286 Hartley — The Action of Heat on the Absorption Spectra and 



would be expected of it if the green colour be caused by simple loss of water. 

 These facts, taken together, suggest that the green solutions of chromium differ 

 from the violet in the following manner : — 



Salts of Chromium, Cr., Violet. Salts of Chromyl,* CrjO, Green. 



Chloride, . . . CrjCle'SHoO. Chloride, . . . Cr,0-Cl4.2HjO. 



Sulphate, . . . Cr,3S04-15HjO. Sulphate, . . . Cr20-2S04-.rH20. 



Nitrate, . . . CrjeNOj-gH^O. Nitrate, . . . Cr30-4N03-HjO. 



The relation of the green to tlie violet compounds is similar to that existing 

 between the salts of uranium and uranyl : as, for instance, the chloride — 



Chloride of Uranium. Chloride of Uranyl. 



TIrCL. UrOCl. 



If one-third the sulpliuric acid in chromic sulphate is neutralised by sodium 

 carbonate, experiment has shown that, though boiling does not precipitate a 

 moderately strong solution, yet ebullition, with about 3000 times its bulk of water, 

 causes a precipitate. This is caused by the formation, first, of chromyl sulphate, 

 which is afterwards decomposed by water at 100"^ C. to produce the insoluble basic 

 salt. 



The view 1 have advanced of the constitution of the chromium salts was first 

 published in the Proceedings of the Royal Institution of Great Britain for 1875, 

 being contained in a report of a Friday evening lecture " On the Action of 

 Heat on Coloured Liquids."! The experimental evidence was not comnmnicated, 

 but only an outline of the conclusions. 



It has been confirmed of late years by the thermo-chemical researches of 

 Recoura.J He found that the sulphate CroOs'SSOs was split into two molecules 

 represented by 2Cr203"5S02 and H2SO4. When the sulphate was decomposed 

 by alkali, a new hydroxide was formed which combines with only 2H2SO4 ; 

 therefore CraOs'SOa, a green sulphate, was obtained, Cr203"3S03'IlH20 in the 

 solid state, which is easily converted by water into the violet modification. It was 

 also shown that, if the compound CrjOs'SSOa is mixed with a sulphate, such as 

 CUSO4, the two instantly combine to form a chromo-sulphate with the formula 

 (Cr24S04)Cu ; and that chrome alum, when 25fii'tially dehydrated, becomes 

 potassium chromosulphate (Cr2'lS04)K2"4H20. 



Cross and Higgin§ also have shown the existence of chromo-sulphates. 



* This was the name originally proposed in the paper from which this is an extract, but the compound 

 CrOaClj is now called chromyl chloride. 



f See also Proc. Eoy. Soc, vol. 33, p. 372, 1875. Abstract, and also Chemical News, vol. 65, p. 15, 

 1892. 



\ Comptes Rendus, vol. 100, p. 1227 ; vol. 101, p. 435 ; vol. 102, p. 515, also pp. 548, 865, and 921 ; 

 vol. 110, pp. 1029, 1193; vol. 112, p. 1439; vol. 113, pp. 857, 1037; vol. 114, p. 477. 



§ Jour. Chem, Soc. Trans., vol. 41, p. 113, 1882. 



