Mishnah Oholot 8

¹Some things bring forth uncleanness and [also] a protect [against it]; [Some things] bring forth uncleanness but do not protect against it; [Some things] protect but do not bring forth; [Some things] do not bring forth nor do they protect. The following bring forth and protect against [impurity]: A chest, a box, a cupboard, a beehive of straw, a beehive of reeds, or the water-tank of an Alexandrian ship, such of which [objects] have [flat] bottoms and can contain [at least] forty seahs liquid measure or two kors dry measure. [Also] a curtain, a leather apron, a leather bedspread, a sheet, a matting underlay or a mat when made into tents. A herd of cattle, unclean or clean, packs of wild animals or birds, a resting bird, a [shady] place that [a woman] makes for her son among the ears of corn; The iris, the ivy, squirting cucumber, Greek gourds and clean foodstuffs. Rabbi Yohanan ben Nuri did not agree with regard to clean foodstuffs except in the case of a cake of dried figs. ²Projections from a wall, balconies, dovecotes, crevices and crags, grottoes, [overhanging] pinnacles, interlaced boughs and protruding stones as long as they are cable of sustaining thin plasterwork, the words of Rabbi Meir. But the sages say a medium plasterwork. What are interlaced boughs? A tree which casts shade over the ground. Protruding stones? [Stones] that project from a wall. ³The following bring [impurity] but do not protect against it: A chest, a box, a cupboard, a beehive of straw, a beehive of reeds, or the water tank of an Alexandrian ship, such of which [objects] have [flat] bottoms but cannot contain forty se'ahs of liquid measure or two kors dry measure. A curtain, a leather apron, a leather undercover, a sheet, a matting underlay or a mat when not made into tents. Cattle or wild animals when they are dead, and foodstuffs that are [liable to become] unclean. In addition to these, a human-powered mill. ⁴The following protect [against impurity] but do not bring it: A loom [with the woof] spread out, the ropes of a bed, waste baskets, and window-lattices. ⁵The following neither bring [impurity] nor protect against it:Seeds, plants [still] attached to the ground, except for the plants mentioned above, A lump of hail, snow, frost, ice and salt. [Anything] that hops from one place to another, or leaps from one place to another, a flying bird. A loosely-flapping garment, or a ship floating [freely] on the water. If the ship were tied with something that can keep it steady, or a stone were [placed so as] to hold down the garment, it can bring uncleanness. Rabbi Yose says: a house on a ship cannot bring uncleanness. ⁶Two [earthenware] jars in which there are two pieces of corpse the size of half an olive, sealed with tightly fitting lids, lying in a house, they remain clean, but the house becomes unclean. If one of them was opened, that [jar] and the house become unclean, but the other remains clean. And similarly with two rooms that open into a house.