Mr. J. Higgin on the Colouring Matters of Madder. 283 



and then filtered; by this means the traces of rubiacine and 

 alizarine are removed, and the xanthine left, if not too much y 

 alumina has been used. To the filtrate is added barytic water , { t 

 to throw down sulphuric and phosphoric acids; and after • 'Vvt 

 the separation of the baryta salts, sufficient subacetate of lead 

 to throw down all the xanthine : some chloride and an or- 

 ganic salt are precipitated at the same time. The red lake is 

 washed with a little cold water to remove some chloride of 

 lead, sugar, &c. ; it is then diffused in water and sulphuretted 

 hydrogen passed through; the xanthine remains attached to 

 the sulphuret of lead. This is thrown on a filter, and washed 

 with cold water to remove the organic acid ; it is then repeat- 

 edly boiled in water and filtered. Xanthine dissolves with 

 a rich yellow colour. The solution is evaporated to a syrup 

 in a water-bath, and, if necessary, neutralized with baryta, the 

 evaporation completed, and the dry mass treated with absolute 

 alcohol. Pure xanthine dissolves, and may be again evapo- 

 rated to dryness. 



As thus obtained it is a dark brown, gummy and deliques- 

 cent mass, perfectly soluble in water; the solution is a beau- 

 tiful yellow when sufficiently dUuie. It has a peculiar dis- 

 agreeable bitter taste, without astringency or sweetness; it is 

 very soluble in alcohol, and but sparingly in aether; soluble 

 in alkalies, with a purplish red colour. Alum throws down 

 from this a dull red lake, which may also be produced by 

 adding a large quantity of hydrate of alumina to the watery 

 solution. Acids make the solution in water lighter in colour, 

 but do not cause any precipitate. When ihe solution in water 

 is boiled with a little sulphuric or muriatic acid, a green powder 

 falls; this is characteristic of xanthine. When the solution is 

 strong and rendered very acid with either of those acids, it 

 slowly becomes green in the cold. When dry, xanthine dis- 

 solves in concentrated sulphuric acid with a fine orange colour ; 

 when heated, this becomes more crimson ; and on the addition 

 of water all the colouring matter is deposited in yellow flocks, 

 which dissolve easily in ammonia, and give a beautiful crimson 

 colour, much more brilliant than that produced by ammonia 

 with the original xanthine. The flocks, as far as I have ex- 

 amined them, have the properties of rubiacine. The solution 

 in sulphuric acid, heated an hour or so, becomes brown; and 

 on addition of water a brown powder falls, which does not 

 dissolve in ammonia and is not coloured thereby. Xanthine 

 is not precipitated by neutral acetate of lead, but entirely by 

 subacetate; the compound is sparingly soluble in cold water, 

 more so in hot, and very soluble in acetic acid. When heated, 

 xanthine fuses, blackens and swells up into a bulky coal, which 



U2 



