Full Idea
The essence of a triangle lies in a very little compass, consists in a very few lines; ...so I imagine it is in substances, their real essences lie in a little compass, though the properties flowing from that internal constitution are endless.
Gist of Idea
The essence of a triangle is simple; presumably substance essences are similar
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.32.24)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.393
A Reaction
This is the clearest evidence I can find that Locke firmly believed in real essence of substances, despite all his sarcasm about anyone who claimed to know what they are. He evidently knows at least one real essence, namely that of the triangle.