In The Merchant of Venice, does Portia's praise of mercy have any effect on Shylock?
Portia's speech about mercy in Act IV, Scene 1 of The Merchant of Venice has two effects on Shylock. The first is to declare that his deeds will stand as they are before God and he will not plead for God's mercy: "My deeds be on my head." The second affect is to compel Shylock to say that no speech will ever sway him from his desire for the letter of the law to be fulfilled: "There is no power in the tongue of man / To alter me: I stay here on my bond."
Portia's Mercy Speech reminds Shylock that mercy is an attribute of God, one that is demonstrated by monarchs (kings and queens). She also reminds him that according to both Christianity and Judaism, mercy is God's salvation from the punishment of inferior human actions. This is when Shylock declares he will let his deeds stand without begging mercy.
Later, after Portia again asks Shylock to show mercy and take "thrice" (three times) the amount due him, allowin that the contract be torn up, Shylock declares that her words--nor anyone else's--will never sway him. He wants justice from the law. Sadly, that is precisely what he gets: no mercy and the full justice of the law.