Full Idea
The standard view was that in a mixture there is only the mixed body and its substantial form (gold). There are no further substantial forms of the elements, because the elements do not actually exist within the body.
Gist of Idea
In mixtures, the four elements ceased to exist, replaced by a mixed body with a form
Source
Robert Pasnau (Metaphysical Themes 1274-1671 [2011], 22.3)
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.500
A Reaction
This seems to me to be the key idea that was overthrown in the seventeenth century, so that corpuscular matter kept aspects of its ingredients, which science could then investigate. With the substantial form, investigation seemed impossible.