So accustomed are we to associate odors with plants and flowers that we can scarcely appreciate the fact thr.t certain minerals and ores give off, under certain circumstances, a distinct and characteristic odor. If an ore of arsenic be rubbed until it becomes slightly heated, a distinct smell of garlic will be observed, which, on heating the substance before the flame of the blow-pipe, becomes still more apparent. This odor, which is characteristic of the compounds of arsenic, is termed alliaceous. When selenium or a selenidc is strongly heated, a distinct smell of decayed horse-radish is perceived. This smell, which is peculiar to burning selenium, is known as the horse-radish odor. The odor proceeding from burning sulphur, or the roastiug of a sulphide, readily reveals the presence of tllF_t substance, and is termed sulphureous. When certain varieties of quartz and limestone are strongly rubbed, they give off the odor of rotton eggs. This peculiar smell is occasioned by the evolution of sulphureted hydrogen ; and substances which possess this property are termed fetid. Clays and other substances containing large quantities of alumina afford a peculiar odor when breathed upon ; whilst a few, such as pyrargillite, present the same phenomena when heated. This odor is termed argillaceous, and may frequently be observed in the open air, just as the ground begins to get moistened by a shower.