For the source text click/tap here: Chagigah 21
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As we learned in the first Mishnah in this perek (chapter) (see 20b), great care must be taken to ensure ritual purity in the cases of terumah (tithes) and kodashim (Temple sacrifices),
but the demands made regarding kodashim are greater than those having to do with terumah.
A Mishnah in Mikvaot is cited in our daf:
Mikvaot can be joined together [if their connection is as big] as the tube of a water-skin in thickness and in space, in which two fingers can be fully turned round.
The hole connecting the two mikvaot must be the size the tube of a water-skin, which is two fingerbreadths in width. As the mishnah explains, one must be able to put one's fingers in the tube and fully turn them around.
If there is a doubt [whether it is as big] as the tube of a water skin or not, it is invalid, because [this is a mitzvah] from the Torah.
If someone immerses in one of these mikvaot, and it alone has less than forty handbreadths and he is not sure whether the connection with the other mikveh is as big as the tube of a water-skin, he remains impure.
This is because the mitzvah to immerse in the mikveh is from the Torah and in cases of doubt concerning toraitic impurity, the law is strict.
We explore the construction of mikvaot and the recent Israeli Supreme Court has ruled that making women immerse under supervision is an invasion of privacy, so why must they fill out ‘permission slips’ to dip?
and the history of mikvah use in 20th century USA.