"But the wonderful spirit of it seized upon him--it was the Marseillaise!" What was the Marseillaise? Why is it referred to in this episode?

La Marseillaise is the French national anthem. It was composed in one night during the French Revolution (April 24, 1792). It is significant because it became the anthem of the common people who rose up against the elite and the rich who were mistreating them. Here the people are inspired to fight a similar struggle against the owners of the factories and plants.