Thus the flavour of peaches can be compounded artificially of aldehyde, acetate, formate, butyrate, valerianate, œnanthylate, and sebate of ethyl, and salicylate of methyl, with glycerine, glycerine being added to the fruit essences, as it is to wines, in order to restrain the evaporation of the volatile bodies. (The fruit essences are used only in the making of flavours. They cannot be employed as perfumes, as they are too irritating to the nose.)
The union of components to form a product different from any one of them is found also in vision. When the colours of the spectrum, for example, are commingled, the resultant white light is devoid of any colour.
Thus the potential responsiveness of the olfactory organ seems to be practically inexhaustible. So far, at all events, it has not yet reached the limits of its capacity.
The number and variety of recognised smells being so great, then, one can readily understand how difficult it is to construct a classification of odours. Many attempts have, in fact, been made, but, depending as they do more or less upon subjective sensation, no two classifiers give us the same classification. Indeed, a division of all smells into “nice,” “neutral,” and “nasty” would be about as good as many much more ambitious efforts.
Zwaardemaker’s is the classification most usually followed at present, and as it is to him we owe most of our knowledge of scientific olfaction, we shall detail it here:
(1) Ethereal or fruity odours; (2) aromatic, including as sub-classes camphrous, herbaceous, anisic and thymic, citronous, and the bitter almond group; (3) balsamic, with sub-groups floral, liliaceous, and vanillar; (4) ambrosial or muscous; (5) garlicky (including garlic), oniony, fishy, and the bromine type of odour; (6) empyreumatic (guaiacol); (7) caprylic (valerianic acid); (8) disgusting; and (9) nauseating.
The subjective character of these classes is obvious, especially in the last two groups, but, apart from that objection, most people will be inclined to protest when they learn that chloroform and iodoform are put into the first, the ethereal or fruity, group, while it is suggested, though to be sure with a query, that coffee, bread, and burnt sugar may belong to the “repulsive” (pyridine) group!
The fact is that Zwaardemaker’s classification is based upon a chemical foundation, that is to say, upon properties which, as we shall see later on, do not necessarily correspond with the odours as we smell them. That, no doubt, explains his inclusion of iodoform among the “fruity” odours.—Iodoform fruity!—Shades of George Saintsbury and his “Cellar Book”!
A shorter classification is that of Heyninx, who, aiming at objectivity, bases his arrangement, to some extent at all events, upon the spectrum analysis of odorous molecules in the atmospheric medium, of which more anon. His list is: acrid, rotten, fœtid, burning, spicy, vanillar or ethereal, and garlicky. But here, also, the coupling of vanillar with ethereal odours seems a little inappropriate.
We stand, perhaps, on rather firmer ground when we turn to the manufacturer’s classification, founded as it is frankly upon subjective sensation, and therefore devoid of any surprises to the logical faculty. Here is Rimmel’s arrangement: rose, jasmine, orange, tuberose, violet, balsam, spice, clove, camphor, sandal-wood, lemon, lavender, mint, anise, almond, musk, ambergris, fruit (pear).