been seen. If refuse matter or bodies of animals are left on the ground, insects flock to them at once, feeding on such substances, and depositing their eggs in them. Scent alone seems to guide them, exclusively of sight even, for, if the object of their desire is hidden, they easily manage to find it. A curious fact as to the scent of insects is furnished by those kinds that prefer decaying substances. A beautiful arum is found in our woods, the cuckoo-pintle, whose white flower diffuses a disgusting odor. Now, the inside of this flower is often filled with flies, snails, and plant-lice, seeking the putrid source of this fetid smell. We may see the little creatures, in quest of their food or of a fit place to lay their eggs, move about in all directions, and quit most unwillingly the flower whose scent has misled them.
II.
Having thus learned what physiologists think of the sense of smell and the conditions of the perception of odors, let its see what naturalists and chemists have ascertained respecting the latter as viewed in themselves, what place they give to odorous bodies, and what character they attribute to them all. The three kingdoms possess odors. Among mineral substances, few solids, but quite a number of liquids and gases, are endowed with more or less powerful scents, in most cases not very pleasant ones, and usually characteristic. Those odors belong to simple substances, such as chlorine, bromine, and iodine; to acids, as hydrochloric and hydrocyanic acid; to carburets of hydrogen, as those of petroleum; to alkaline substances, ammonia, for instance, etc. The odors observable among minerals may almost all be referred either to hydrocarbonic or hydrosulphuric gases, or to various solid and liquid acids produced by the decomposition of fats, or to peculiar principles secreted by glands, such as musk, ambergris, civet, and the like. Vegetables present quite another variety of odors, from the faintest to the rankest, from the most delicious to the most disgusting. Absolutely scentless plants are very rare, and many, that seem to be so while they are fresh, gain, on drying, a very decided perfume.
The odor of plants is due to principles very unequally distributed throughout their different organs; some solid, as resins and balsams, others which are liquid, and known by the name of essences or essential oils. In most cases the essence is concentrated in the flower, as occurs with the rose and the violet. In other plants, as in bent-grass and Florence iris, only the root is fragrant. In cedar and sandal wood, it is the wood that is so; in mint and patchouli, the leaves; in the Tonquin bean, the seed; in cinnamon, the bark, which is the seat of the odorous principle. Some plants have several quite distinct fragrances. Thus the orange has three: that of the leaves and fruit, which gives the essence known by the name of "petit-grain;" that of the flowers, which furnishes neroli; and again the rind of the fruit,
