Didn't know they used tracer bullets in ww1.
Information re Tracer Bullets: Fighting at night was especially unproductive because there was no way to see where you were shooting.
Tracer bullets are rounds which emitted small amounts of flammable material that left a phosphorescent trail. The friction of air molecules against the fast travelling bullet was enough to ignite the pyrotechnic coating. The first attempt, in 1915, was not actually that useful, as the trail was “erratic” and limited to 100 meters, but the second tracer model developed in 1916 which emitted a regular bright green-white trail which was successful. Tracer rounds were mainly used for against aerial attacks so the round could be spotted hitting the plane. Link: http://msmhsmaritimehistory.weebly.com/tracer-bullets.html
Also. Tracer ammunition (tracers) are bullets or cannon calibre projectiles that are built with a small pyrotechnic charge in their base. Ignited by the burning powder, the pyrotechnic composition burns very brightly, making the projectile trajectory visible to the naked eye and very bright during night time firing. Link: https://www.google.co.uk/search?q=Tracer+Bullets+ww1&rlz=1C1DSGL_enGB426GB426&oq=Tracer+Bullets+ww1&aqs=chrome..69i57j0.10814j0j8&sourceid=chrome&ie=UTF-8
I read in another article that The flame could ignite hydrogen which made it perfect for taking down German Zeppelins.