The next paragraph commences with a shift in tone — no less pragmatic but even more ruthlessly efficient — as Lady Macbeth switches her attention to the details of the murder itself. Her plan to drug the guards with alcohol is couched in metaphorical language derived from the ancient science of alchemy. The words "receipt," "fume," and "limbeck" specifically refer to this process, whose purpose was to turn base metal (such as lead) into gold. It is heavily ironic that, in the Macbeths' experiment, that which is gold — the king himself — will become base and doubly ironic that Macbeth's golden reputation will be reduced to worthlessness.
Macbeth has been convinced. In words that uncannily recall his wife's, he now puts on the mantle of murderer: the monosyllabic "False face must hide what the false heart doth know" has a certainty to it that completely overturns his earlier vacillation.
