THE SIMULTANEOUS ACTION OF TWO ODOURS. 1257 



organs simply by their presence, even when in the gaseous condition, 

 they must he moved over the olfactory surface. 



The simultaneous action of two odours. — When two odours 

 diffuse into a room, we can generally recognise them both, or the odour 

 partakes of the nature of each. If one of them (A) is small in amount, 

 and the other (B) greatly preponderates, we may only just recognise A. 

 On increasing the quantity of A the sensation changes, and we may 

 recognise the two odours ; and on increasing A still further, we may lose 

 the sensation of B in the preponderance of that of A. But this is not 

 always the case, for certain odours appear to antagonise each other ; and 

 if two such antagonistic odours are mixed, either one of them alone is 

 smelt or nothing at all. Thus iodoform may be antagonised by balsam 

 of Peru, the odour of musk by that of the smell of almond. This 

 antagonism may of course be merely a chemical or physical action 

 outside the body of one odour upon the other. Zwaardemaker, however, 

 maintains that there is in some cases a purely physiological antagonism. 

 He has constructed a double olfactometer, consisting of two olfactometers 

 of the form already described. Two odours in any desired quantity can 

 thus be led (without mixing) one into each nostril, so that each odour 

 will stimulate its own portion of olfactory mucous membrane. The best 

 results were obtained when the vapour of a 2 per cent, solution of 

 acetic acid was led into one nostril, and the vapour of a 1 per cent, 

 solution of ammonia into the other. If ammonia was alone smelt, he 

 increased the strength of the acetic acid vapour, until a point was 

 reached at which there was no odour perceptible. 



The following is a list of physiologically antagonistic odours : — 



Musk and oil of bitter almonds. 

 Caoutchouc and paraffin. 

 Volatile oils and iodoform. 

 Ammonia and acetic acid. 



If these statements are true, and we can hardly doubt the results 

 obtained by so experienced an investigator as Zwaardemaker, we have 

 here what appears to be a unique fact in the physiology of the senses. 

 We have to suppose that two stimuli, each one capable by itself of pro- 

 ducing an effect on consciousness, when they act together mutually 

 antagonise one another's action. There are other cases, in other sense- 

 organs, in which one stimulus may modify the effect of another ; but in 

 no other case that I am aware of do two separate stimuli destroy each 

 other's effect on consciousness. This discovery modifies, too, one defini- 

 tion of modality as used by Helmholtz. Modality is the difference 

 which exists between the sensations of the different sense-organs, and 

 these are distinguished by their inability to pass into each other by 

 intermediate stages. By quality, however, is meant the difference which 

 exists between sensations of the same sense-organ, and these are dis- 

 tinguished by their power of passing into each other by intermediate 

 links. As this is not the case with the smell of ammonia and acetic 

 acid — for, according to Zwaardemaker, you either smell the one or the 

 other, or nothing at all — the differences between them must be one of 

 modality rather than of quality, although they belong to one and the 

 same sense-organ. 



The psychology of smell. — We detect the presence of most of 



