maker has put forward the hypothesis that stimulation takes place by direct 

 contact of the gaseous molecules with the cilia of the olfactory cells. The fact 

 that fishes e. g., the dogfish have a well-developed sense of smell speaks 

 pretty definitely in favor of Miiller's view. 



Zwaardemaker has constructed a small apparatus, the olfactometer (Fig. 192). 

 for the purpose of testing the acuteness of the sense of smell. The essential parts 

 of this apparatus are a paper cylinder and a tube through which one may inhale. 

 The cylinder, which can also be made of filter paper, is dipped in the scented 

 fluid, and when its pores are filled with this, it is withdrawn, dried out and 

 hastily blown through. The smelling tube, which fits exactly into the tubulure 

 of the cylinder, is then inserted and the other end placed in the nasal opening. 

 The small wooden shield serves to keep the odor out of the other nasal opening. 



When air is inhaled through the tube from the cylinder impregnated with 

 the odorous substance the number of- odoriferous particles reaching the nose 

 will vary inversely as the depth to which the smelling tube is inserted into the 

 cylinder. By graduating the tube, cne can thus make very rapid and very exact 



