What "law" is Danforth referring to in his assertion: "I should hang ten thousand that dared to rise against the law, and an ocean of salt tears. . ."
. . . of salt tears could not melt the resolutions of the statutes."
This line can be found in Act IV, Scene 1. To find out what law Danforth is referring to, just look a little bit farther back in this short speech that he gives to Rev. Hale. The law that Danforth is talking about here is God's law. Danforth is saying that he does not care at all how many people go against God's law or how many people cry for him to be merciful -- he is going to uphold God's law as he sees it. This is why he will not pardon anyone or let anyone off.
Here is the relevant quote:
While I speak God's law, I will not crack its voice with whimpering. If retaliation is your fear, know this -- that I should hang ten thousand...