If so, what about smells coming from solid matter, like wood?
No, smell is a sense. That is like asking if taste is a food.
Some gasses can be detected by our sense of smell as can some aerosols. I don't know if the scent of wood is carried by a gas or an aerosol.
As DaleSpam said, smell is a sensory perception. It is an interpretation by our brain of the interaction of molecules (from gasses, aerosols, whatever) that hit the olefactory nerves.
There are some sensations that people THINK of as smells that actually are not. The best example I can think of is the reaction your nose has to ammonia. There is a bit of smell involved, but it is swamped by the predominant perception which is that of your mucus membranes being irritated by the chemical reaction they have with ammonia.
That is interesting. I did not know that.
Sometimes, certain smells can overwhelm your senses.
In low concentrations, hydrogen sulfide smells like rotten eggs. If the concentration of this gas goes above a certain limit, you lose the ability to perceive the rotten egg smell any longer, which can be dangerous. You might be surrounded by an atmosphere which has little or no oxygen available, and asphyxiation might result.
Wood contains many organic compounds such as turpentine, oil and resin. Each wood has a characteristic smell when cut. Smell is your sense detecting a few of the gas molecules of organic compounds that evaporate when exposed. These compounds often protect the tree from decay or digestion by insects.
Smell if you can, a piece of old wood, then scratch it and smell it again. You will smell the exposed organic molecules as they evaporate from the damaged cell structure.
Smell is something we detect with our sense of smell, much like sound is what we detect with our sense of hearing and light is what we detect with our sense of vision.
So I guess this sums up what a smell is.
I don't think smell is something tangible but rather what we perceive in our brains. Indeed, such senses are aroused by stimuli and in the case of olfactory sense, it should be some thing in the air,
I believe that most smells are from gaseous molecules like benzene related or lipid based fragrances, bad smells of Sulphur dioxide or ammonia gas, which arise from volatile liquids or fumes. In many cases, the smell of the solid objects are from fumes released from the solid, for example the fungus on woods or volatile compound.
I am not sure about metals though if there isn't such thing described above.
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