COMPOUND ODOURS. 321 



substances of definite composition do not necessarily possess simple 

 odours. Several different odours may co-exist in the same com- 

 pound, and ,Q;ive to the sense of smell the impression of a simple 

 odour when it is really a mixture. He states that such odours 

 may be dissociated in such a way as they can be recognised or 

 perceived individually. His description of the method of analysis 

 may be abstracted as follows : — " If several odours exist in the 

 same body, each of them should possess its individual or particular 

 perceptible minimum or lowest quantity by which its presence is 

 perceptible, which will not necessarily coincide with that of any 

 of the others ; and that if, from this point, the quantity of the 

 substance be progressively diminished or attenuated, the separate 

 odours ought to disappear or become imperceptible one after the 

 other." This theory is said to be verified by experience : as for 

 example with tertiary amylic alcohol. " Starting in the reverse 

 way with a quantity too small to be perceived by the sense of smell 

 and gradually increasing the quantity, the following series of 

 sensations are perceived : tttq-^o^o o o ^^ ^ gramme, odour of benzene 

 and isomylic alcohol ; -ywoVijwo of a gramme, — a camphoraceous 

 odour. At length, on increasing the quantity, an alcoholic odour 

 is perceived, which is not, strictly speaking, an odour at all, but an 

 impression on the nervous system generally, and which overpowers 

 the preceding impressions. An analogous succession of impressions 

 is produced by a great number of bodies, such as benzaldehyde, 

 benzylchloride, &c. Most perfumes which are pleasant when 

 smelled in small quantity, are extremely unpleasant when in bulk. 

 This observation holds good in a great number of instances, the 

 bodies possessing — 1'^, an agreeable perfume which is not very 

 intense, and which alone is perceived when the quantity is minute ; 

 2°, an odour which is very intense and unpleasant, and which 

 masks or overpowers the perfume when a larger quantity of the 

 body is smelled. These peculiar variations in the quality of an 

 odour are well known to persons who are accustomed to handle 

 perfumes." Some of the points mentioned by Passy in the 

 Memoire above quoted, are recognised facts, but others require 

 confirmation by experiment. Certain it is that pure otto of Bosa 

 centifolia in bulk is unpleasant because it is overpowering, but 

 either in bulk or in a very dilute state, it is always the same, 

 and does not remind of anything else, hence it is pure ; it is 

 also inimitable. The odours of most flowers can be imitated by 



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