Niddah

If a woman is alternately unclean for nine days and clean for nine days she may have marital intercourse on eight days out of every eighteen days.  If she is alternately unclean for ten days and clean for ten days, the days in which she is permitted marital intercourse are the same in number as the days of her zibah.  And the same  applies to cycles of a hundred  and so also to cycles of a thousand. MISHNAH. THE BLOOD OF A MENSTRUANT AND THE FLESH OF A CORPSE CONVEY UNCLEANNESS WHEN WET AND WHEN DRY. BUT THE ISSUE, PHLEGM AND SPITTLE OF A ZAB, A DEAD CREEPING THING, A CARCASS AND SEMEN CONVEY UNCLEANNESS WHEN WET BUT NOT WHEN DRY. IF, HOWEVER, ON BEING SOAKED, THEY ARE CAPABLE OF REVERTING TO THEIR ORIGINAL CONDITION THEY CONVEY UNCLEANNESS WHEN WET AND WHEN DRY. AND WHAT IS THE DURATION  OF THEIR SOAKING?  TWENTY-FOUR HOURS IN LUKEWARM WATER.  R. JOSE RULED: IF THE FLESH OF A CORPSE IS DRY, AND ON BEING SOAKED CANNOT REVERT TO ITS ORIGINAL CONDITION, IT IS CLEAN. GEMARA. Whence are these rulings  deduced? — Hezekiah replied: From Scripture which says, And of her that is sick with her impurity,  her impurity  is like herself, as she conveys her uncleanness so does her impurity convey similar uncleanness. Thus we find the law concerning wet blood,  whence the deduction concerning dry blood? — R. Isaac replied: Scripture said, Be,  it shall retain its original force.  But might it not be suggested that this  applies only to blood that was wet and then dried up; whence, however, the deduction that it applies also to blood that was originally  dry? And, furthermore, with reference to what we have learnt, 'If a woman aborted an object that was like a rind, like earth, like a hair, like red flies, let her put it in water and if it dissolves she is unclean', whence is this  deduced? — 'Be'  is an inclusive statement.  If [it be argued:] As she causes couch and seat to convey uncleanness to man and to his garments  so should her blood also cause couch and garment to convey uncleanness to man and his garments. [it can be retorted:] Is then her blood capable of using a couch or a seat?  — But according to your argument  [it could also be objected]: Is a leprous stone  capable of using a couch or a seat that a text should be required to exclude it?  For it was taught. 'It might have been presumed that a leprous stone should cause a couch and a seat to convey uncleanness to man and to his garments, this being arrived at logically, for if a zab who does not convey uncleanness by means of entry  causes couch and seat to convey uncleanness to man and to his garments, how much more then should a leprous stone, which does convey uncleanness by means of entry,  convey uncleanness to couch and seat to convey it to man and his garments, hence it was specifically stated, He that hath the issue,  implying only 'he that hath the issue' [is subject to the restriction]  but not a leprous stone'. Now the reason  is that Scripture has excluded it, but if that had not been the case it would have conveyed the uncleanness, would it not?  — A reply may indeed be forthcoming from this very statement,  for did you not say. 'He that hath the issue  [is subject to the restriction] but not a leprous stone'? Well here also Scripture said, Whereon she sitteth,  only she but not her blood.