140 Dr. Scbunck on the Colouring Matters of Madder. 



acin in combination with baryta. This is separated by filtration, and 

 washed with cold water until the sulphuret of barium and hyposul- 

 phite of baryta are removed, and then decomposed with muriatic 

 acid, which leaves the rubiacin behind. After being washed with 

 water, it is dissolved in boiling alcohol, from which it crystallizes on 

 the solution cooling. It is obtained by these means in beautiful 

 yellow crystalline plates and needles, having a strong lustre. The 

 colour is a little darker than that of neutral chromate of potash, and 

 inclines more to green than to red. If there is a tinge of red in it, 

 it is impure, and must be recrystallized. It has the following pro- 

 perties : — 



It is a little soluble in boiling water, to which it communicates a 

 reddish-yellow tinge. The solution deposits it on cooling in shining 

 yellow scales. The alcoholic solution has a pure yellow colour, 

 without a tinge of red. It is precipitated from its alcoholic solution 

 by water in yellow flocks. When heated on platinum foil it melts, 

 and burns with a smoky flame, leaving no fixed residue. When 

 heated in a tube closed at one end, it melts and gives yellow fumes, 

 which condense in the colder parts of the tube in yellow crystals. 

 If it be heated gradually at a moderate temperature, it may be almost 

 entirely volatilized, leaving only a film of charcoal. Concentrated 

 sulphuric acid dissolves it with a yellow colour, and water precipitates 

 it again in yellow flocks. The solution in sulphuric acid may be 

 heated to the boiling-point without becoming in the least degree 

 black, and water still precipitates it. Concentrated nitric acid slowly 

 decomposes it on boiling with an evolution of nitrous acid. Dilute 

 nitric acid dissolves it with a yellow colour on boiling, and on the 

 solution cooling, it crystallizes out again in shining needles. It 

 appears therefore that nitric acid does not convert it into rubiacic 

 acid. Boiling acetic acid dissolves it and deposits it again on cooling 

 in yellow flocks. It is not affected by chlorine. It dissolves in a 

 concentrated solution of perchloride or pernitrate of iron, forming 

 a dark brownish-red solution, which on the addition of acid becomes 

 yellow, and deposits yellow flocks. These flocks are no longer un- 

 changed rubiacin, but rubiacic acid. It is a remarkable circum- 

 stance, that though rubiacic acid differs from rubiacin merely in 

 containing a few atoms more oxygen, it is not formed from the 

 latter by the action of nitric acid either dilute or concentrated, but 

 is easily produced by the agency of the persalts of iron, which seems 

 to indicate that something besides the influence of oxygen is brought 

 into play during the process. Rubiacin is soluble in carbonate of 

 potash with a blood-red colour ; the solution deposits after some time 

 red flocks. It dissolves in caustic alkalies with a reddish-purple colour, 

 and is reprecipitated by acids in yellow flocks. The solution in am- 

 monia gives with the chlorides of calcium and barium dirty red pre- 

 cipitates. The alcoholic solution gives with an alcoholic solution of 

 sugar of lead a dark red precipitate. If into the boiling alcoholic 

 solution hydrate of alumina be introduced, the alumina acquires an 

 orange colour, and the fluid is deprived of all its rubiacin. The com- 

 pound with alumina thus formed dissolves entirely in caustic potash, 



