Charles Spurgeon Commentary Genesis 18:26-30

"And Jehovah said, If I find in Sodom fifty righteous within the city, then I will spare all the place for their sake. And Abraham answered and said, Behold now, I have taken upon me to speak unto the Lord, who am but dust and ashes: peradventure there shall lack five of the fifty righteous: wilt thou destroy all the city for lack of five? And he said, I will not destroy it, if I find there forty and five. And he spake unto him yet again, and said, Peradventure there shall be forty found there. And he said, I will not do it for the forty`s sake. And he said, Oh let not the Lord be angry, and I will speak: peradventure there shall thirty be found there. And he said, I will not do it, if I find thirty there." — Genesis 18:26-30 (ASV)

This time the patriarch has advanced by ten; before, it was by fives. Pleading men grow bolder and braver in their requests. A man who is very familiar with God will, eventually, venture to say what, at first, he would not have dared to utter.