For polluted, read, with the Revised Version, weltering, the primary meaning of the verb being that of stamping or treading, and omit "when thou wast," as weakening the condensed force of the original.
The marvel of that unlooked for pity is emphasized by the iteration of the word of mercy, Live. The commentary of the Chaldee Targum is sufficiently curious to be quoted: "And the memory of my covenant with your fathers came into my mind, and I was revealed that I might redeem you, because it was manifest to me that ye were afflicted in your bondage, and I said unto you, 'I will have compassion on you in the blood of circumcision,' and I said unto you, 'I will redeem you by the blood of the Passover'" (Rosenmuller).
The thought underlying this strange interpretation is that blood might be the means of life as well as of pollution, and in that thought there is a significance at once poetical and profound, almost, as it were, anticipating the later thoughts that the blood of Jesus cleanseth from all sin (1 John 1:7; Revelation 1:5), that we make our robes white in the blood of the Lamb (Revelation 7:14).
There is no reason, however, for believing that such thoughts were present to the prophet's mind.