That's not a reference to chemistry. Rather it's to philosophy; specifically to the first of Plato's dialogues that appears in the Signet edition of Plato's collected works. My brother is reading the Republic and several others currently, and in a fit of nostalgia for freshman year of high school, I picked up his book and started from the beginning. I hadn't read Ion since way back then, and it's a much faster read at 20 than it is at 14, I can assure you.
For one thing, it's one of the shortest Socratic dialogues that I've ever read. Well actually, probably the shortest - even Laches seemed longer. I believe Cleitophon is the shortest that exists, but I've never read that one. Anyway, its remarkable conciseness: Socrates manages to turn the very pliable Ion's way of thinking completely around using even fewer arguments and examples than he usually needs to correct the generic dull-minded interlocutor. This brevity makes it quite an easy dialogue to follow, and the ubiquitous irony of the set-up and of the argument's course is much easier to pin down than usual.
In questioning Ion about the nature of his art of rhetorician, Socrates is basically doing what we would be if we tried to define the work of a literary critic; at least, so one might think of it. "The reciter," Socrates observes to Ion, "must be the interpreter of the poet's mind to the audience; and to do this, if he does not understand what the poet says, is impossible". This makes plenty of sense to Ion, who, though enthusiastic about his craft, hardly strikes one as the cleverest reciter of poetry to sing verse. Within a few paragraphs, he has Socrates running intellectual circles around him (as is Socrates' habit), as both begin to conclude that the reciter is altogether less qualified to judge the merit of the poet when, for instance, the poet sings of horsemanship than a skilled horseman, when the poet speaks of healing than a doctor. In other words, Ion is soon admitting, the reciter or actor has much less knowledge of anything that a poet writes about than the professional whose skill is that described; and any knowledge that the reciter does have is in virtue of his skill in that art. That is, a reciter may know the standards of good horsemanship because he possesses that skill to some degree himself, but only in virtue of that art does he know it, not in virtue of his art as a reciter.
Clearly this leaves them with the idea that there is no such thing as the art of reciting or acting, at least not such as Ion understands them and Socrates had just defined them at the beginning. Ion's proposed defense of the reciter's art is that he knows "what is proper for a man to say" - that is, he knows what a fisherman, a weaver, a slave, or a ruler would say in a given situation. But the argument Socrates has just used easily quashes that suggestion when he points out that surely a fisherman, a weaver, a slave, or a ruler would certainly know better than the reciter whether what the poet has written corresponds to the reality of the situation.
The dialogue ends with Socrates and Ion agreeing that the reciter really has no art or special knowledge at all: he is either a cheat, or when interpreting the great poets is "possessed by divine dispensation". Ion is serious. Socrates is most certainly not.
At least, it seems most unlikely that Plato, master of the literary art of the dialogue, would have so little respect for art that he would find it uninterpretable - or so little rationality that he would consider interpretation to be possible only through a divinely-induced frenzy. And of course, he never leaves the reader with a hard and fast conclusion to savour: these dialogues are meant to keep you thinking.
Well one of the first things this gets me thinking is that perhaps Plato wants to make a point about the nature of poetic art as compared to that of manual crafts or theoretical skills. The close connection he draws between the work and the interpreter necessarily makes us think, as he discusses the nature of the interpreter's understanding, also of what is being understood. Poetry is not one of the arts with a concrete, useful result, like a plow made by a blacksmith that can help create a garden, or a mathematical formula that allows certain buildings to be built; it is not, as Josef Pieper would say, one of the "servile arts". Thus it's not something that is properly encountered as a blacksmith would approach a "how-to-forge-a-good-plow" manual. You don't just want to judge poetry on the skill with which it describes isolated activities. It's a thing much more organic and coherent than a how-to manual; if it is good poetry, at any rate, it will say something not so much about fishing or smithing, as it will say something about the meaning of life. Yes, accuracy in the details is important, but only insofar as this will help give a truer picture of what life as a whole is truly like and how human beings will interact with this world.
The accuracy of description is not the end of poetry, though Ion quite misses the point and easily falls for Socrates' trap when the latter suggests that it is through his incessant questioning. And if there's some other type of truth present than the purely utilitarian, there is something more in poetry than a fisherman, a blacksmith, a horseman, or any other type of artisan will know by virtue of their art. There's nothing preventing the busiest artisan from being an interpreter of art on some level, but now the tables are turned from when Socrates suggested that the reciter only can judge the virtue of poetry by means of other skills that he possesses. It is not the case that the artisan will truly understand parts of Homer because he is a seaman and Homer talks about ships; rather he will understand all of Homer and be capable of judging his work, insofar as he understands human nature and something of what life in the world is all about.