1. Beis Hillel originally disagreed with the 18 decrees (13b).
2. The Gemara relates that Shamai and Hillel argued only about three issues.
3. There were various decrees enacted regarding the Tum'ah caused by the land outside of Eretz Yisrael.
4. The Gemara discusses why it was decreed that a glass container can become Tamei.
5. If a glass vessel has a hole in it, it becomes Tahor.
A BIT MORE
1. The next day, however, Beis Hillel agreed that these decrees were correct.
2. The Gemara clarifies that this means there were only three arguments that originated between Shamai and Hillel where neither one conceded that the other was correct. The other arguments found throughout the Mishnayos involve their students (Beis Hillel and Beis Shamai).
3. The Gemara explains that different decrees were made throughout the years before the destruction of the second Beis ha'Mikdash, leading to the final decree in Usha that if Terumah from Eretz Yisrael touches land outside of Eretz Yisrael, it must be burned.
4. The Rabanan decreed that glass can become Tamei, since it is made from sand and is therefore similar to earthenware which can become Tamei.
5. Since it becomes Tamei like an earthenware vessel, it also has the law that it becomes Tahor like an earthenware vessel, through being broken.