116 "RohiciXxei 071 Aromd. 



different cases the ammonia lends as it were its volatility to 

 bodies, of which the odour, without this auxiliary, would 

 scarcely be sensible. I am far from generalizing this observa- 

 tion, but if we ought to admit with Fourcroy, that every 

 odorous emanation is the result of a vaporization in the portion 

 of air which affects our organs, I do not think, with that 

 illustrious chemist, that odour is constantly occasioned by a 

 pure and simple solution of the odorous body in this elastic 

 fluid; but I believe that this solution frequently cannot be 

 effected, except by the aid of an intermede, and that this 

 medium may vary according to the substance exactly in the 

 same manner as is the case with colouring matters which can- 

 not be fixed on cloth, except by means of mordants appro- 

 priated to their particular nature. To support this opinion on 

 some positive data, T will notice what takes place relative to 

 the essential oil of some cruciform plants, and particularly that 

 of the mustard-seed. 



I have remarked, whilst repeating some of the experiments of 

 M. Thibierge, that the essential oil lost its odour by remaining 

 on metallic surfaces, and that a sulphuret and an inodorous 

 oil resulted. It is, therefore, by the intermedium of sulphur 

 that this oil acquires so penetrating an odour. Perhaps it may- 

 be necessary to add to these sort of combinations another 

 vehicle, for it is known, that the addition of a little acetic acid 

 considerably heightens the odour of mustard. 



It results, according to my opinion, from the facts men- 

 tioned, that the odours which diffuse themselves in the air 

 ought not to be generally attributed to a simple volatilization or 

 emanation produced by the odorous body itself, but in many 

 cases to a gas or vapour resulting from its combination with 

 the vehicle appropriate and susceptible of diffusing it through 

 space according to known laws. As to odorous distilled wa- 

 ters, many of them are pure solutions of these combinations ; 

 and, approximating to Macquer's idea, I will willingly suppose 

 that the essential oils owe their odour to the combination of a 

 variable vehicle with an inodorous oil. Thus a problem will be 

 resolved which has long occupied those distillers who would 



