Mikvah

Flowing [water] sources are like a spring, and dripping [water] sources are like a mikveh. Rabbi Tzadok testified regarding flowing sources that [their waters] were greater than [those of] dripping sources [in a case where the two became mixed], that they are valid. And regarding a dripping source that was made into a flowing source [i.e. if it was somehow breached], one may put even a stick, or even a reed, or even a zav or a zavah beside it [the breach, in order to seal it off], and one may [then] descend and immerse [as into a mikveh of gathered waters], according to Rabbi Yehuda. Rabbi Yose says: anything which can be rendered impure, one cannot use it to stop the flowing.

That purification from all kinds of defilement shall be effected by immersion in the waters of a Mikvah, as it is said, "…. then he shall wash all his flesh in water" (Lev. 15:16). It has been learnt by tradition that this means that the washing must be in a sufficient quantity of water, so that the entire body is totally immersed at one time.

It is a mitzvah for every man to immerse in a mikvah on erev Yom Kippur,⁵It is best to do so before the Minchah prayers because the Vidui, (confession of sins) is said at Minchah. Others have the custom to immerse after the final meal because then it is closer to Yom Kippur. (Ibid 606:18) to purify himself from nocturnal emission, and also as an element of repentance, just as a convert is required to immerse in a mikvah. Therefore, even boys and girls⁶This refers to boys of thirteen years of age and girls who are twelve years old. (Ibid 606:23) should immerse themselves in a Mikvah. You should be certain that there is no chatzitzah on your body [a particle intervening between the body and the water] (see Chapter 161 below). The most appropriate time for the immersion is after midday. A woman after intercourse is likely to discharge semen within three days. She is then considered as one who has had a nocturnal emission, and the immersion is pointless in this case. The practical thing for her to do, is before immersing, to wash herself well with warm water; she will then experience no further discharge. However, if she had intercourse soon after her monthly immersion in the mikvah, or immediately before her menstrual period, when she is likely to become pregnant, then she is forbidden to destroy the semen of conception, and she should, therefore, not wash herself in warm water. Nevertheless, she should immerse in cold water. A mourner, may the Merciful One protect us, even during the first seven days of mourning⁷Even if the funeral was held on erev Yom Kippur. (Ibid 606:23), may wash and immerse himself⁸He may even wash with warm water in order to be sure that there is no chatzitzah on his body. (Ibid 606:24) about one or two hours before nightfall, even before the Minchah service, but all the other laws of mourning, such as sitting on the floor, and not wearing [leather] shoes, must be observed until nighttime.

...R. Yochanan used to go and sit at the gates of the mikvah. He used to say: ...'When the daughters of Israel ascend from the mitzvah of immersion let them chance upon me, that they may bear sons as beautiful and as learned in Torah as I.'