Heptazine, or tri-s-triazine or cyamelurine, is a chemical compound with formula C6N7H3, that consist of a planar triangular core group or three fused triazine rings, with three hydrogen atoms at the corners.

[3] Berzelius discovered in 1815 that the ignition of mercuric thiocyanate Hg(SCN)2 yielded, besides cinnabar HgS, carbon disulfide, and nitrogen, a yellow insoluble residue.

[4] In the same year, Wöhler described the characteristic twisted yellow column that forms when the salt is ignited and burns (which is the popular "Pharaoh's serpent" school chemistry demo).

[4] This foam has recently been found to consist of highly and randomly folded sheets of a material with composition close to C3N4.

The sheets are very thin (less than 20 nm) and have been conjectured to consist of heptazine or tri-s-triazine cores linked at their corners by nitrogen atoms.

[7] In the following years, Leopold Gmelin and Wilhelm Henneberg prepared novel salts that were eventually recognized as related to the compounds described by Liebig, and named melonates and cyamelurates.

It is suggested that Pauling failed to solve the double helix structure of DNA before Watson and Crick because he viewed uracil as an amide and not as the tautomeric hydroxy compound.