Niddah, Chapter 7, Mishnah 2. Our mishnah takes up a basic question about the various substances that serve as sources of tumah: does such a substance keep its power to convey tumah even after it has dried out, or is it only tamei so long as it is still moist? The mishnah draws a line between two groups.
The substances that are tamei even when dry
"Dam haniddah uvesar hameis metamin lachin umetamin yeveishin." The blood of a niddah and the flesh of a corpse convey tumah in either condition: while they are still moist, and equally after they have dried out. Drying does not diminish them at all; their status as a source of tumah remains fully intact.
The substances that are tamei only while moist
"Aval hazov vehani'a veharok vehasheretz vehaneveilah veshichvas zera metamin lachin ve'ein metamin yeveishin." Here the mishnah lists a second group: the flow of a zav, the mucus of a zav, the saliva of a zav, a dead crawling creature (a sheretz), a neveilah (the carcass of a dead animal), and shichvas zera. All of these convey tumah while they are moist, but once they have dried out they no longer convey tumah at all.
The test of soaking
"Ve'im yecholin lehisharos velachzor lechmoseihen metamin lachin umetamin yeveishin." Dryness by itself is not the final word. If one of these dried substances could be placed in water and, after absorbing the water, return to the consistency it originally had, then it conveys tumah in either state, dry as well as moist. The reasoning is straightforward: since the substance is still capable of being restored to what it once was, its present dryness is not a real change in its nature.
"Vechama hishriasan? Beposhrin me'eis le'eis." The mishnah then tells us how long a soaking we are speaking about, since the whole test depends on it. The measure is water at room temperature for a full twenty four hours, from one point in time to the same point the following day. If that soaking would bring the substance back to its original consistency, it is treated as though it were still moist.
The view of Rebbi Yosi
The mishnah closes with a dissenting opinion, opened by the words "Rebbi Yosi omer." His subject is "besar hameis," the flesh of a corpse, which the mishnah had grouped among the substances that transmit tumah in any condition. Rebbi Yosi holds that the test of soaking governs here too. Where such flesh has become "yaveish" to a degree that immersion in water would no longer bring it back to the consistency it once had, he rules it "tahor": it has lost its ability to convey tumah.
The mishnah thus leaves us with a clear principle to carry along: for most of these sources of tumah, everything hinges on whether the substance is still what it was, either because it remains moist or because water could bring it back to its former consistency.