(Bamidbar 5:17) "And the Cohein shall take consecrated water: This refers to water consecrated in a vessel, the waters of the laver. "in an earthen vessel": We are hereby taught that all vessels are not equated with earthen vessels. For it would follow: Since soil and water consecrate in the instance of the red heifer and soil and water consecrate in the instance of sotah, then if I derive that all vessels are equated with earthen vessels re the red heifer, then re sotah, too, all vessels should be equated with earthen vessels; it is, therefore, written (specifically) "in an earthen vessel" (to negate the above derivation). "in an earthen vessel": a new one.
For it would follow: If I have derived that re the red heifer both a new and an old vessel are permitted, I should derive the same for sotah. It is, therefore, written here "in an earthen vessel," and, elsewhere (Vayikra 14:5) "into an earthen vessel." Just as there, a new one, here, too, a new one. These are the words of R. Yishmael. "and of the soil that shall be on the floor of the mishkan the Cohein shall take": Scripture hereby teaches us that if there were no soil there, he brings soil from elsewhere and places it there; for it is the place which consecrates.
Issi b. Yehudah says ("that shall be"): to include (the same for) the soil of the Temple (in Jerusalem). Issi b. Menachem says (in demurral): If in respect to a lesser form of tumah (e.g., dead-body tumah or sheretz tumah), the Temple (mikdash) was equated (vis-à-vis kareth liability for entry) with the sanctuary (mishkan), then, in respect to a graver form of tumah, sotah, (where death is the punishment,) how much more so, should the (strictures of the) mikdash be equated with (those of) the mishkan!
Why, then, need it be written "that shall be on the floor of the mishkan"? — That he not bring soil in his basket, (but shall use soil that is already there.) R. Shimon says: It is written here "afar" ("and of the afar that is on the floor of the mishkan"), and, elsewhere (Ibid. 19:17) "And they shall take for the unclean one of the afar of the burning of the (heifer) for cleaning." Just as "afar" here," "afar on the face of the water" (i.e., visible on the surface of the water), so, there, afar on the face of the water.
And, just as there if the afar preceded the water, it is valid, so, here. "the Cohein shall take (the afar) and place it on the water": so that it be visible. Three "things" in the Torah must be visible: the ashes of the heifer (Ibid.), the afar of the sotah, and the spittle of the yevamah (viz. Devarim 25:9). R. Yishmael says: Also the blood of the (slaughtered) bird (viz. Vayikra 14:6).