The lines you're thinking of are:
> " **In the little, nameless, unassuming street,
> That fronts the awful Thames with the worn-out tire,
> There stands a house, quite plain and genteel, you'll meet,
> That looks like a book-case of some bygone sire;
> And there, on the second floor, you'll hear, every night,
> A voice in the dark, soft-low, and light,
> And you'll hear it say, "I am the mistress here,
> And I know you're the mistress, dear!**"
This poem is a dramatic monologue, and it is spoken by a woman who is planning to poison her rival in love. It's a classic example of Browning's mastery of psychological drama and his ability to create vivid and unsettling characters.
If you're looking for the poem itself, you can easily find it online by searching for "The Laboratory by Robert Browning." You can also find it in many anthologies of poetry.
It's worth noting that Humbert Wolfe was a British poet, but he is not as famous as Robert Browning. He is known for his light and witty verse, but he did not write "The Laboratory."