to which a person has an antipathy, may have been originally sweet, or lie so near the confines of pleasure, as to propagate their vibrations much farther than original fœtids can. For these seem to revive from fainting by making a vigorous impression on the nose, which yet is not propagated freely over the whole system; or, if it be, will occasion immediate sickness and fainting. Fœtids in this resemble other pains, which, if moderate, excite; if very violent, overpower.
If it be objected to this, that such fragrant smells as a person has an antipathy to, are disagreeable to him in the highest degree, and that upon the first perception; also that the smell of those fœtids which revive, as of asafœtida, spirit of hartshorn, &c. is agreeable to many; I answer, that these two opposite changes seem to arise merely from association. The faintness and revival, attending these smells respectively, must, by association, transfer the vestiges and miniatures of themselves upon the first perception of the smells, whose associates they are.
Sixthly, It is agreeable to the notion of vibrations, that spirituous liquors and opium should produce their appropriated effects by smell, as well as by being taken into the stomach, as they are found to do in fact. For, if these effects arise from specific vibrations, the mere impression of small active particles may be sufficient for the purpose of producing them. We must, however, suppose that the exhalations of odoriferous bodies are imbibed in some small degree by the absorbing vessels of the membrana schneideriana. We might shew by parity of reason, that the great subtlety of odoriferous effluvia favours the doctrine of vibrations.
Though odoriferous particles are more subtle than the sapid ones, yet they are perhaps grosser than the rays of light. For the smoke of a tallow candle ceases to smell, when it begins to shine, i.e. when it is more attenuated by heat. Since therefore the vibrations from heat are probably smaller than those from light, we may range the vibrations of the medullary substance in the following order, in respect of subtlety; heat, light, smell, tastes, tangible impressions, and the vibrations of the air, from which sound arises. But it is to be observed, that these last may excite much more frequent vibrations in the auditory nerve, than those of the sounding body, to which they correspond; just as the vibrations from friction are much more numerous than the strokes of friction; and the tremors of the particles of an anvil much more numerous than the strokes of the hammer.
This proposition is analogous to the thirty-eighth, in which the agreement of the specific differences of tastes with the doctrine of vibrations is considered; and may be illustrated by it. One
