Yadayim, Chapter 4, Mishnah 1. Here we begin the last chapter of Maseches Yadayim, and it begins by turning to a new topic. Everything until this point has concerned the impurity that attaches to a person's hands. Now the Mishnah moves off in a different direction and gathers together halachos that were decided "bo bayom," on that day: the day Rabbi Elazar ben Azaryah was installed as the head of the academy, when the open questions of halachah were settled one after another. The Bartenura tells us what that phrase is pointing back to. It refers us to the story of Rabbi Shimon ben Azzai at the end of the previous chapter, which unfolded on the very day that Rabbi Elazar ben Azaryah was placed at the head of the yeshivah. And the Bartenura gives us a rule to carry with us: any time the Mishnah uses the words "bo bayom," that same day is what it has in mind.
The mishnah opens: "Bo bayom nimnu vegamru," on that day they put the matter to a vote and reached a conclusion. And what was the subject? "Al areivas haraglayim," a footbath, of a size described as "shehi mishnei lugin," holding from two lug, "ve'ad tisha kabin," and going as high as nine kav. What happened to it? "Shenistekah," it split open. And what did they conclude? "Shehi temei'ah midras," that it can still receive the impurity of midras.
Let us get a feel for the measurements. A lug is roughly eighteen and a half ounces, so two lug comes to something in the range of thirty seven to thirty eight ounces. A kav is four lug, which makes a single kav about seventy four and a half ounces, and nine kav therefore comes to approximately six hundred and seventy ounces. The mishnah is thus speaking about a fairly wide bracket, from a vessel holding under forty ounces up to one holding well over six hundred.
The case is a footbath that has cracked, so that it no longer holds water. Ordinarily a vessel that can no longer serve its purpose is no longer a vessel for the purposes of tumah. Nevertheless, the sages voted that such a footbath can still contract midras impurity, the form of tumah that a person with that status conveys to an object by sitting on it.
The Bartenura explains why. An areivah of this sort is a tub made for a person to soak his feet in. Even before it cracked, people were in the habit of sitting on these footbaths, so the vessel served for sitting alongside its primary function. Since sitting was already one of its uses, the loss of its water holding ability does not remove it from the category of an object designated for sitting, and it remains susceptible to midras.
Rabbi Akiva takes issue: "SheRabbi Akiva omer, areivas haraglayim kishmah." Rabbi Akiva says that a footbath is exactly what its name says it is. The Bartenura spells out his position: he disagrees, holding that the vessel is defined by its name, and it is nothing more than a footbath, a vessel for washing feet and not an object for sitting. Accordingly, in his view, the designation of midras impurity does not apply to it at all.
The Bartenura closes with the practical bottom line: "Ve'ein halachah keRabbi Akiva." The halachah in this matter does not follow Rabbi Akiva, but rather the ruling that was voted on that day, that the cracked footbath remains susceptible to tumas midras.