Dr. Schunck on the Colouring Matters of Madder. HI 



forming a purple solution, a characteristic which distinguishes it from 

 alizarin, the alumina compound of which, as mentioned above, is 

 insoluble in caustic potash. 



The aqueous and alcoholic solutions of rubiacin communicate a 

 slight tinge to mordanted cloth, the latter more than the former. 

 It seems to be incapable itself of dyeing, but in conjunction with 

 alizarin it assists in the process of madder-dyeing in a very essential 

 manner, as I shall afterwards show. 



Rubiacic Acid. — I have before described the method of obtaining 

 this acid through the instrumentality of the persalts of iron. There 

 is no reason for supposing that it exists as such in the plant. It is 

 doubtless a product of the oxidation of rubiacin. To obtain it in a 

 state of purity, pure crystallized rubiacate of potash is dissolved in 

 water, and muriatic acid or any strong acid is added, which precipi- 

 tates the rubiacic acid in the shape of a yellow powder. It has a pure 

 lemon-yellow colour. It cannot be obtained in a crystalline form. 

 It is slightly soluble in boiling water, to which it imparts a yellow 

 colour. It is also slightly soluble in boiling alcohol with a yellow 

 colour. The solution deposits nothing on cooling, but water produces 

 in it an iridescent appearance, caused by the deposition of minute 

 crystals. When heated on platinum foil it melts and burns with a 

 bright flame, leaving no residue. When heated in a tube it melts, 

 and gives fumes which condense on the colder parts of the tube to 

 an oil which solidifies, but does not crystallize. Concentrated sul- 

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

 it again in yellow flocks. On heating the solution in sulphuric acid 

 it becomes darker, but no gas is given off; some decomposition seems 

 however to have taken place in consequence of the heating, for water 

 now produces no precipitate. Concentrated nitric acid dissolves it on 

 boiling, and slowly decomposes it with an evolution of nitrous acid. 

 It dissolves in a solution of perchloride of iron with a dark reddish- 

 brown colour, and is reprecipitated by acids in yellow flocks. It 

 is not affected by bichromate of potash and sulphuric acid even on 

 boiling. 



The potash salt is the only salt of this acid which I have as yet 

 been able to examine. It crystallizes from a hot concentrated solu- 

 tion in water in needles and prisms of alight brick-red colour. On 

 being heated it h Instantly decomposed, giving rise to a slight deto- 

 nation. The aqueous solution has a blood-red colour, but is quite 

 transparent. The addition to it of caustic potash causes it to assume 

 a beautiful dark purple colour, and a concentrated boiling solution 

 does not crystallize on cooling after the addition of caustic potash. 

 The watery solution gives with solutions of the earthy and metallic 

 salts the following reactions. With chloride of calcium it gives an 

 orange-coloured, crystalline precipitate; with chloride of barium a 

 yellow precipitate ; with sulphate of magnesia a yellow precipitate ; 

 with alum an orange-coloured precipitate ; with protosulphate of iron 

 a greenish-gray precipitate ; with perchloride of iron a slight reddish- 

 brown precipitate, which is soluble in the boiling fluid ; with sugar 

 of lead a red precipitate ; with sulphate of copper a dull red preci- 



