We continue with Maseches Terumos, chapter 8, mishnah 5, dealing with uncovered water - water that was left exposed, a law that applies equally to wine and to milk. In the previous section we discussed the length of time required for the exposure to matter, and now we turn to the quantity of the water.
The measure of uncovered water:
The mishnah states: "kedei shetovad bahen ha'marah" - it is enough that the bitterness, meaning the venom, would be lost within the water. According to the Bartenura this is the plain meaning: if there is a large enough quantity of water in that place, so that even if a snake drank from it whatever venom remained would be diluted to the point where it is no longer dangerous, the water may be drunk, even though it was left uncovered.
It is worth noting that other commentators understood this differently: it is not a question of how much the venom is diluted, but the opposite - a large quantity is worse than a small one. In a small quantity one can detect by eye whether any trace of venom remains mixed in the water, since it is not spread out, and therefore if no poison is visible in it, the water is safe to drink. In any case, for now we will follow the plain approach of the Bartenura: when the water is plentiful it becomes safe to drink and may be drunk. As for the exact measure, the Bartenura writes that in those days they were expert in it.
"Rabbi Yosei omer: bekeilim" - Rabbi Yosei says: in vessels:
Rabbi Yosei distinguishes between water held in a vessel and water in the ground:
In vessels: the quantity of water in the vessel makes no difference. As long as the water is in a vessel, it is dangerous at any level of dilution, and therefore one may not drink water from a vessel that was left uncovered.
In the ground: the measure is forty se'ah, the measure of a mikvah - meaning that more than forty se'ah is safe to drink.
The Bartenura writes, however, that the halachah does not follow Rabbi Yosei but rather the first tanna, and therefore we go according to the plain approach stated above: the safety of the water is determined by the quantity of water relative to the venom that a snake might have left behind, and these measures were known in their days.
Additional laws brought by the Bartenura:
A natural spring: the prohibition of uncovered water does not apply to it, since it produces fresh water at every moment, and there is no problem at all.
Other uses: water that became forbidden because it was left uncovered may not only not be drunk, it may not even be used for other purposes: one may not give it to animals to drink, one may not wash with it, and one may not use it to wash one's hands and face (since the venom can penetrate through the skin), and one may not even mix it with cement to make concrete and the like. All of these are forbidden because of the dangers involved in using poisoned water.
In summary: in this mishnah we learned the required quantity for uncovered water - "kedei shetovad bahen ha'marah" - and the two ways of explaining it; Rabbi Yosei's distinction between vessels and the ground (forty se'ah) and the ruling in accordance with the first tanna; and also that the law does not apply to a natural spring, and that water forbidden because it was left uncovered is forbidden for other uses as well.