chlorides and iodides form similar compounds--for instance, the salt HgKI(CN)_{2} crystallises very well, and is produced by directly mixing solutions of potassium iodide and mercuric cyanide. Wells (1889) and Vare obtained and investigated many such double salts, and showed the possibility of the formation, not only of HgCl_{2}MCl and HgCl_{2}2MCl where M is a metal of the alkalis--for example, Cs--but also of HgCl_{2}3MCl,2(HgCl_{2})MCl, and in general _n_HgX_{2}_m_MX, where X stands for various haloids. Mercury has a remarkable power of forming very unstable compounds with ammonia, in which the mercury replaces the hydrogen, and, if a mercuric compound be taken, its atom occupies the place of two atoms of the hydrogen in the ammonia. Thus Plantamour and Hirtzel showed that precipitated mercuric oxide dried at a gentle heat, when continuously heated (up to 100°-150°) in a stream of dry ammonia, leaves a brown powder of _mercuric nitride_, N_{2}Hg_{3}, according to the equation 3HgO + 2NH_{3} = N_{2}Hg_{3} + 3H_{2}O.[24 bis] This substance, which is attacked by water, acids, and alkalis (giving a white powder), is very explosive when struck or rubbed, evolving nitrogen, proving that the bond between the mercury and the nitrogen is very feeble.[25] By the action of liquefied ammonia on yellow mercuric oxide Weitz also obtained an explosive compound, dimercurammonium hydroxide, N_{2}Hg_{4}O, which corresponds with an ammonium oxide, (NH_{4})_{2}O, in which the whole of the hydrogen is replaced by mercury. A solution of ammonia reacts with mercuric oxide, forming the hydroxide, NHg_{2}.OH, to which a whole series of salts, NHg_{2}X, correspond; these are generally insoluble in water and capable of decomposing with an explosion. But salts of the same type, but with one atom of mercury, NH_{2}HgX, are more frequently and more easily formed; they were principally studied by Kane, although known much earlier. Thus, if ammonia be added to a solution of corrosive sublimate (or, still better, in reverse order), a precipitate is obtained known as white precipitate (_Mercurius præcipitatus albus_) or _mercurammonium chloride_, NH_{2}HgCl, which may also be regarded not only as sal-ammoniac with the substitution of H_{2} by mercury, but also as HgX_{2}, where one X represents Cl and the other X represents the ammonia radicle, HgCl_{2} + 2NH_{3} = NH_{2}.HgCl + NH_{4}Cl. When heated, mercurammonium chloride decomposes, yielding mercurous chloride; when heated with dry hydrochloric acid it forms ammonium chloride and mercuric chloride. Other simple and double salts of mercurammonium, NH_{2}HgX, are also known. Pici (1890) showed that all the compounds HgH_{2}NX may be regarded as compounds of the above-named Hg_{2}NX with NH_{4}X because their sum equals 2HgH_{2}X.[25 bis]