Mutual relationship in science (9)

I believe the answer is:

chemistry

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'science' is the definition.
(I know that chemistry is a type of natural science)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for chemistry that I've seen before include "The science of matter" , "Science - mutual attraction" , "Scientific study of substances - by pharmacist?" , "A science (of personal attraction?)" , "scientific discipline" .)

I've seen this clue in The Times.

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