A wad of oakum will give better results than the sponge.
Natural gas is produced as simply.

A T-shaped gas pipe has three or four gas tips on the cross-piece.

The long end is placed in the mouth, which already holds concealed a sponge, or preferably a ball of oakum, saturated with pure gasoline.

Blowing through the pipe will force the gas through the tips, where it can be ignited with a match.

It will burn as long as the breath lasts.
In a London periodical, The Terrific Record, appears a reprint from the Mercure de France, giving an account of experiments in Naples which led to the discovery of the means by which jugglers have appeared to be incombustible.