208 PHILOSOPHY OF ZOOLOGY. 



to their escape, as arsenic or phosphorus with oxygen. 

 Heat assists the escape of odorous particles from a variety 

 of bodies which are termed volatile. Light likewise influen- 

 ces the emanation of odorous particles. Hence we find some 

 plants give out their smell during the day, while others per- 

 fume the air only at night. Dampness, in many cases, as- 

 sists the escape of such particles, as appears by the fra- 

 grance of a garden after a summer shower, or clay when 

 breathed upon. These odours emanate from bodies in all 

 directions, with greater or less rapidity, penetrate only 

 where air can enter, and obey the motions which it experi- 

 ences. 



The air, in passing through the nostrils to the lungs, 

 comes in contact with the olfactory membrane, which lines 

 the narrow passages, and enables the odorous particles which 

 it conveys to act upon the olfactory nerves. We are igno- 

 rant of the manner in which this action is produced. Has 

 the olfactory membrane an affinity for odorous particles ? 

 Does the nasal mucus retain these by its adhesive power, 

 or is it employed in attracting them ? Are odorous particles 

 deposited on the membrane, or do they merely act mecha- 

 nically^ as they move along its surface ? These are questions 

 of difficult solution, and hitherto treated in a superficial 

 manner. 



In the case of fishes, the water impregnated with odor- 

 ous particles, traverses the cavity of the nose, urged by the 

 muscular action of the apertures and walls, and brings 

 them into contact with the olfactory membrane. 



3. Knowledge obtained by the sense of Smell. The sen- 

 sations produced by the different kinds of smells, are clas- 

 sified with difficulty. Some are agreeable, such, in general, 

 as those which serve for food ; others are disagreeable, as 

 the most of those substances which are deleterious. They 



