The philosopher calculates the velocity and intensity of light, he can analyze it, he knows from what substance a given colour emanates, and if this substance exists in the star, the rays of which he is observing; he demonstrates in the vibrations of bodies the origin of the sonorous waves, and sees in light as in sound, not particles of matter traversing space, but a movement excited in the surrounding media. Some learned men have thought that odours also result from a vibratory movement transmitted to the ambient air by the molecules of odoriferous substances, but Fourcroy demonstrated the origin of odoriferous emanations in the volatility of the immediate materials of vegetables; and odours are now generally considered as bodies existing by themselves, and not as a purely physical result comparable to sonorous or luminous waves; they are extremely minute material particles, volatilized in the atmosphere. But here matter seems to become intangible. The chemist can extract from a body the essential oil which gives it its odour, but he cannot separate the odoriferous principle from the oil itself, and he can only recognize its presence by the special impression received by the olfactory nerve.

Nothing gives us a more exact idea of the divisibility of matter than the diffusion of odours. Three-quarters of a grain of musk placed in a room cause a very powerful smell for a considerable length of time without any sensible diminution in weight, and the box in which musk has been placed retains the perfume for an almost indefinite period. Haller relates that some papers which had been perfumed by a grain of ambergris, were still very odoriferous after a lapse of forty years.

Odours are transported by the air to a considerable distance. A dog recognizes his master's approach by smell even when he is far away; and we are assured by navigators that the winds bring the delicious odours of the balmy forests of Ceylon to a distance of ten leagues from the coast.

Simple experiments prove that odoriferous bodies emit a stream of particles so small as to seem to be immaterial. When a morsel of camphor or a small body saturated with ether, or minute portions of benzoic acid, are thrown upon water, they are animated by a peculiar movement, which is due to the propulsion produced by the invisible vapour which emanates from these substances.

Heat, light, and other influences modify the production of odours, and their transmission in space. Certain plants are odoriferous only at night, and it is especially in the morning and evening, when the dew is scanty, that flower-gardens perfume the atmosphere. Rain destroys the perfume of flowers, probably by its mechanical action, and by lowering their temperature. It is remarkable also that animal or vegetable odours are feebler, as the countries are colder in which the plants or animals live from which they emanate. Hence perfumes come principally from tropical countries.

It has been stated that substances absorb and retain odours according to their colour. Thus, the experiments of Stark tend to prove that black garments are more quickly impregnated with an odour, and retain it longer, than light-coloured garments. On the other hand, A. Dumenl assures us he has ascertained that white stuffs absorb odours as quickly as others, but that the odoriferous particles are sooner evaporated from them. There must consequently be in this respect a difference in odours like that in luminous rays, but the first of these phenomena has not been at all so clearly demonstrated as the latter.

Under the influence of a shock, or from friction, certain vegetable and mineral bodies emit odours more or less powerful. Such are several varieties of wood, especially lilac and Saint Lucy, the leaves of mint, lemon verbena, and southernwood, and certain calcareous or silicious rocks. Other plants, on the contrary, lose their aroma on being bruised, like the mignonette, the violet, etc The contact of water, or of vapour, also develops odours in argillaceous rocks and several vegetable substances.

Odours have a very marked effect on the nervous system; but some persons are far more impressible in this respect than others. There is no doubt that certain odours may cause grave disturbance in the nervous system; but the imagination sometimes plays a prominent part in the discomfort produced by a bouquet of roses or violets; the sight of artificial flowers is sometimes sufficient to excite persons painfully who believe them to be natural. People often ascribe to this influence of odours on the brain, what is really due to the effects of carbonic acid gas, or of poisonous emanations absorbed by the lungs; and how many persons there are who do not believe the open combustion of charcoal is innocuous, because it does not emit so much smell as coaL

But even after making due allowance for the effects of the imagination, it is certain that odours act as an excitant on the brain, which may be dangerous when long continued. They are especially dreaded by the Roman women. It is well known that in ancient times the women of Rome indulged in a most immoderate use of baths and perfumes; but those of our rimes have nothing in common with them in this respect; and the words of a lady are quoted, who said on admiring an artificial rose, "It is all the more beautiful that it has no smell"

We are warned by the proverb not to discuss colours or tastes, and we may add odours also. Men and nations differ singularly in this respect The Laplander and the Esquimaux find the smell of fish-oil delicious. Wrangel says his compatriots, the Russians, are very fond of the odour of pickled cabbage, which forms an important part of their food; and assafcetida is, it is said, used as a condiment in Persia, and, in spite of its name, there are persons who do not find its odour disagreeable any more than that of valerian.