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 durmg 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 
