Full Idea
Form alone cannot be a composite substance's essence. For a thing's essence is expressed by its definition, and unless the definition of a physical substance included both form and material, the definition wouldn't differ from mathematical objects.
Gist of Idea
The definition of a physical object must include the material as well as the form
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.93)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.93
A Reaction
This is the sort of thoroughly sensible remark that you only get from the greatest philosophers. Minor philosophers fall in love with things like forms, and then try to use them to explain everything.