The Experimental Error blog considers the difference between disciplines (via Tom):
I often contemplate the differences between these two areas of study. Also, I hear fellow undergrads argue for one or the other, usually divided along the lines of their respective major. Anymore, I think they’re so interrelated that I find it hard to find a difference between the two, except for the phases of matter that they most often deal with.
Back in the days when science was new, Physics dealt with understanding the fundamental laws of the universe, and it was Chemistry that was making the attempt at understanding the fundamental pieces that the universe was composed of. Both of these fields also grew out of a long standing philosophical tradition that can be traced back to the days of the pre-Socratics, and exemplified by Aristotle. Buuuut… that’s going a bit further back than I think is necessary to understand what’s different, anymore, about these two sciences, if indeed they ever really were different.
He goes on through a nice discussion of the history of the two fields, and the similarities and differences between their approaches to the world. It gets complicated, and it’s hard to draw the line.
My own view, as someone with a degree in Chemical Physics (which gives me just enough experience to appreciate the joke attributed to Art Schawlow: “A diatomic molecule is a molecule with one atom too many.”), is that you can divide the different sub-fields based on their usual level of approximation. That has its messy points as well, but it works for me.