What is a dramatic device used in Act IV, Scene 1 of The Merchant of Venice?

In Act IV, Scene 1 of The Merchant of
Venice, Shakespeare prominently uses the dramatic device of contrasts to
starkly draw the picture of the horror and bassness of Shylock's intended action.
Shylock starts a series of contrasts with his speech in which he
says:

Some men
there are love not a gaping pig;
Some, that are mad if they behold a
cat;
And others, when the bagpipe sings i' the nose, . .
.

Antonio takes up the device
of contrasts in his speech to Bassanio begging him not to try to reason with Shylock.
Antonio's contrasts start:

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You may as well go stand upon the
beach
And bid the main flood bate his usual height;
You may as well
use question with the wolf
Why he hath made the ewe bleat for the lamb; .
.

Antonio takes up the device
of contrasts again in his rejoinder to the Duke in his speech that starts: "You have
among you many a purchased slave, / Which, like your asses and your dogs and mules,  . .
.."

Shakespeare uses another dramatic device, that of
surprise when he introduces the surprise presence of Belthasar, who is Portia in
disguise. Surprise is a href="http://swift.tahoma.wednet.edu/">dramatic device used to provide an
unexpected twist in the plot. Contrast is a dramatic device used to arouse strong
emotions by painting opposition that gives emphasis or clarity.