Full Idea
I understand the word 'form' to mean, not a real substance distinct from matter, but only the matter itself of a natural body, with its peculiar manner of existence [corpuscular structure], which may be called its 'essential modification' or 'stamp'.
Gist of Idea
Form is not a separate substance, but just the manner, modification or 'stamp' of matter
Source
Robert Boyle (The Origin of Forms and Qualities [1666], p.324), quoted by Jan-Erik Jones - Real Essence §3
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.8
A Reaction
I don't think Aristotle ever thought that a form was separate from its matter, let alone qualifying as a substance. On the whole, Boyle attacks scholastic philosophy, rather than Aristotle.