Sotah, Chapter Two, Mishnah 2. Continuing with the procedure of the sotah, we now turn to the cup of water into which the written passage of the sotah, containing the Name of Hashem, is erased, so that the sotah will drink from it.
The cup and the amount of water:
"Peyali" - a Greek word meaning cup, and it is made of earthenware, that is, of clay, and it must be a new vessel.
"Chatzi log mayim min hakiyor" - the cup is filled with half a log of water, taken from the basin in the Beis HaMikdash.
Rabbi Yehudah disagrees and says "revi'is" - half a log is not required, but only a quarter of a log. The Mishnah notes that his words are consistent with his position below: Rabbi Yehudah requires a smaller amount of writing on the parchment to be erased into the water, and since the amount of writing that is erased is smaller, correspondingly he also requires a smaller amount of water.
Taking the earth from the floor of the Heichal:
The Mishnah continues its description of the procedure: the kohen enters the Heichal, into the sanctuary, and turns to his right. There was a spot in the floor there, one cubit by one cubit, with a marble slab (tablet) over it that had a ring in it. The kohen lifts the slab, takes earth from beneath it and places it into the water, in an amount such that the earth will be visible on the surface of the water.
The source of the law in the verse:
From where is this derived? The verse states that one takes from the earth that is on the floor of the Mishkan. The Torah speaks of the Mishkan and not of the Beis HaMikdash, but in the Beis HaMikdash they would reach the ground after removing the tile, and it is that earth that the kohen places upon the water.
In summary: In this Mishnah we learned the order of preparing the bitter waters: a new earthenware cup, half a log of water from the basin (and according to Rabbi Yehudah a quarter of a log, consistent with his position that requires less writing to be erased), and earth taken from beneath the marble slab in the floor of the Heichal and placed on the surface of the water - all in preparation for her drinking of these waters, known as the bitter waters.