ODORS AND LIFE. i 49 



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 leaimed what physiologists think of the sense of smell 

 and the conditions of the perception of odors, let its see what natural- 

 ists and chemists have ascertained respecting the latter as viewed in 

 themselves, what place they give to odorous bodies, and what char- 

 acter 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 re- 

 ferred 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 per- 

 fume. 



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 essen- 

 tial 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 fra- 

 grances. 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, 



