THE chemistry of the sulphur nitrides began a long time ago. As early as 1835 Gregory discovered a sulphur nitride formed by reaction of sulphur monochloride (S2C12) and ammonia. Today inany sulphur-nitrogen compounds are known which are derived from this nitride, and their chemistry has been thoroughly investigated. It shows hardly any analogy with that of the nitrogen-oxygen compounds. It is governed by the great stability of the linkage between nitrogen and sulphur, by a tendency to formation of negatively charged ions, and by the ease of polymerisation which can lead to large molecules containing long-chain or six-membered or eight-membered ring systems.Tetrasulphur Tetranitride, S,N,.-In 1835 it was found that reaction between sulphur monochloride and ammonia gave, besides other substances, a compound which contained only sulphur and nitrogen ; 1s but it was not until 1850 that the exact composition of this compound was worked out, and not until 1896 that a determination of the molecular weight gave the formula S,N4.39 4, 5 Tetrasulphur tetranitride, S,N,, is solid a t room temperature. It forms orange-yellow crystals of the class C, , and space group P2,/a6 It is diamagnetic and strongly endothermic. The melting 10s 4-4NH3 + 6HZS + S,N, .-' M .