We are not only without names for smells, but we are far from being agreed as to the qualities of them. To one person the odor of sweet peas is delightful, while to another it is quite the reverse. Sometimes we consider a smell pleasant merely because of the associations it brings. The odor of pine lumber is grateful to one who has spent a season in the lumber districts where sawmills abound; and so the smell of an ordinary lumber pile gives pleasure to one where to another it is somewhat disagreeable. The sense of smell is one that tires most readily. After smelling certain odors for awhile one loses temporarily the power to notice them at all. The sense does not tire as a whole, but it merely becomes inoperative with respect to the odor continually present. Almost any perfume held to the nose soon loses its charm, and is only effective again after a temporary absence. But while one perfume is not sensed a new one presented to the nostrils is eagerly appreciated, showing the sense to be fatigued only with regard to what has been there for some time. The owner of a large rendering establishment in a city was called upon by a committee of citizens who objected to the smells arising from his plant. He went out with the committee to inspect the premises and declared with evident honesty that he could detect nothing disagreeable in the air nor any sort of a scent that did not properly belong to a rendering establishment. Those who work where there are strong and disagreeable odors soon become so accustomed to peculiar smells that they do not notice them at all, although they are keen to detect any unusual odor, as when the liquor in a tanner's vat has not in it the proper admixture of materials. All the lower animals seem to be positive as to the direction of the source of any scent, but man is powerless in the matter. He merely knows an odor is present, but is unable to tell without moving about whether it comes from one side of him or another. A blindfolded boy cannot tell which side of his nose is nearest to a suspended orange. To affect this sense a substance must be dissolved or scattered through the atmosphere to be breathed. Whether such substances are divided and used up in giving out odors is still a question. Some of them, as the essential oils, waste away when exposed to the air, but a grain of musk remains a grain of musk with undiminished power after years of exposure. The experiment is such a delicate one in connection with the musk that it has never been settled to the satisfaction of science. Substances which scatter themselves readily through the air are usually odorous, while those which do not are generally without smell. But many of these when transformed into vapors, as by the application of heat, become strongly odorous. Bodies existing naturally in the gaseous state are usually the most penetrating and effective as odors. Sulphuretted and carburetted hydrogen are examples of these.