Yadayim, Chapter 1, Mishnah 1. With this masechta we reach the next to last tractate in all of Shas. Maseches Yadayim is devoted to netilas yadayim, the washing of the hands, and the opening chapter begins with the most basic practical question of all: how much water does one need?
The Mishnah answers by stages. "M'revi'is nosnin layadayim, l'echad af lishnayim": with a revi'is of water, roughly three ounces and a bit, one may wash hands, and that quantity serves one person and even two. "Machatzis lug lishlosha o l'arba": half a lug, which is the equivalent of two revi'is, suffices for three people or four. "Milug lachamisha, v'la'asara, v'lame'ah": with a full lug, that is, four revi'is, one may wash five people, ten people, even a hundred. In other words, once the vessel holds a lug, its size no longer limits how many people can wash from it, though obviously it will have to be refilled again and again as the line moves along.
The Revi'is Itself
The Bartenura gives us the exact measure: one revi'is is a fourth of a lug, and in bulk it amounts to the volume of one and a half eggs. That is the minimum with which the washing of hands can be performed at all.
Why does a single revi'is cover two people? The Bartenura explains that the requirement is that a revi'is be present at the start. If the vessel held a revi'is, the first person washed from it, and the second then washed from what remained, the washing of the second person is valid even though a full revi'is is no longer there. The leftover water carries the status of water of taharah, and one may rely on it.
The Rambam understood the leniency more narrowly. Since hand washing is done by pouring water over the hands twice, the Rambam applies this halachah only to the second pouring: there, the second person need not have a full revi'is, because his hands have already been rendered pure by the first pouring. For the first pouring, however, the Rambam holds that each individual requires a complete revi'is of his own. The Bartenura records that his own teachers did not learn the Mishnah in the Rambam's way.
Why Three People Need Two Revi'is
On the Mishnah's next clause the Bartenura poses a pointed question. Do the arithmetic: if a single revi'is takes care of two people, then three people ought to manage with one and a half revi'is. So why does the Mishnah insist on a full half lug, a full two revi'is, before three or four may wash?
His answer is a comment on human nature. When a group of people is washing from one vessel, we are concerned that they will not be careful to pour over the entire hand up to the joint, because each one will try to economize with the water in order to leave enough for the person after him. Precisely because people are considerate, the earlier washers are liable to shortchange themselves. The Mishnah therefore requires a margin of extra water so that nobody feels pressed.
The View of Rebbi Yose
Rebbi Yose attaches a stipulation, introduced with the word "uvilvad", provided that. His condition is "shelo yifchos", that one not fall short, "la'acharon shebahen mirevi'is", for the last member of the group, below a revi'is. Rebbi Yose grants that a lug can serve five, ten or a hundred, but in his view the one who washes last must still find a complete revi'is waiting in the vessel when his turn arrives. The Bartenura points out that we do not rule in accordance with Rebbi Yose.
Topping Off the Later Water, but Never the First
The final ruling of the Mishnah concerns supplementing: "mosifin al hashniyim", one may top off the latter water, "v'ein mosifin al harishonim", whereas the earlier water may not be topped off. The Bartenura explains what lies behind the distinction.
The rule of netilas yadayim is that the water must reach up to the joint of the wrist. Now, once a person has poured the first water over his hands, his hands themselves are clean, yet the water still lying on them is not. If he were to wipe his hands on a cloth or towel and then touch the moisture that had been transferred to that cloth, his hands would become impure again, and were he then to handle a loaf of terumah, the loaf would become impure. It is for this reason that the standard practice is a second pouring: the second water comes to purify the first water that remains on the hands.
With that in mind, the distinction of the Mishnah becomes clear. If the second pouring did not reach all the way to the wrist, one may simply add more water and pour it over the area that was missed, and the first water on his hands is still purified. For example, if the first pouring covered the hand up to the wrist but the second pouring only reached the knuckles, he pours again and covers the stretch from the knuckles to the wrist.
The first water is different. If the first pouring did not reach the wrist, he cannot go back and add water to complete it. He must dry his hands and start over, pouring water that reaches the joint in a single act. One cannot wash half the hand and then the other half for the first water; it has to come all at once.
So Mishnah 1 has given us both the quantities of water required for one person or for a crowd, and the fundamental difference between the first pouring, which must be complete from the outset, and the second, which may be filled in.