1. Rav Nachman differentiates between different types of washing pools for clothes.
2. Rabah bar bar Chanah: One may urinate next to his neighbor's wall.
3. The preceding law applies despite the fact that the Mishnah says that one must distance himself three Tefachim from his neighbor's wall.
4. The Gemara rejects Rabah bar bar Chanah's ruling.
5. Any object that can become Tamei does not protect another area from becoming Tamei.
A BIT MORE
1. He explains that the Mishnah -- which says that one must distance his washing pool three Tefachim from the wall of his neighbor's pit -- is referring to a pool where one soaks clothing (in order to clean them) and they become foul smelling. However, if he has a washing pool where he beats the clothes and water sprays very far, he must distance his pool four Amos from his neighbor's wall.
2. This is evident from the verse that says that Hashem will eliminate even a "Mashtin b'Kir" -- "one who urinates against the wall" from the house of Achav. This implies that it is normal to urinate against someone else's wall.
3. The Mishnah refers to spilling out a bucket of urine, which indeed damages a wall if it is done within three Tefachim of the wall. However, a person may urinate next to the wall.
4. The Tosefta explicitly says that one may not urinate next to his neighbor's wall. The Gemara explains that the verse regarding Achav was talking about a dog that often urinates against a wall.
5. Accordingly, when two houses are connected by a window and there is a dead person in one of the houses, and there are items that can become Tamei filling up that window, both houses are deemed Tamei. This is because the window connecting the two houses makes them as one. Only when there are items that cannot become Tamei filling the window does the Tum'ah not pass (for they effectively block the Tum'ah).