How does John Stuart Mill distinguish between "wealth" and "riches"?
I can only speculate on what Mill meant, but it seems to me that

This is mostly a figure of speech. People sometimes say things like &quot;I have no money, but I am wealthy in other ways&quot;, meaning they have something they would not trade for money, i.e. monetary wealth.
If there is a distinction, it is in the concept of ownership. Air is not wealth because I do not own it. There are several things that are beneficial to have (that enrich us) that we do not own and could not easily trade. Health, loved ones, air, etc. One can have, but not own these things, so they are not wealth.
(Except in Spaceballs.)