398 



SENSE OF SMELL : ODOROUS SUBSTANCES. 



powerful rasp (resembling that in fig. 107), by which it rubs 

 down the sea- weeds on which it feeds ; whilst the tongue of 

 the Bee (fig. 289) forms a channel through which it draws-up 

 the juices of flowers. In most Insects, the palpi, small jointed 

 appendages in the neighbourhood of the mouth ( 172), seem 

 to answer the purpose of an organ of taste ; being observed 

 to be in incessant motion whilst the animal is taking food, 

 touching and examining it before it is introduced into the 

 mouth. 



Sense of Smell. 



504. Certain bodies possess the property of exciting in us 

 sensations of a ^peculiar nature, which cannot be perceived by 

 the organs of taste or touch, and which seem to depend upon 

 emanations that spread from them through the air, pro- 

 ducing what we term odours. It appears probable that odours 

 are, in reality, very finely-divided particles of the odoriferous 

 substance ; and this idea derives support from the fact that 

 most volatile bodies are more or less odorous, whilst those 

 which do not readily transform themselves into vapour, have 

 little or no fragrancy in their natural state, but possess strong 

 odorous properties as soon as they are converted into vapour 

 by the aid of heat, for instance. The most powerful and 

 penetrating odours are for the most part those of bodies 

 already in a gaseous state, such as sulphuretted and carbu- 

 retted hydrogen; or of fluids which readily pass into the 

 state of vapour, as the vegetable essential oils. But there are 

 some solid substances, as musk, which are very strongly 

 odorous ; and which yet do not appear to diffuse themselves 

 through the air in the state of vapour. The odoriferous 

 particles of these must be of extreme minuteness ; for the 

 substances do not seem to lose weight by freely imparting 

 their peculiar scent to an unlimited quantity of air. Thus the 

 experiment has been tried, of keeping a grain of musk freely 

 exposed to the air of a room of which the doors and windows 

 were constantly open, for a period of ten years ; the air, thus 

 continually changed, was completely impregnated with the 

 odour of musk ; and yet at the end of that time, the particle 

 was not found to have suffered any perceptible diminution in 

 weight. 



505. In order that we should become sensible of odours, it 



