Full Idea
In the Enquiries Hume clearly suggests that causes and effects are entities that can be named or described by singular terms; probably events, since one can follow another; but in the Treatise it seems to be the quality or circumstance which is the cause.
Gist of Idea
At first Hume said qualities are the causal entities, but later he said events
Source
report of David Hume (Enquiry Conc Human Understanding [1748]) by Donald Davidson - Causal Relations §1
Book Reference
Davidson,Donald: 'Essays on Actions and Events' [OUP 1982], p.149
A Reaction
A quality would have to have an associated power if it was going to trigger an effect. But then so would an event (unless inertia carried across?). Qualities are more distinct. Events can last for years.