Full Idea
Whilst they exist united together, the mass consisting of the same atoms must be the same mass, ...but if one of those atoms be taken away, or one new one added, it is no longer the same mass, or the same body.
Gist of Idea
A mass consists of its atoms, so the addition or removal of one changes its identity
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.27.03)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.330
A Reaction
This is clearly a 'strict and philosophical' usage, rather than a 'loose and popular' one - indeed, so strict as to be ridiculous. Knowing what we do now of quantum activity (emission of photons etc), we would abandon 'identity' totally.