Over at Philosopher's Playground, Steve Gimbel asks why the philosophy of chemistry is such a recent discipline given how long there has been serious activity in the philosophy of biology and the philosophy of physics.
He floats a few possible answers -- as it happens, the same options those of us who actually do philosophy of chemistry encounter fairly regularly. After responding briefly to these possible reasons for thinking that there shouldn't be a distinct philosophy of chemistry, I'll offer a brief sketch of what a philosophy of chemistry might be about.
(1) There are no big questions in chemistry that are not already somewhere else.
I suppose it all depends on what you count as a "big question" for a subfield of philosophy. Steve mentions the origin of the universe as a big question for philosophy of physics (and we can add to that questions about the nature of time, space, and matter) and the origin of life as a big question in the purview of philosophy of biology (to which we might also add questions about our fundamental human nature).
What big questions are left for chemistry?
Surely there are interesting questions to be asked about the nature of change. Chemistry is a science that concerns itself with transformations of matter. To the extent that different substances are part of our reality, there are interesting and important questions about the nature of substances -- what it is that defines a substance and its characteristics, what features persist in transformations and which do not. (Some of these play out in particular questions like "Is there any salt in the sea?" Joe Earley says there isn't, while Paul Needham argues that there is.)
Not every philosopher will view these as earth-shaking questions, but then again, there are plenty of philosophical questions to which serious scholars have devoted their whole careers but whose pull is ... let's say "elusive" to others of us. (For example, the problem of universals, to me, has never seemed a "problem" worth losing sleep over.)
(2) Chemistry does not actually exist -- or at least it won't forever.
That would explain why physics departments are so huge and well-funded, while chemistry departments have shrunk down to nearly nothing.
Oh, wait! That hasn't happened!
Maybe chemistry won't survive as a distinct discipline to the moment at which the human race ceases to be. But I think it's an empirical question. And given that string theory has not so far revolutionized organic synthetic design, I wouldn't bet on chemistry's obsolescence in the foreseeable future.
(3) It's the chemists -- they just aren't a philosophical bunch.
The chemists who attend the meetings of The International Society for the Philosophy of Chemistry are an interesting bunch, but I don't think they're complete outliers. Many chemists of my acquaintance are at least somewhat interested in the philosophy of science -- and this was the case even before I crossed the line to become a philosopher.
Anyway, for any X, it strikes me that there could well be a philosophy of X even if it were only the philosophers and not the practitioners of X themselves who were interested in any of the philosophical implications of topics within X. Philosophers are competent to decide what issues of theory and practice are of interest to them without seeking affirmation from the folks who happen to use those theories or engage in those practices. I'm not saying engagement between practitioners of X and philosophers of X is not often productive, but it's not a requirement for there to be a philosophically interesting "there" there.
(4) It's the philosophers -- they just don't know any chemistry.
For awhile, this might have been closer to the truth. The Vienna Circle was all about the physics, after all. Not that this didn't stop philosophers from turning to chemistry as a source of example to help support the philosophical claims they were trying to make. What kind of support those examples actually provided, it turns out, depends on how fully the audience to be persuaded grasps the piece of chemistry being waved around. (This is why nearly every philosopher who has done any work in the philosophy of chemistry, and many a chemically literate who has not, has at some point written a paper on Hilary Putnam's Twin Earth water example. A large number of these papers point out that water is not H2O -- at least not in the unproblematic way Putnam thought it was.)
However, there are significant numbers of philosophers out there nowadays who have significant training in chemistry (from an undergrad minor to a B.S. to an M.S. to the odd Ph.D.). Not all of them are doing work in philosophy of chemistry, but a bunch of them are. And, perhaps more importantly, these folks may be reaching a critical mass to overcome the "Physics rules, chemistry drools!" biases of the old guard in the philosophy of science. (That old guard, as well, is starting to die off. It's the circle of life.)
There are distinctive and important philosophical questions about how chemistry engages with the world. Here are just a handful of the ones I find interesting (as I described them in an abstract I wrote a couple years ago):
Chemical models and chemical phenomena. In trying to understand the range of chemical entities and phenomena (atoms, bonds, molecules) and phenomena (such as chemical reactions), chemists use an astounding array of models. These range from the very abstract and mathematical to the "tinker-toy" models of molecular structure familiar to chemistry students. The prevalence of models here illuminates the different roles models play in the practice of science, as tools for predicting and explaining, but also for creating novel molecules and for reasoning about molecules whose existence is deemed impossible. It also presses the question of what sort of correspondence is required between a model and the entity or phenomenon being represented. (Chemical bonds are understood to be vastly different from sticks, yet the ball-and-stick models of molecules still convey useful information. How?)
Measurements, explanations, realism, and instrumentalism. Chemistry is a science whose experimental prowess has at times outrun its theoretical resources. Yet it has made use of the proximity of physics to augment those resources. As a result, chemists sometimes have multiple models for the same entities or phenomenon - models that seem to make contradictory claims (e.g., about what a bond is "really" like). Do chemists require only that their models fit with the observables while making no commitments about whether these models get at the "true structure" of chemical stuff? Do chemists try to choose from among multiple models that fit equally well with the observables to get closer to such truths? The ways chemists navigate comparisons of models and measurement illustrates methodological advantages and disadvantages of realism and instrumentalism.
Policing the border: reduction, autonomy, or some intermediate position? One solution to the question of what makes for a good model in chemistry might be to impose constraints from the best physical theory. However, this is not what chemists do - even quantum chemists' descriptions of atoms and bonding violate strict quantum mechanical theory - nor is it clear that chemistry would be better off in accepting this constraint (since, for example the features of quantum chemical models that are explanatory are the classical features, not the quantum ones). Is chemistry accountable to the claims of physical theory as well as to empirical features of chemical phenomena? Where these come into conflict, how does chemical theory and practice respond? What lessons might the relation between chemistry and physics have for questions of reduction and the unity of science?
I suppose I haven't provided a positive answer to Steve's question about why there wasn't a critical mass of philosophers of chemistry before the 1990s. However, I hope I've provided at least a reason to think that there are important philosophical questions worth pursuing in it, even if there's still some mystery about why they weren't noticed for so long.