IRON, COBALT, AND NICKEL 365 



=Co 2 3 4- 2BaS0 4 -f HC1 4- 2C0 2 . Under these circumstances nickelous- 

 oxide does not immediately form black sesquioxide, but after a consider- 

 able space of time it also separates in the form of sesquidxide, Ni 2 3 , 

 but always later than cobalt. This is due to the relative difficulty of 

 further oxidation of the nickelous oxide. It is, however, possible to- 

 oxidise it ; if, for instance, the hydroxide NiH 2 O 2 be shaken in water 

 and chlorine gas be passed through it, then nickel chloride will be. 

 formed, which is soluble in water, and insoluble nickelic oxide in the 

 form of a black precipitate: 3NiH 2 2 + Cl 2 =NiCl 2 + Ni 2 3 ,3H 8 O, 

 Nickelic oxide may also be obtained by adding sodium hypochlorite 

 mixed with alkali to a solution of a nickel sialt. Nickelic and cobaltio 

 hydrates are black. Nickelic oxide evolves oxygen with all acids, and 

 in consequence of this it is not separated as a precipitate in the presence 

 of acids ; thus it evolves chlorine with hydrochloric acid, exactly like 

 manganese dioxide. When nickelic oxide is dissolved in aqueous 

 ammonia it liberates nitrogen, and an ammoniacal solution of nickelous 

 oxide is formed. When heated, nickelic oxide loses oxygen, forming 



longer acts like water in a free state. We see the same phenomenon in many other 

 cases for example, the chlorine in chlorates no longer gives a precipitate of chloride of 

 silver with AgN0 3 . Thus, although the 'instance which is found in the difference 

 between the roseo- and purpureo-salts deserves to be fully studied on account of its sim- 

 plicity, still it is far from being exceptional, and we cannot expect it to be thoroughly 

 explained unless a mass of similar instances, which are exceedingly common among 

 chemical compounds, be conjointly explained. (Among the researches which add to 

 our knowledge respecting the complex ammoniacal compounds, I think it indispensable 

 to call the reader's attention to Prof. Kournakofi's dissertation ' On complex metallic- 

 bases,' 1893.) 



Kournakoff (1894) showed that the solubility of the luteo-salt, CoCl 3) 6NH 3 , at 

 = 4-80 (per 100 of water), at 20 = 7'7, that in passing into the roseo-salt, CoCl 3 H 2 05NH 3 , 

 the solubility rises considerably, and at = 16'4, and. at 20= about 27,- whilst the 

 passage into the purpureo-salt, CoCl 3 ,5NH 3 , is accompanied by a great fall in the 

 solubility, namely, at = 0'28, and at 20 = about 0'5. And as crystallohydrates with a- 

 smaller amount of water are usually more soluble than the higher crystallohydrates (Le 

 Chatelier)/ whilst here we find that the solubility falls (in the purpureo-salt) wkh a loss 

 of water, that water which is contained in the roseo-salt cannot be compare^ with the 

 water of crystallisation. Kournakoff, therefore, connects the fall in solubility (in the 

 passage of the roseo- into the purpureo-salts) with the accompanying loss in the reactive 

 Capacity of the chlorine. 



In conclusion, it may be observed that the elements of the eighth group that is, the 

 analogues of iron and platinum according to my opinion, will yield most fruitful results 

 when studied as to combinations with whole molecules, as already shown 1 by the examples 

 of complex ammoniacal, cyanogen, nitro-, and other compounds, which are easily formed 

 In this eighth group, and are remarkable for their stability. This faculty of the elements 

 of the eighth group for forming the complex compounds alluded to, is in all probability 

 connected with the position which the eighth group occupies with regard to the others. 

 Following the seventh, whi'ch forms the type RX 7 , it might be expected to contain the 

 most complex type, RX 8 . This is met with in OsO 4 . The other elements of the eighth 

 group, however, only form the lower types RX 2 , RX 3 , RX 4 .... and these accordingly 

 should be expected to aggregate themselves into the higher types, which is accom- 

 plished in the formation of the above-mentioned complex compounds. 



