CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 287 



The reduction of chromic oxide for instance, in a solution by zinc 

 and sulphuric acid leads to the formation of chromous oxide, CrO, and 



soluble hydrate of chromic chloride (hydrochloride of oxide of chromium), and the 

 original anhydrous chromic chloride will pass into chromous chloride. The chromous 

 chloride re-formed in this manner will then act on a fresh quantity of the chromio 

 chloride, and in this manner transfer it entirely into solution as hydrate. This view is 

 confirmed by the fact that other chlorides, capable of absorbing chlorine like chromous 

 chloride, also induce the -solution of the insoluble chromic chloride for example, ferrous 

 chloride, FeCl 2 , and cuprotfs chloride. The presence of zinc also aids the solution of 

 chromic chloride, owing to its converting a portion of it into chromous chloride^ The 

 solution of chromic chloride in water obtained by these methods is perfectly identical 

 with that which is formed by dissolving chromic hydroxide in hydrochloric acid. On 

 evaporating the green solution obtained in this manner, it gives a green mass, con- 

 taining water... On further heating it leaves a soluble chromic oxychloride, and when 

 ignited it first forms an insoluble oxychloride and then chromic oxide ; but no anhy- 

 drous chromic chloride, Cr 2 Cl 6 , is formed by heating the aqueous solution of chromio 

 chloride, which forms an importa.nt fact in support of the view that the green solu- 

 tion of chromic chloride is nothing else but hydrochloride of oxide of chromium. At 

 100 the composition of the green hydrate is Cr 2 Cl 6) 9H 2 O, and on evaporation at the 

 ordinary temperature over H 2 S0 4 crystals are obtained with 12 equivalents of water ; 

 the red mass obtained at 120 contains Cr 2 O 3 ,4Cr 2 Cl 6 ,24H 2 O. The greater portion of 

 it is soluble in water, like the mass which is formed at 150 The latter contains 

 Cr 2 O 3 ,2Cr 2 Cl 6 ,9H 2 O = 8(Cr 2 OCl 4 ,3H 2 O) that is, it presents the same composition as 

 chromic chloride in which one atom of oxygen replaces two of chlorine. And if the 

 hydrate of chromic chloride be regarded as Cr 2 O 3 ,6HCl, the substance which is ob- 

 tained should be regarded as Cr 3 3 ,4HCl combined with water, H 2 0. The addition 

 of alkalis for example, baryta to a solution of chromic chloride immediately produces 

 a precipitate, which, however, re-dissolves on shaking, owing to the formation of one of 

 the oxychlorides just mentioned, which may be regarded as basic salts. Thus we may 

 represent the product of the change produced on chromic chloride under the influence 

 of water and heat by the following formulae . first Cr 2 O 3 ,6HCl or Cr-jCl^SHoO is formed, 

 then Cr 2 O 3 ,4HCl,H 2 O or Cr 2 OCl 4 ,3H 2 O, and lastly Cr 2 O 3 ,2HCl,2H 2 O or Cr 2 O 2 Cl 2 ,3H 2 0. 

 In all three cases there are 2 equivalents of chromium to at least 3 equivalents of 

 water. These compounds may be regarded as being intermediate between chromio 

 hydroxide and chloride ; chromic chloride is Cr-jCl^, the first oxychloride Cr 2 (OH) 2 Clj, 

 the second Cr 2 (OH) 4 Cl 2 , and the hydrate Cr 2 (OH) 6 that is, the chlorine is replaced by 

 hydroxyl. 



It is very important to remark two circumstances in respect to this . (1) That the 

 whole of the chlorine in the above compounds is not precipitated from their solutions 

 by silver nitrate ; thus the normal salt of the composition Cr 2 Cl 6 ,9H 2 O only gives up 

 two-thirds of its chlorine , therefore Peligot supposes that the normal salt contains the 

 oxychloride combined with hydrochloric acid: Cr 2 Cl 6 + 2H 2 O = Cr 2 O 2 Cl 2 ,4HCl, and that 

 the chlorine held as hydrochloric acid reacts with the silver, whilst that held in the 

 oxychloride does not enter into reaction, just as we observe a very feebly-developed 

 faculty for reaction in the anhydrous chromic chloride; and (2) if the green aqueous 

 solution of CrCl 3 be left to stand for some time, it ultimately turns violet ; in this form 

 the whole of the chlorine is precipitated by AgN0 3 , whilst boiling re-converts it into the 

 green variety. Lbwel obtained the violet solution of hydrochloride of chromic oxide by 

 decomposing the violet chromic sulphate with barium chloride. Silver nitrate precipi- 

 tates all the chlorine from this violet modification ; but if the violet solution be boiled 

 and so converted into the green modification, silver nitrate then only precipitates a portion 

 of the chlorine. 



Recoura (1890-1893) obtained a crystallohydrate of violet chromium sulphate, 

 Cr 2 (S04) 3 , with 18 or 15 H 2 By boiling a solution of this crystallohydrate, he 



