Chapter 13 of El Filibusterismo is titled “The Class in Physics.” The chapter begins with a description of a physics class being taught by Padre Millon, a Spanish friar. The students in the class are all Filipino, and they are struggling to understand the concepts being taught.

The main character, Simoun, is also in the class. He is a wealthy and influential man who is secretly plotting a revolution against the Spanish government. Simoun is frustrated with the slow progress of the revolution and is looking for ways to speed things up.

During the class, Padre Millon demonstrates an experiment involving a pendulum. Simoun uses this opportunity to plant a bomb in the classroom, which he plans to detonate during a later event.

After the class, Simoun meets with his friend, Basilio, who is also involved in the revolution. They discuss their plans and Simoun reveals his intention to use the bomb to create chaos and confusion, which he hopes will lead to a successful revolution.

The chapter ends with Simoun leaving the school and heading to a nearby church, where he plans to meet with other revolutionaries and continue his plotting.