Lewis on metaphors for sets

Comparing collections and fusions, Michael Potter, Set Theory and its Philosophy, p 22, says

A collection, by contrast, does not merely lump several objects together into one; it keeps the things distinct and is a further entity over and above them.  Various metaphors have been used to explain this---a collection is a sack containing its members, a lasso around them. an encoding of them---but none is altogether happy.  We need to be aware that collections are metaphysically problematic entities if they are entities at all, and need to be handled with care.

He goes on to say that David Lewis's Parts of Classes has an 'excellent discussion of the difficulty of making good metaphysical sense of such metaphors.'

I'd very much like to read this bit of Lewis.  Not having access to an academic library, can anyone send me a scan of the relevant pages?