Parah, Chapter 5, Mishnah 7. This chapter is occupied with the vessels used in preparing the mei chatas, the water into which the ashes of the parah adumah are placed. Only a genuine kli, a vessel, may be used to draw the water, to sanctify it with the ashes and to sprinkle from it. Our Mishnah applies that principle to a case that looks like a vessel but is really part of the ground.
The opening case is a basin that is part of the bedrock itself: "Hashoket shebasela", a trough hollowed out of a rock. Concerning it the Mishnah records five rulings. "Ein memalin ba": one does not draw the water into it. "V'ein mekaddshin ba": one does not combine the ashes with water inside it. "V'ein mazin mimena": one does not sprinkle out of it. "V'eina tzricha tzamid patil": it needs no lid sealed tightly around its mouth in order to protect its contents from tumah. And "v'eina poseles es hamikveh": it has no power to spoil a mikveh.
Picture a hollow scooped out of a boulder, a natural basin cut into the stone itself. It holds water beautifully, but it is not a vessel at all; it is simply a shaped piece of the earth. Since it is not a kli, none of the steps of the mei chatas may be performed in it: not the drawing of the water, not the placing of the ashes into the water, and not the sprinkling. For the same reason the rule of a lid sealed tightly all around, tzamid patil, has nothing to do with it, because that rule speaks of vessels.
The final clause of this list carries a practical consequence. If rainwater collects in such a rock trough and then flows out of it into a mikveh that holds less than forty se'ah, the mikveh remains valid. Water that has passed through a vessel becomes mayim she'uvim, drawn water, which disqualifies a mikveh, but this trough is no vessel, so the water that runs out of it is still perfectly natural rainwater.
Now the Mishnah turns the case around. "Haysa kli": the trough had originally been a genuine vessel, "v'chibrah b'sid", and afterward its owner fixed it to the rock with plaster. Every one of the earlier rulings is now inverted. "Memalin ba": water is drawn into it. "Umkaddshin ba": the ashes are put into the water there. "Umazin mimena": one sprinkles from it. "U'tzricha tzamid patil": a sealed covering does shield what is inside it. And "u'poseles es hamikveh": it ruins a mikveh. The reason is straightforward. This basin entered the world as a kli, and mortaring it to the ground does not strip away what it already is. It therefore serves for each stage of preparing the mei chatas, it falls under the halachah of the tightly sealed lid, and rainwater that passes through it turns into drawn water capable of invalidating a mikveh that does not yet contain forty se'ah.
Having established a trough that qualifies as a vessel, the Mishnah takes up what happens when it springs a leak and is patched. "Nikva milmatan": a hole opened in its floor, "ufekakah b'smartut", and the owner plugged the hole with a scrap of cloth. In that situation "hamayim sheb'socha psulin", the water standing in it may not be used, "mipnei she'einam agulim bikli", since the water is not ringed round by a vessel. A rag never becomes part of the kli. Water resting directly above an opening whose only stopper is cloth is being supported by the cloth rather than by the utensil, and water like that is disqualified for the mei chatas.
Against that the Mishnah sets a second scenario. "Min hatzad": the puncture appeared in the wall of the trough, "ufekakah b'smartut", and again he stuffed it with a rag. Here "hamayim sheb'socha k'sheirin", the water in the trough may be used, "mipnei sheheim agulim bikli", for the water is surrounded on all sides by the vessel. When the gap is in the side wall and sits some distance above the base, whatever water lies below the level of that gap rests on the floor of the utensil and is bounded by its walls. The cloth plays no real role in holding it. Such water is truly contained by a kli, and it is fit.
The concluding case deals with something built onto the trough. "Asu lah atarah", they fashioned a rim for it, "shel tit", out of clay, "v'halchu hamayim l'sham", and the water rose up into that area: "psulin", the water is disqualified. They had heightened the trough's edge with a band of clay running around the top, enlarging its capacity. Once the water climbs into that clay collar, it is no longer resting inside the vessel but inside an added ring that never became part of the kli, and it is unusable.
The Mishnah ends with the qualification. "Im hayah bari", if the clay rim was fastened strongly, "k'dei sheyital imah", to the point that the trough itself would be picked up along with it, then "kasher", the water is fit. Where the bond is so firm that taking hold of the clay collar and raising it would bring the entire trough up as one piece, the clay has effectively merged into the vessel. Water at that upper level is then being held by the kli, and it may be used for the mei chatas.