For this I shall have time enough to mourn.
In poison there is physic, and these news,
Having been well, that would have made me sick,
Being sick, have in some measure made me well.
And as the wretch, whose fever-weakened joints,
Like strengthless hinges, buckle under life,
Impatient of his fit, breaks like a fire
Out of his keeper's arms, even so my limbs,
Weakened with grief, being now enraged with
Are thrice themselves. Hence therefore, thou
nice crutch! [He throws down his crutch.]
A scaly gauntlet now with joints of steel
Must glove this hand. (1.1.149-162)
When Northumberland learns two bits of terrible news (his son was killed in battle and the king's forces are on their way to Northumberland to arrest and/or kill him), he's suddenly feeling well enough to dispose of his "crutch" and get his battle on. (Funny how that happens. Too bad he wasn't inspired to fight sooner – his son might still be alive if he had.) Northumberland, who we have established is a big faker, indulges in a bit of crafty word play here as he insists that "in poison there is physic." In other words, the terrible news is like a "poison" to his system and makes him "sick" to hear it. The news has also left him feeling whoozy and weak in the knees. At the same time, he insists, this awful, sickening news has the effect of curing his (supposed) physical ailments (his "fever-weakened joints" and what not) because he's suddenly inspired to leap up and grab his weapon so he can fight the king.