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XIII. On two New Salis of Chromic Acid. 

 By Archibald Duncan, Jwn., Esq.^ 



IN 1827, Dr. Thomson described in his paper on the Com- 

 pounds of Chromium (Phil. Trans. 1827, p. 223), the 

 double salt — potash chromate of viagnesia — (KO CrOg, MgO 

 CrOy, 2HO) obtained by digesting a solution of bichromate 

 of potash over carbonate of magnesia. I obtained a corre- 

 sponding lime-salt about two years ago by the following pro- 

 cess. A boiling solution of bichromate of potash was poured 

 over newly-slaked lime in a tall vessel. The undissolved 

 lime having subsided, the supernatant fluid, which was of a 

 lemon-yellow colour, was drawn off by a syphon, and slowly 

 evaporated in a hot-air stove at 80°. During the first two 

 days of the evaporation, crystalline crusts of an orange salt 

 were formed on the surface of the liquor, and required to be 

 frequently removed. After this time, however, these crusts 

 ceased to be produced, and crystals of a yellow salt began to 

 make their appearance at the bottom of the evaporating bason, 

 and in two or three days more a mass of beautiful crystals was 

 obtained. The proportion of the orange to the yellow salt 

 depends a good deal on the temperature employed in the eva- 

 poration. In one experiment the heat was raised to boiling, 

 and no yellow crystals were obtained at all, orange crusts 

 continuing to separate as fast as they could be removed. 



Yellow Potash Chromate of Lime. — This salt crystallizes in 

 lemon-yellow four-sided oblique prisms. It is soluble in water, 

 but insoluble in cold alcohol, and is formed in the latter part 

 of the process described. 



The salt, when ignited, fuses; and on cooling, the mass has 

 a crystalline aspect, and is quite soluble in water. 



The mean of several analyses gave the following result: — 



Experiment. Calculation. 

 Chromic acid . . . 51*840 52-52 



Potash 23-900 24-24- 



Lime 14-950 14-14 



Water 9-600 9*10 



100-290 100-00 



This corresponds nearly with the formula KO CrOg, CaO 

 Cr03 + 2HO, the water being slightly in excess. It therefore 

 is a parallel compound to the magnesian salt described by 

 Dr. Thomson. 



Orange Potash Chromate of Lime. — The mode of formation 



• Communicated by the Author, having been read before the Philoso- 

 phical Society of Glasgow, Dec. 13, 1848. 



