8. Odours differ very much as to the permanence of the impression which they produce. While that of some is very transient, in others the scent remains for hours after the application of the substance. They also differ, as we have seen, as to the extent to which their influence extends. Moisture in the atmosphere is favourable to the diffusion of odours, which would seem to show that vapour is a good conductor. For example, a flower garden is never more grateful to the smell than in the morning, when the dew is evaporating, or after a warm summer shower. So also the plants of a green house are more fragrant, just after they have been watered. Some flowers give off their odour only at certain times, generally after they are fully expanded, and their parts in the greatest activity.

9. Though the air is the usual vehicle for odours, yet we find that they adhere to solid bodies, and can even be conveyed through water. But though the whole art of perfumery is founded on this fact, it has been strenuously denied that they could be conducted through water, and consequently that fishes could smell. Some physiologists state that fishes have no olfactory organ ; that the part commonly considered as such is the organ of taste. This opinion is, however, erroneous. " Not many naturalists of the present day," says Dunglison, " will be hardy enough to deny, that fishes have an organ, or sense of smell. At all events, few anglers, who have used their oil of rhodium, or other attractive bait, will be disposed to give up the results of their experience, without stronger grounds than any that have been assigned by the advocates of that view of the subject."

10. Fishes are furnished with organs of smell, but they have no communication with the mouth or gullet. They are insulated cavities, covered with a valvular lid, and lined with a plaited membrane, similar to the under surface of some mushrooms. This serves to extend the surface, while it is covered with a viscid mucus. In the cod fish, the nerves of smell are spread out in a cavity filled with fluid, and larger than that which contains the brain itself. "That the cod is guided by smell in the selection of food," says Aitkin, "must be well known to every one who has taken it withv bait, in circumstances where he could watch the conduct of the fish. If not very hungry, it may frequently be observed to approach the bait, apparently attracted by the sight, till, at a closer distance, it seems distinctly to smell at it; and if not satisfied, turns aside and neglects it." Every person who has been in the habit of fishing much, must have often observed the same fact, in catching common pond fish, such. as perch, roach, etc.

11. It was an opinion formerly entertained, that odoure possess nutritive properties, as savoury smells seem to have the effect of allaying hunger, or at least of satisfying the appetite in some degree. This effect, however, is best explained, by referring it to the influence of odours on the nervous system, as we see the appetite often instantly destroyed by unwelcome news. In persons whose digestive organs are weak, the appetite is often destroyed instantaneously by a nauseous odour. We read that Democritus lived three days on the vapour of hot bread ; and Bacon speaks of a man, who was supported several days by inhaling the odour of a mixture of aromatic and aliaceous herbs. In 1638, Dr. Wilkins,. the Bishop of Chester, published a book, the object of which was, to show that the moon is inhabitable, and that it is possible for us to find a passage thither. In this work, he says, " If we must needs teed, upon something, why may not smells nourish us ? Plutarch and Pliny, and divers other ancients tell us of a nation in India, that lived only upon pleasing odours ; and it is the common opinion of physicians, that these do strangely both strengthen and repair the spirits."

12. We read an amusing anecdote in Fuller, who lived at the same time with the bishop, in relation to this supposed nourishing property of odours. A poor man being very hungry, staid so long in a cook's shop, who was dishing up the meat, that his stomach was satisfied with only the smell thereof. The choleric cook demanded of him to pay for his breakfast; the poor man denied having had any; and the controversy was referred to the decision of the next man that should pass by, who chanced to be the most notorious idiot in the whole city ; he, on the relation of the matter, determined that the poor man's money should be put between two empty dishes, and that the cook should be recompensed with the jingling of the money,* as he was satisfied with the cook's meat.

18. The sense of smell is closely associated with that of taste. It seems indeed as a sentinel standing on guard, to see that no enemy approaches the citadel ; it tells us whether the aliment placed before us, is agreeable or disagreeable ; of course whether it will agree or disagree with the stomach. The taste and the smell are hardly ever at loggerheads; it does, however, sometimes happen that a substance that is repugnant to the smell, is agreeable to the taste. In such a case they soon come to an understanding, and the smell chooses, to make the best of it, and drop its repugnance. At any rate, its aversion, some way or other, is soon neutralized. The smell is, however more useful to animals, as a sentinel, than it is to man, whose reason is more than a match for the instinct of the brute creation. In them, indeed, it rarely fails to guide aright. How wonderful is this provision which leads them with unerring certainty to choose the innocent herb from the poisonous plant; the nutritious vegetable from that which is destitute of nutriment; and to reject instinctively, every thing which would prove noxious or disagreeable. It is not, however, infallible even in the brute creation ; for we see the flesh fly attracted by a certain species of mushroom which admits a cadaverous odour, similar to putrid flesh; in these they deposit their eggs, which, when hatched, perish for want of suitable food to nourish them.