IRON. COBALT. AKD NICKEL 351 



tion, being formed accordirig to the equation: 3FeCl 2 +2K 3 FeC 6 N ( } 

 .=6KCl+Fe 3 Fe 2 a i2 N l2 , or 3FeC 2 N 2 ,Fe 2 6 N 6 ; in Prussian blue we 

 have Fe 7 Cy 18 , and here Fe 5 Cy, 2 . A farric salt ought to form, ferric 

 ^cyanide Fe 2 C 6 N 6 , with red priissiate, but ferric cyanide is solublei 

 and therefore no precipitate is obtained, and the liquid only becomes 

 brown. 31 



If chlorine ancl sodium are representatives of independent groups 

 of elements, the same may also be said of! iron. Its nearest ana- 

 logues show, besides a similarity in character, a likeness as regards 

 physical properties and a proximity in atomic weight. Iron, occupies a, 

 medium position amongst its nearest analogues, both with respect to 

 properties and faculty of forming saline oxides, and also as regards 

 -atomic weight. On the one hand, cobalt, 58, and 'nickel, 59, approach 



S! It is important to mention a series of readily e.rystallisable salts formed by the- 

 action of nitric acid on potassium and other ferrocyanides and ferricyanides. Thes* 

 salt contain the elements of nitric oxide, and are therefore called nitrO'(nitro9o} 

 ferricyanides (nitroprussides). Generally a crystalline sodium salt is obtained 

 Na 2 FeC 5 N 6 O,2H 3 O. In its composition this salt differs from the red sodium salt, 

 Na 3 FeC 6 N 6 , by the fact that in it one molecule of sodium, cyanide, NaCN, is replaced by 

 nitric oxide, NO. In order to prepare it, potassium ferrocyanida in powder is mixed 

 with five-sevenths of its weight of nitric acid diluted with 'an equal volume of water. 

 The mixture is at first left at the ordinary temperature, and then heated x>n a. 

 water-bath. Here ferricyanide is first of all formed (as shown by the liquid giving a. 

 precipitate with ferrous chloride), which then disappears (no precipitate with ferrous, 

 chloride), and forms a green precipitate. The liquid, when cooled, deposits crystals. 

 of nitre. The liquid is then strained off and mixed with sodium carbonate, boiled*, 

 filtered, and evaporated ; sodium nitrate and the salt described are deposited in crystals. 

 It separates in prisms of a red colour. Alkalis and salts of the alkaline earths do not 

 give precipitates: they are soluble, but the salts of iron, zinc, copper, and silver form 

 precipitates where sodium is exchanged with these metals. It is remarkable that the- 

 sulphides of the alkali metals give with this salt an intense bright purple coloration. 

 This series of compounds was discovered by Gmelin and studied by Playfair and others. 

 (1849). 



This series to a certain extent resembles the nitro-sulphide series described by 

 Eoussin. Here the primary compound consists of black crystals, which are obtained aa 

 follows : Solutions of potassium hydrosulphide and nitrate are mixed, and the mixture 

 is agitated whilst ferric chloride is added, then boiled and filtered;- on cooling, black 

 Crystals aro deposited/having the composition Fe 6 S 3 (NO) 10 ,IL)O (Rosenberg), or, acfcord- 

 ing to Demel, FeNO^NHgS. They have a slightly metallic, lustre, and are soluble im 

 water, alcohol, and ether. They absorb the latter as easily as calcium chloride absorbs, 

 water. In the presence of alkalis these crystals remain' unchanged, but with acids they 

 evolve nitric oxides. There are several compounds which are capable of interchanging,, 

 and correspond with Roussin's salt. Here we enter into the series of the nitrogen 

 compounds which have been as yet but little investigated, and will most probably in 

 time form most instructive material for studying the nature of that element. These- 

 Beries of compounds are as unlike the usual saline compounds of inorganic chemistry as. 

 are organic hydrocarbons. There is no necessity to describe these series in detail, because-, 

 their connection, with othei compounds is not yet clear, and they have nob yet any 

 application. 



