Page:An introduction to physiological and systematical botany (1st edition).djvu/107

Rh flinty earth in the form of a most exquisite powder, and this accounts for the utility of burnt straw in giving the last polish to marble. How great is the contrast between this production, if it be a secretion, of the tender vegetable frame, and those exhalations which constitute the perfume of flowers! One is among the most permanent substances in Nature, an ingredient in the primæval mountains of the globe; the other the invisible untangible breath of a moment!

The odour of plants is unquestionably of a resinous nature, a volatile essential oil, and several phænomena attending it well deserve our attentive consideration. Its general nature is evinced by its ready union with spirits or oil, not with water; yet the moisture of the atmosphere seems, in many instances, powerfully to favour its diffusion. This I apprehend to arise more from the favourable action of such moisture upon the health and vigour of the plant itself, thus occasionally promoting its odorous secretions, than from the fitness of the atmosphere, so circumstanced, to convey them. Both causes however may operate. A number of flowers which have no