ODOUR OF PLANTS. 59 



tares, for even brass cannot resist its action. Common 

 Wheat straw, when burnt, is found to contain a por- 

 tion of 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 primaeval 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 phenomena 

 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 scent in the course of the day, 

 smell powerfully in an evening, whether the air be 

 moist or dry, or whether they happen to be exposed 

 to it or not. This is the property of some which Lin- 

 neeus has elegantly called fares tribes, melancholy 



