Mikvaot, Chapter Seven, Mishnah Two
1) These invalidate the mikveh and do not raise it up to [the required quantity]:
a) Drawn water, whether clean or unclean, water that has been used for pickling or for boiling, and grape-skin wine before it becomes vinegar.
b) How do they make the mikveh invalid and do not raise it up [the required quantity]?
c) If a mikveh contained forty seahs less a kortov, and a kortov of these fell into it, it does not raise it up [the required quantity];
d) And if there were three logs of any of these, they would invalidate the mikveh.
2) But other liquids, and the juice of fruits, brine, and liquid in which fish has been pickled, and grape-skin wine that has fermented sometimes raise it up to [the required quantity] and sometimes do not raise it up.
a) How so? If a mikveh contained forty seahs less one, and a seah of any of these fell in it, it does not raise it up to [the required quantity].
b) But if the mikveh contained forty seahs and a se’ah of any of these was put in and one seah was removed, the mikveh is still valid.
Section one: The mishnah now explains the types of materials that do invalidate a mikveh.
As we have learned many times, drawn water certainly invalidates a mikveh, at least one that doesn’t already have forty seahs of valid water. This is true regardless of whether the water is pure or impure. Similarly, water that has been used for pickling or for boiling invalidates the mikveh. Grape-skin wine is really water with a little bit of flavor in it. This type of wine invalidates a mikveh only before it turns into vinegar. Grape-skin wine that has fermented into vinegar is dealt with in section two.
If one of these substances falls into a mikveh and thereby raises the level of the mikveh to forty seahs, it does not cause the mikveh to have the requisite amount. And if three logs or more fall in, they invalidate the mikveh.
Section two: Other liquids like milk, wine and the liquids listed here are neutralthey neither raise the level nor invalidate the mikveh.
If the mikveh has less than forty seahs and a seah of one of these substances spills in, it does not raise it up to the requisite level.
But if the mikveh has the requisite level and one of these falls in and then one seah is removed, the mikveh remains valid because when the seah of the liquid fell in, there were already forty seahs. In this way, the liquid can, in a sense, aid in raising the mikveh to the requisite level.