Just that really.
How does something give off a smell?
Does everything have a smell?
What happens to the air to make it carry a smell.
How do we ‘ingest’ the smell?
How do we recognise different smells, and why some as pleasant and others unpleasant?
Smells are produced by molecules. The molecules come off of whatever it is that is smelly.
Example: Stinky cheese is actively losing cheese molecules into the air. These molecules carry along until they hit receptors in your nose. The receptors generate a specific nervous signal that gets carried up to your brain where you recognize, or at least acknowledge the smell.
So, when you smell shit from a freshly laid dog log, molecules of shit are literally touching your nose and tickling your brain. Makes one’s head explode, no?
Not everything has a smell, though everything does lose molecules of itself into the air at various rates. For example, rocks don’t normally have a smell, either because they just aren’t composed of smelly molecules or because there aren’t enough of those molecules diffusing into the air. However, when it rains, the presence of water helps more rock and dirt molecules diffuse into the air. Hence, the wet dirt smell after it rains.
Pleasantness and unpleasantness is mostly a matter of opinion, but could conceivably have an evolutionary component. Things that are rotting smell due to bacteria dissolving the food and producing, mostly, sulfur containing molecules that are stinky. Most animals won’t eat food that smells wrong, either because evolution or personal experience has taught them that such food will shorten their lifespans.
This post is sort of disjointed, but I hope it helps.
Missed the edit window:
Nothing really “happens” to the air to make it carry a smell. Everything on the planet is diffused into everything else at a chemical level. How much it diffuses depends on the properties of the molecules each thing is made of. So, the molecules from a rock like other rock molecules and tend to stick together. A rock molecule only leaves the other rock molecules and diffuses into the air really rarely. On the other hand, a bunch of helium molecules will happily get away from each other as much as they can. If you release a bunch of helium into a room, the helium molecules will spread out until they’ve filled up every corner, mixed in with the air molecules. They try to do this when you fill up a balloon, but the latex keeps them from getting too far. Hence, the balloon expands.
OK, thanks for that. I understand that rocks give off very few molecules, so there’s not much to smell, but do those (few) molecules *have *a smell?
And - as far as I remember - helium doesn’t have a smell, does it? Despite diffusing readily.
Whether something has a smell or not, at the molecular level, depends partly on structure and partly on atomic composition. Things that contain sulfur tend to smell bad, no matter what the structure of the molecule is. Even sulfur all by itself smells bad.
The molecular structure dictates how the electrons are spread through the molecule and, therefore, how the molecule interacts with your nose receptors. Things with Benzene ring shapes tend to have fruity smells. Helium, IIRC, doesn’t have a smell because it is a noble gas and doesn’t interact with other molecules at all.
That’s about the most I know. Chemistry class kicked my ass.
OK, in general you’ve got the spirit of the thing, but I have to point out that the above is incorrect and highly misleading. There is no such thing as “a molecule of shit”. Shit is a mixture of various things. What you smell is organic molecules like indole, skatole, hydrogen sulfide, mercaptans, and a bunch of other stuff that only a shitologist would know. These are volatile compounds that easily evaporate and diffuse in air to reach your nose. It’s not as if any actual septic parts of the feces are physically touching your nose… it’s only the gases that we have learned to regard as smelling septic. Thus we know to avoid the actual feces, thus we avoid actual bacterial contamination, thus is fulfilled the purpose of the nose.
It’s interesting, I think, that the smell nerves (the olfactory nerves) go right into the central nervous system, including the amygdala*, which is the “seat” of emotions, the processing and memory thereof. The amygdala, in turn, triggers the hypothalamus to release stress hormones and work the trigeminal nerves in your face to control expressions of fear.
Wassat mean? It means that it’s no coincidence that a whiff of a smell like your grandfather’s shaving creme can bring back a wave of emotional memory 40 years old. If it’s an emotion tied into fear, anger or stress, the smell alone can trigger that feeling again. It’s not about conscious connection - the frontal lobe is entirely uninvolved. It’s literally much deeper in the brain and not under your conscious control.
*Yes, that part of River’s brain that was messed with in *Firefly *and *Serenity *which made her batshit sane.
Every solid and evaporates somewhat and gives its molecules to the passing breeze. But, for it to have a smell, our olfactory nerves have to be able to sense it. There are some things that we don’t have the ability to sense, like helium for instance.
Apparently, which of the zillions of chemical compounds we can sense is one of the most genetically variable things we can play with.
Noses aren’t our only chemical sensors. Our tongues do it too, though with different (and fewer) compounds.
Housecats have a third chemical sense organ, between the upper lip and the front of the upper gums. When they use it, they usually adopt a dopey look for a few seconds, with their mouth and their eyes half-open.
Well, rock dust can certainly have a smell.
I can’t figure out how it goes, but there’s a good joke here involving Carbon, Radium, and Phosphorus
A team of British researchers claim to have combined sulfur, hydrogen, and iodine with the rare trace metal tellurium to create SHITe.
Geeze, spoil a girl’s fun some more, will ya? 
So does that mean that if I’m not there to smell it, then the smell doesn’t exist? (This smells like quantum uncertainty :dubious: ) If not, what is it in the molecules that smells. Surely molecules don’t have an inbuilt smell?
The “smell” isn’t in the molecules, it exists in your brain as a response to outside stimulation. Based on the stimuli, different responses will be sent from your nose receptor into your brain to be interpreted as a smell. Some stuff doesn’t provoke a response at all, and those things are odorless.
So what actually are those stimuli that are created, presumably, by the molecules that we respond to?
The general idea is that volatile compounds coming from the thing you are smelling enter your nose. There, they interact with olfactory receptor neurons that reside in the olfactory epithelium, a mucousy thing with the surface area the size of a stamp that is at the top of your nose. These neurons have cilia that extend into the mucous that is on the epithelium, and these cilia are covered with olfactory receptor proteins. You have a set number of different types of olfactory receptor proteins - about 350 or so for humans (though with pseudogenes that number is up to about 850, and there is some evidence that pseudogenes play a role here too, but I digress) - but you have many many copies of each protein. Molecules of some volatile chemical X will bind to varying degrees with a few types of the olfactory receptor proteins that you have. The generally accepted theory is that this binding is determined by the volatile chemical’s shape, though there is another, less accepted, theory on this (see Luca Turin). It will bind strongly to some and weakly to others. Any chemical that will interact with your olfactory receptor proteins can then be characterized as a set of binding strengths (specific to certain OR proteins). Skipping all the details, this info is brought up through the olfactory bulb which sits above your nose, to which the olfactory receptor neurons are connected, and eventually some mumbo-jumbo that’s less clearly understood happens, and you derive some sort of subjective sensation of smell.
See my previous post for an explanation of this. But generally, you may be interested in knowing that the foundational work in this was done by Richard Axel and Linda Buck, for which they received the 2004 Nobel Prize in Medicine
>So does that mean that if I’m not there to smell it, then the smell doesn’t exist?
Well, I guess so, just for reasons of semantics, not for any scientific reason. The molecules are really there in any case, and they would smell like one thing or another if somebody was there to observe them, but by “smell” is generally meant the sensation.
This is a chemical version of the “if a tree falls and nobody is there to hear it” question.
I think it’s technically correct and significant to say that the molecules given off by things into the air have some chemical nature, some ability to react with certain things and so forth, but to say they have a smell is to only say that they would elicit a certain sensation if sniffed. It may be more convenient to think of them as having a smell whether there is somebody there or not, but in doing so we only propose that they would have a smell if somebody sniffed them.
**ArmenE ** - thanks for the link. As far as I can see, the report concentrates on the work that Richard Axel and Linda B. Buck have done on the ‘receiving end’ (so to speak) and only mentions in passing ‘odorous molecules’ without going in to more details about them. I’d still like to know what it is about the molecules that makes us perceive them as being ‘odorous’.
The best basic answer, as outlined above, is “their shape” - a molecule’s shape makes it bind to olfactory receptor proteins in a way specific to that molecule. Let’s talk about light for a minute - you perceive red light as being red and not blue based on its wavelength. molecule shape is the olfactory equivalent of wavelength. red light is not inherently red, it’s just that your brain interprets it as such. The same is true of the smell of gasoline, or shit, or what have you. The perception part of it is all handled later on in the brain, past the nose, and we don’t understand that enough to be able to explain why shit smells exactly the way that shit does, and not like roses.