1823.] Mr. Or urn on Indigo. 89 



It seemed then only necessary to dissolve in water some salt 

 of potash, to enable it to wash this substance without dissolving 

 it. I found the acetate to answer extremely well, and this salt 

 possesses the advantage over the muriate or sulphate of not 

 being precipitated by alcohol from a weak solution in water, as 

 they are. It may consequently be afterwards removed by wash- 

 ings with alcohol. 



Such edulcorations with acetate of potash, I repeated so often 

 upon a quantity of precipitate, taking it every time off' the filter, 



and agitating it well in a phial with the liquid, that not r^rJh 



of a grain of the original soluble matter could be left in it. I 

 then washed away the weak solution of acetate of potash as well 

 as possible by means of alcohol, without, however, being able to 

 remove all traces of that salt, although I used the alcohol some- 

 what dilute. The small quantity that remained could not mate- 

 rially affect the experiments 1 made upon it. 



A portion of the substance thus prepared, when burnt in a large 

 platina crucible, left a considerable quantity of ashes, slightly 

 reddish coloured, which dissolved almost entirely in water. 

 What remained was of a deep red, or rather brown colour, and 

 was principally oxide of iron. The solution of the ashes was 

 not at all alkaline ; it gave a dense white precipitate with 

 muriate of barytes ; a very slight one with oxalate of ammonia ; 

 and with sulphate of alumina, large crystals of alum were 

 formed in a few hours. It was sulphate of potash. 



To remove all doubt of the precipitate having been sufficiently 

 well washed to take away from it every thing that was not really 

 insoluble in the menstrua I employed, and consequently the 

 sulphate of potash, if that substance were not chemically com- 

 bined with the cerulin, I mixed a large quantity of muriatic acid 

 with the original solution in sulphuric acid, and formed the pre 

 cipitate by means of muriate of potash. Then, having washed it 

 as before, with the solution of acetate of potash, and afterwards 

 with alcohol, I found that the solution of the ashes when the 

 substance was burnt, gave indications with muriate of barytes, 

 of abundance of sulphuric acid ; while with nitrate of silver, no 

 precipitate appeared ; a quantity of small shining crystals alone 

 being deposited after some time, which were sulphate of silver. 



From these facts, I have no hesitation in concluding that this 

 precipitate is a combination of cerulin with sulphate of potash. 

 That salt forms more than a fourth of its weight. It may, 

 therefore, be called ceritleo-su/phate of potash. 



The salts of soda also form precipitates in the solution of 

 cerulin with sulphuric acid, and these are likewise insoluble in 

 solutions of potash or soda, though soluble to a certain extent in 

 pure water. When heated, these ceruleo-sulphates dissolve 

 even in solutions of their salts. On cooling, the greater part falls 

 down again in blackish grains; a portion, however, remaining in 



