1) THE 12-HOUR PERIOD BEFORE THE EXPECTED ONSET OF "DAM NIDAH"
QUESTION: The Gemara cites a Beraisa that discusses the Halachah in the case of a woman who bleeds every other day (Rashi points out that she always bleeds in the daytime, but not at night). The Beraisa states that she is permitted to her husband on the eighth day after she first bled, and also on the following night. RASHI (DH Meshameshes) explains that she is Tahor on the eighth day because on the evening of the seventh day (i.e. the evening after the seventh day), she immersed in a Mikvah. Since she bleeds only on odd-numbered days, she may assume that she will not bleed again until day 9 after she first bled. Therefore, she is permitted on both the night and the day of day 8. She is also permitted on the evening of day 9, because she does not expect to see blood until the daytime.
The Beraisa continues and says that she is permitted on four additional nights within the 18 days after she first bled. Rashi (DH v'Arba'ah) writes that she is permitted on these four nights since she will never become a Zavah, because she never sees three days in a row. When she bleeds on day 9, she must be Shomeres day 10 (and make sure that she does not see blood on that day), and she is permitted to be with her husband on the night going into day 11. She is expected to see blood again during the daytime of day 11. This cycle continues so that she is also permitted on the night before day 13, the night before day 15, and the night before day 17. Accordingly, there are four additional nights on which she is permitted to be with her husband during the 18-day cycle.
Why does the Gemara not say also that she is permitted to be with her husband on the evening of day 19, which is the next in the series of odd-numbered nights? She should be permitted on that night because she does not expect to see blood until the daytime on day 19!
(a) The RASHBA cites RABEINU CHANANEL who says that, mid'Oraisa, this woman indeed may immerse on day 18 and be permitted that night (of day 19) in the same way that she was permitted at night after the days on which she was Shomeres for Zivah before she was expected to bleed (that is, between days 8 and 18, when she would have been a Zavah Gedolah if she would have seen blood on three consecutive days; see Tosfos DH v'Arba'ah). However, the Rabanan (see 63b) decreed that a man must separate from his wife before her Veses (the period when she expects to see Dam Nidah), as the Torah states, "You shall separate the people of Yisrael from their impurity" (Vayikra 15:31). Rabeinu Chananel explains that a Veses is a 12-hour period. Therefore, since day 19 has arrived, which is the day when the new Nidah cycle starts (because every set of 18 days is comprised of 7 days of Nidah followed by 11 days of Zivah), her husband is required to separate from her on the night of day 19, the 12-hour period before she expects to see blood.
(However, during the days of Zivah (days 8-18), he is permitted to be with her on the night before she expects to see blood, because the Gemara (39a) says that a woman cannot establish a Veses during her days of Zivah. See footnote #312 to CHIDUSHEI HA'RITVA.)
The MEROMEI SADEH writes that Rabeinu Chananel's proof from the Gemara here for the length of a period of a Veses is a source for the opinion of the AVI'ASAF and the OR ZARU'A, cited by the SHACH (YD 184:7). They maintain that a husband must separate from his wife in the 12-hour period (either day or night) before the night or day when her Veses occurs. (Others rule (see (b) below) that it suffices to separate only during the 12-hour period itself that she expects to see Dam Nidah.)
(b) The RASHBA and RITVA disagree with Rabeinu Chananel and rule that the Mitzvah to separate from one's wife is only in the day or night when her Veses occurs, but not in the 12-hour period prior to that day or night. They explain that the reason why the Gemara does not say that she is permitted on the night of day 19 is that the Gemara counts only the nights she is permitted before the Nidah cycle restarts. The night of day 19 belongs to the new cycle.
This also seems to be the intention of Rashi (DH v'Arba'ah), who writes that the last time she is permitted "is not within the 18 days, and when she sees on the next day, day 19, it will be the beginning of her Nidah cycle." (D. BLOOM)
2) TURNING DRIED ITEMS BACK INTO LIQUIDS
QUESTION: The Mishnah states that Dam Nidah and the flesh of a Mes are Metamei whether they are moist or dry. In contrast, Zov, various forms of saliva, Sheretz, Neveilah, and Shichvas Zera are Metamei only when they are still moist. The Mishnah concludes that if any of these items becomes re-hydrated by being soaked in lukewarm water for a period of 24 hours, it is not considered dried out and it is Metamei.
The Gemara quotes the Mishnah earlier (21a) and asks what the source is for the Mishnah's statement. The Mishnah there says that if a woman miscarries a scab-like piece of dry blood, it is Metamei if it is dissolvable in water.
Why does the Gemara question the statement of the Mishnah earlier? According to the teaching of the Mishnah here, that anything that dissolves in water is considered a liquid (and is Metamei), it is obvious that a piece of dry blood should be Metamei.
(a) TOSFOS in Zevachim (79b, DH Af b'Rotbo) discusses a similar case. Tosfos suggests that the Mishnah earlier is referring to a case in which the objects are so dry that they re-hydrate only by being soaked for more than 24 hours. The Mishnah here, in contrast, considers a dry object to be moist with regard to Tum'ah only when it re-hydrates by being soaked for less than 24 hours.
(b) The CHIDUSHEI HA'RAN explains that the rule of the Mishnah here that an object that re-hydrates in 24 hours is considered moist applies only to a Neveilah and a Sheretz. It does not apply to Zov, the various forms of saliva, and Shichvas Zera. The Mishnah earlier is referring to dried blood, which is not considered a liquid even if it re-hydrates after being soaked.
(c) The RA'AVAD (Hilchos Tum'as Mishkav u'Moshav 2:1) rules that only a Sheretz and Neveilah can become re-hydrated by being soaked for 24 hours. His words imply that all substances are considered liquids when they re-hydrate, but a Sheretz and Neveilah are different in that even if they take 24 hours to re-hydrate, they are considered liquids with regard to Tum'ah. The other substances are considered liquids only when they re-hydrate within a shorter period of time. Accordingly, the Mishnah earlier might be referring to a situation in which the dried blood dissolved in water within 24 hours, but only after its allotted times.