Mikvaot, Chapter Ten, Mishnah Six
The first half of our mishnah discusses the purification of impure water through a process known as “kissing.” This is when the edge of a vessel with impure water is brought up and allowed to “kiss” or “touch” a body of purifying water.
The second half deals with immersing a vessel that has some sort of liquid already in it.
1) Bet Shammai say: hot water may not be immersed in cold, or cold in hot, foul in fresh or fresh in foul.
a) But Bet Hillel say: it may be immersed.
2) A vessel full of liquids which one immersed, it is as if it has not been immersed.
a) If it was full of urine, this is reckoned as water.
b) If it contained hatat waters, [it is unclean] unless the water [of the mikveh which enters the vessel] exceeds the hatat waters.
3) Rabbi Yose says: even if a vessel with the capacity of a kor contains but a quarter-log, it is as if it had not been immersed.
Section one: According to Bet Shammai, when one does “kissing” in order to purify water, both the pure and the impure water must be the same. If one is of a different temperature or a different quality from the other, then the immersion doesn’t work. It is as if by having the waters touch, they become one body and this can only be true if they are of the same quality. Bet Hillel says that this is not essential. To Bet Hillel, all water is the same, be it foul or fresh, cold or hot.
Section two: Liquids other than water cannot be immersed in order to purify them. Therefore, a vessel full of liquid cannot be immersed. If the vessel is not full, however, it can be immersed because the water that enters swishes around and has contact with the whole vessel.
Interestingly, urine is treated like water in this manner. A vessel can be immersed even if full of water.
Hatat waters are the waters used in the red heifer purification ritual. If the vessel needs to be purified then the waters of the mikveh that enter the vessel must be greater than the hatat waters that are already in the vessel. In other words, the law is more stringent here than it is with other types of liquids.
Section three: Rabbi Yose adds that it doesn’t matter how large the vessel is—if there is even a small amount of liquid in it, it cannot be immersed. Rabbi Yose seems to be speaking about all liquids, not just water. Whereas the previous opinion allowed the immersion of vessels with liquids in them, as long as the vessel wasn’t full, Rabbi Yose is stricter. The vessel must be completely empty.