288 PRINCIPLES OF CHEMISTRY 



its salts, CrX 2 , of a blue colour (see Notes 7 and 7 b!s ). The further 



converted it into the green salt, which, when treated with alkalis, gave a precipitate 

 of Cr 2 O 3 ,2H 2 O, soluble in 2H 2 SO 4 (and not 3), and only forming the basic salt, 

 Cra(OH) 2 (SO 4 )2. He therefore concludes that the green salts are basic salts. The 

 cryoscopic determinations made by A. Speransky (1892) and Marchetti (1892) give a 

 greater 'depression' for the violet than the green salts, that is, indicate a greater 

 molecular weight for the green salts. But as Etard, by heating the violet sulphate to 

 100, converted it into a green salt of the same composition, but with a smaller amount 

 of H 2 O, it follows that the formation of a basic salt alone is insufficient to explain the 

 difference between the green and violet varieties, and this is also shown by the fact that 

 Bad, precipitates the whole of the sulphuric acid of the violet salt, and only a portion 

 of that of the green salt. A. Speransky also showed that the molecular electro- 

 conductivity of the green solutions is less than that of the violet. It is also known that 

 the passage of the former into the latter is accompanied by an increase of volume, and, 

 according to Recoura, by an evolution of heat also. 



Piccini's researches (1894) throw an important light upon the peculiarities of the 

 green chromium trichloride (or chromic chloride) ; he showed (1) that AgF (in contra- 

 distinction to the other salts of silver) precipitates all the chlorine from an aqueous 

 solution of the green variety ; (2) that solutions of green CrCl 3 ,6H 2 O in ethyl alcohol 

 and acetone precipitate' all their chlorine when mixed with a similar solution of AgNO 5 ; 

 (3) that the rise of the boiling-point of the ethyl alcohol and acetone green solutions of 

 CrClsjGHoO (Chapter VII., Note 27 bis) shows that i in this case (as in the aqueous solu- 

 tions of MgSO 4 and HgCl 2 ) is nearly equal to 1, that is, that they are like solutions of 

 non-conductors ; (4) that a solution of green CrCl 3 in methyl alcohol at first precipitates 

 about J of its chlorine (an aqueous solution about ) when treated with AgNO 3 , but 

 after a time the whole of the chlorine is precipitated ; and (5) that an aqueous solution 

 of the green variety gradually passes into the violet, while a methyl alcoholic solution 

 preserves its green colour, both of itself and also after the whole of the chlorine has 

 been precipitated by AgNO 5 . If, .however, in an aqueous or methyl alcholic solution 

 only a portion of the chlorine be precipitated, the solution gradually turns violet. 

 In my opinion the general meaning of all these observations requires further elucidation 

 and explanation, which should be in harmony with the theory of solutions. Recoura, 

 moreover, obtained compounds of the green salt, Cr 2 (SO 4 ) 3 , with 1, 2, and 3 molecules of 

 'H 2 SO 4 , K 2 SO 4 , and even a compound Cr 2 (SO 4 ) 3 H 2 CrO 4 . By neutralising the sulphuric 

 acid of the compounds of Cr 2 (SO 4 ) 3 and H 2 SO 4 with caustic soda, Recoura obtained an 

 evolution of 83 thousand calories per each 2NaHO, while free H 2 SO 4 only gives 80'8 

 thousand calories. Recoura is of opinion that special chromo sulphuric acids, for 

 instance (CrSO 4 )H 2 3O 4 = $Cr 2 (SO 4 ) 3 H 2 SO 4 , are formed. With a still larger excess of 

 sulphuric acid, Recoura 6btained salts containing a still greater number of sulphuric 

 acid radicles, but even this method does not explain the difference between the green and 

 violet salts. 



These facts must naturally be taken into consideration in order to arrive at 

 any complete decision as to the cause of the different modifications of the chromic salts. 

 We may observe that the green modification of chromic chloride does not give double 

 salts with the metallic chlorides, whilst the violet variety forms compounds Cr 2 Cl 6 ,2RCl 

 (where R = an alkali metal), which are obtained by heating the chromates with an excess 

 of hydrochloric acid and evaporating the solution until it acquires a violet colour. As 

 the result of all the existing researches On the green and violet chromic salts, it appears 

 to me most probable that their difference is determined by the feeble basic character of 

 chromic oxide, by its faculty of giving basic salts, and by the colloidal properties of its 

 hydroxide (these three properties are mutually connected), and moreover, it seems to me 

 that the relation between the green and violet salts of chromic oxide best answers to the 

 relation of the purpureo to the luteo cobaltic salts (Chapter XXIL, Note 85). This 

 subject cannot yet be considered as exhausted (see Note 7). 



We may here observe that with tin the chromic salts, CrX 3 , give at low temperatures 



