Odours are conveyed to the olfactory end-organ in the air we breathe. Before they can rise into the air from the odorivector (the odorous body) and be transported they must, it is clear, pass into the vaporous or gaseous state. (In the case of fish, of course, the odour must undergo solution, that is pass into the liquid state.) Many of the natural properties manifested by smells have been related to this transformation into vapour.
Everybody knows how rich garden scents become after a shower. It has been claimed that this results from the lightening of the atmosphere by the storm, in consequence of which the diffusion of odorous vapours, following the law that governs the diffusibility of gases, is facilitated. But some of the effect must be due, one would think, partly to the impact of the raindrops breaking up and dispersing the halo of perfumed air that surrounds each flower, and partly also to the evaporation of the rain-water that has absorbed these floral emanations.
We are told also that during the night and in the chill of early morning the air is less charged with odours because cold checks the diffusion of gases. This may be true enough for some odours, but I am inclined to think that the fact is not stated with perfect accuracy, as there are certain perfumes, that of the tobacco-plant for one and that of the night-scented stock for another, which are most prevalent after nightfall. And it has always seemed to me that Mother Earth is never so nicely perfumed as on a cool September morning, although I should never be inclined to call any morning “incense-breathing,” like Gray, for anything less like incense could scarcely be imagined.
There is no doubt, however, that frost seals up all odorivectors and renders the air quite odourless.
A physical law appertaining to gases is also invoked to explain the “clinging” of odours. Many, if not all, solids and liquids when exposed to air and other gases adsorb (cause to adhere) to their surfaces a thin, dense layer or film of the gas. If now that gas happens to contain an odour, or is itself odorous, the odour must also be adsorbed, and so in the case of porous materials, such as fabrics, permeated by the odour, it lingers tenaciously in their depths.
Odorous bodies in the solid or powdered form are known to retain their perfume for prolonged periods. Look how long a sandal-wood box remains aromatic. This property is supposed to depend upon the lowered vapour tension of the odorous molecules in the depths of the solid or powder, in virtue of which they rise into the air, or evaporate, but slowly.
It would seem to be natural to suppose that, as vaporisation plays such an important part in the dissemination of odours, the volatile bodies and liquids would be more odorous than the nonvolatile. But, as Zwaardemaker has pointed out, this is by no means always the case. Many substances of low volatility are nevertheless highly odorous, and vice versâ.
We turn now for a moment to consider the behaviour of the odorous vapour in the nose.
As it passes through the nose the current of inspired air sweeps along the lower and middle regions only; the upper or olfactory region is not directly traversed. But almost certainly some of the air is diverted up into the olfactory region in light eddies, and the act of sniffing, which is a short inspiration abruptly begun and ended, and which we instinctively resort to when trying to detect a faint odour, is obviously of a nature to propel side-streams or eddies up into the olfactory zone. One is reminded of the production of smoke rings from a box.
We smell not only during inspiration, however, but also during expiration, the latter conveying to the olfactory region the flavours of food and drink.