What purpose does mercury dichloride have in fireworks? My local newspaper ran an article covering the local rules about fireworks.  One of those rules specifically calls out "snakes and fountains containing mercury bichloride" as forbidden.

Neither Wikipedia's article on mercury(II) chloride nor its article on mercury lists any fireworks-related use.  What purpose does the mercury dichloride serve?  Is it just a way to color the sparks, or does it have some more fundamental purpose?
Most likely, mercury was used as a colorant, specifically red.  This list shows mercury providing a red color in flame tests, and the same would presumably be seen in fireworks.

Several other, less dangerous elements also give red or reddish colors.  This site identifies lithium as providing the color in red fireworks, and also mentions lithium carbonate specifically.

Be safe this 4th, handle fireworks with care if you do so at all!