"The Laboratory" begins with some scene-setting imagery and a moment of pitch-black humor. The poem's speaker, a lady from the court of the 17th-century French king Louis XIV, puts on a protective "glass mask" as she sits in an apothecary's laboratory—that is, a kind of old-fashioned chemistry lab. As she does so, she glances around the room, peering through the "faint smokes" that "curl[] whitely" up from the apothecary's mysterious simmering kettles, and has one question for him:
Which is the poison to poison her, prithee?
This, then, is a lady with murder on her mind. Over the course of this dramatic monologue, readers will learn exactly what has driven this speaker to plot an act of crazed violence.
In these first lines, though, the poem keeps things mysterious. Readers new to the poem won't know anything much about the speaker as she begins this speech: there's much more detail about the setting than the characters in this first stanza.
But that setting is revealing in its own right—and so is the speaker's voice. The speaker sees the apothecary's lab as a kind of "devil's-smithy": that is, a workshop from Hell itself, a place for manufacturing sinister tools. But that doesn't seem to disturb her; in fact, it only piques her interest. Listen to the sounds and language of lines 3-4 again:
As thou pliest thy trade in this devil’s-smithy—
Which is the poison to poison her, prithee?
Those popping alliterative /p/ sounds feel rather prim—especially alongside the speaker's dainty little "prithee" (an old-fashioned, polite tag for a question, rather like "if you please"). For that matter, rhyming the grim "devil's-smithy" with the mannerly "prithee" sets up a darkly funny contrast between nefarious deeds and civilized language.
The poem's form similarly compresses madness into an orderly shape. The poem's regular quatrains (or four-line stanzas) are broken into neat little rhymed couplets. And the meter sticks to tetrameter (that is, four strong stresses per line), like this:
Which is the | poison to | poison her, | prithee?
But there's madness in this order. This line, like much of the poem, is written mostly in dactyls, metrical feet with a DUM-da-da rhythm (though the last foot here is a trochee, with a DUM-da rhythm). That means a whole lot of syllables get compressed into one four-beat line!
All these formal choices mean that the quick-talking speaker sounds at once controlled and manic, refined and crazed.