Mishnah for Yadayim 4:1
בּוֹ בַיּוֹם נִמְנוּ וְגָמְרוּ עַל עֲרֵבַת הָרַגְלַיִם, שֶׁהִיא מִשְּׁנֵי לֻגִּין וְעַד תִּשְׁעָה קַבִּין שֶׁנִּסְדְּקָה, שֶׁהִיא טְמֵאָה מִדְרָס. שֶׁרַבִּי עֲקִיבָא אוֹמֵר, עֲרֵבַת הָרַגְלַיִם כִּשְׁמָהּ:
On that very day [on which Rabbi Elazar ben Azaria was inducted into his seat as head at the Academy], they voted and concluded regarding a trough for [the washing of] feet, [capable of containing] between two <i>log</i> [a specific measure of volume] to nine <i>kav</i> [a specific measure of volume, equivalent to four <i>log</i>, that if it became cracked it may [still] be rendered impure with <i>midras</i> impurity [a type of impurity due to being sat on by certain types of impure individuals, rendering something an origin of impurity]. For Rabbi Akiva [contrary to the conclusion reached that day] says: a trough for feet is as its name indicates [i.e. even though a cracked trough could technically be used to sit upon, its utility with regard to whether or not in can be rendered impure is determined by whether it can still be used for washing one's feet].