When cyanogen is prepared by heating mercuric cyanide, a residue known as para-cyanogen, (CN)x, is left; this is to be regarded as a polymer of cyanogen.
The next salt to be obtained was the mercuric salt, which was prepared in 1 799 by Edward Charles Howard, who substituted mercury for silver in Brugnatelli's process.
The use of mercuric fulminate as a detonator dates from about 1814, when the explosive cap was invented.
The now well-known fact that small doses of poisonous substances may act as stimuli to living protoplasm, and that respiratory activity and growth may be accelerated by chloroform, ether and even powerful mineral poisons, such as mercuric chloride, in minimal doses, offers some explanation of these phenomena of hypertrophy, wound fever, and other responses to the presence of irritating agents.
It is also prepared by digesting precipitated mercuric sulphide with an alkaline sulphide fox some hours; it is said that Chinese vermilion owes its superiority to being made in this way.