90 Mr. Crura on Indigo. [Feb, 



solution. The soda compound is evidently more soluble than 

 that of potash. The washings of the former precipitate, with a 

 cold solution of its acetate, are a little more coloured than those 

 of the potash precipitate, with its acetate of the same strength. 

 This last substance is totally insoluble in water containing one 

 per cent, of acetate of potash, or even a half per cent, after two 

 or three edulcorations. 1 have generally employed a solution of 

 two parts dry acetate of soda in a hundred of water ; and any 

 quantity of such a liquid may be used without the quantity of 

 the substance being materially diminished. 



The salts of ammonia likewise form precipitates in the sul- 

 phuric solution of cerulin, when not much diluted. The preci- 

 pitate dissolves readily in hot solutions of ammoniacal salts, and 

 again separates when cold, the whole mass becoming curdy. I 

 believe this substance to be a combination of cerulin with 

 sulphate of ammonia, from the quantity of that salt which I 

 found in it, when as well washed as possible. It is much more 

 soluble, however, than the ceruleo-sulphates of potash and soda, 

 and, therefore, cannot be washed so freely as these fixed alka- 

 line compounds. Potash and soda, and their salts, decompose 

 it. It dissolves in great quantity in boiling pure water, and in 

 40 or 50 parts of cold water. It has the same general proper- 

 ties with the more insoluble compounds. 



A corresponding combination with barytes may be formed by 

 decomposing ceruleo-sulphate of potash by muriate of barytes. 

 The compound formed is extremely insoluble. An abundant 

 blue precipitate is thus formed in solutions of ceruleo-sulphate 

 of potash, containing so little sulphuric acid, that they are not 

 troubled in the slightest degree by a barytic salt, when the 

 cerulin has been previously destroyed by nitric acid. 



Similar compounds may be formed with other bases, whose 

 sulphates are difficultly soluble in water ; but these I have not 

 particularly examined. 



The salts of magnesia have no power of precipitating cerulin 

 from its solution. The whole of the sulphuric acid in the original 

 solution may be saturated with magnesia, without any precipitate 

 being formed. 



Ceruleo-sulphate of Potash. 



This substance is of so deep a blue, when wet with water, as 

 to appear absolutely black. When dry, it has a shining strong 

 copper-red colour. By transmitted light it is blue. It attracts 

 water from the air with great rapidity. In two hours, a portion 

 which had been dried attracted a tenth of its weight. 



It is soluble to a considerable extent in hot water. Cold water 



takes up rr^th of its weight of this substance, and forms a solu- 

 1 140 ° ' 



tion so deeply coloured, that when diluted with 20 parts of water 



in a phial of an inch in diameter, it may just be seen to be 



