Complex sentence: This new gunpowder called "Poudre B" ("Powder B") was made of stabilized nitrocellulose and had just been invented and developed in 1884 by Paul Vieille (1854--1934).
Split into simpler sentences: This new gunpowder called "Poudre B" ("Powder B") was made of stabilized nitrocellulose. It had just been invented and developed in 1884 by Paul Vieille (1854--1934).