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 
