188 PHYSIOLOGY CHAP. 



If this surprising phenomenon, discovered by Zwaardemaker, 

 took place in the open air, it could easily be explained as the 

 effect of the chemical combination of the two odours, which would 

 form ammonium acetate. But this explanation will not hold for 

 the double olfactometer, because the two substances are separated 

 during the entire period of excitation by the nasal septum. It is 

 therefore a physiological effect, analogous to, but more complete 

 than, the compensations of gustatory sensations studied by 

 Kiesow. 



Zwaardemaker 's assertion that the simultaneous excitation of 

 smell by two or more different odours never elicits a compound or 

 mixed sensation was contradicted by later researches of Nagel, 

 who came to the following conclusions : 



(a) It is possible for any two odours to fuse into a mixed 

 sensation, which, at least for a few seconds, gives the impression 

 of a simple odour of a new quality. 



(&) The mixed odour is a persistent or transitory sensation, 

 according as the fatiguability of the olfactory organ is ap- 

 proximately equaL or different for the component odours. 



(c) If instead of only two, a large number of substances are 

 mixed, as by perfume makers, it is easier to obtain a more 

 persistent and decided mixed odour. 



(d) The mixed odour may resemble the component odours 

 without being identical with them ; in other words, it is always a 

 qualitatively new sensation. 



Zwaardemaker took up this question again, and admitted the 

 existence of true mixed odours, but declared that they only appear 

 when the component odours are very nearly allied, i.e. when they 

 belong to the same or to an allied class. When, on the other 

 hand, two odours of different and dissimilar classes are brought 

 together, there is not a mixed odour, but a neutralisation or con- 

 flict between the two sensations neutralisation if the stimuli 

 are weak, conflict if they are strong. Moreover, there are 

 certain variations in the strength of the stimuli, within which 

 the effects of compensation or of struggle do not disappear. To 

 produce a conflict the stimuli need not necessarily be equal in 

 intensity, or of the same value in olfacties. When weak stimuli 

 are used the intensity can only be varied within narrow limits ; 

 but there is a wider range of variation when stronger stimuli are 

 employed. 



To this Nagel replied that compound or mixed odours may be 

 formed by mixing odours not only with similar but also with 

 dissimilar substances. He obtained unmistakable mixed smells 

 with vanilline and bromine, amyl acetate and iodine, turpentine 

 and xylol, etc. It is true that owing to the different volatile 

 properties of the odorous substances, and to the unequal 

 fatiguability of the olfactory organ to different stimuli, the 



