Part One taught you to name what you smell. This part teaches you to take it apart.
It is a much harder step, because the nose cannot do it on its own. The nose gives you a single impression, already mixed, with no list of ingredients.
What the nose does and does not do
Inside your nose there are a few hundred different kinds of olfactory receptor. An odour molecule binds to some of them, each to a different degree, creating a pattern of activation.
The brain reads that pattern and returns one result: this is the smell of coffee. It does not return the list of eight hundred volatile compounds that took part in creating the pattern.
This is quite unlike sight. Looking at a painting, you can separate the red areas from the green. Smelling a bottle of essential oil, you cannot separate linalool from linalyl acetate — unless you have learned each compound on its own beforehand.
Why taking it apart is still worth learning
Three practical reasons.
You can predict. Knowing a sample is rich in beta-caryophyllene means knowing it will still be on your collar tomorrow. Knowing it is rich in limonene means knowing it will be gone in ten minutes.
You can compare. Two bottles with the same name but different lists are two different things, and you know where they differ instead of merely feeling that they do.
You can check. The whole of Part Four, on detecting fraud, rests on being able to read a list and see what is unusual in it.
The tool for taking it apart
Humans have no sense that can separate a mixture of smells. Machines do.
Gas chromatography makes the mixture race down a long tube; whatever sticks least comes out first. At the other end, mass spectrometry shatters each molecule and weighs the fragments, giving a fingerprint by which to name it.
The result is a list: compound name, CAS number, and share. It is something your nose will never give you on its own.
What the list does not give you
It does not tell you how the sample smells. A list and an experience are two different things, linked by a complicated relationship that the remaining nine lessons of this part will gradually untangle.
Anyone who says reading an analysis is enough to know whether a bottle smells good has not understood either end.
Look at a sample analysis before reading on — the next lesson uses it as an example.