Dr. Schunck on the Colouring Matters of Madder. 137 



I shall now proceed to give a more detailed account of the pro- 

 perties of some of the substances, the methods of obtaining which I 

 have just described. 



Alizarin. — It is necessary to state that the substance which I 

 called alizarin in the paper read by me on this subject at the South- 

 ampton meeting of the British Association, I now call rubiacin. I 

 am of opinion that the substance discovered by Robiquet, and called 

 by him alizarin, does not agree in its properties with that which I 

 call rubiacin ; but after carefully comparing his account with the 

 results obtained by myself, I have come to the conclusion that the 

 substance to which I have applied the name of alizarin is identical 

 with his. 



Alizarin has the following properties : — When heated on platinum 

 foil it melts and burns with a bright flame. When heated in a glass 

 tube closed at one end it melts and gives yellow fumes, which con- 

 dense on the colder parts of the glass, forming an oil, which soon 

 congeals to a mass of orange-coloured crystals possessing consider- 

 able lustre, which are unchanged alizarin. A carbonaceous residue 

 is usually left in this case, but I have no doubt that by carefully 

 heating it might be entirely volatilized. Alizarin is slightly soluble 

 in boiling water. The solution has a yellow colour, but so small is 

 the quantity dissolved, that the alkali or earth usually contained in 

 filtering paper is sufficient to render the solution pink, on being fil- 

 tered to separate it from the excess of alizarin. Hence no doubt 

 arises the statement found in books, that alizarin dissolves in water 

 with a pink colour. The boiling solution deposits it on cooling in 

 yellow crystalline flocks. It is soluble in boiling alcohol. The 

 solution has a deep yellow colour and deposits nothing on cooling, 

 but on evaporation the alizarin is left in long needle-shaped or pris- 

 matic orange-coloured crystals possessing a considerable lustre. The 

 colour of the crystals very much resembles that of bichromate of 

 potash. Concentrated sulphuric acid dissolves alizarin in the cold 

 with a blood-red colour. It is precipitated from this solution by water 

 in flocks of a dull orange colour. Dilute nitric acid decomposes it 

 on boiling with an evolution of nitrous acid. If it is still mixed with 

 fat, then the fat remains behind after the alizarin has been decom- 

 posed and dissolved by the nitric acid. It is not affected by muriatic 

 or acetic acid. On passing chlorine into water in which alizarin is 

 suspended, the colour of the latter is changed to yellow, but it is 

 seemingly not destroyed. It is decomposed by bichromate of potash 

 and sulphuric acid. A boiling solution of perchloride or pernitrate 

 of iron decomposes it with a copious evolution of gas, a pungent smell 

 resembling that of aldehyde being at the same time given off. The 

 product of this decomposition I shall describe presently. Chloride 

 of gold is not reduced by it on boiling, but on the addition of cau- 

 stic potash metallic gold is deposited in shining scales. It is soluble 

 in caustic and carbonated alkalies with a splendid purple colour, and 

 is reprecipitated by acids in flocks of a dull orange colour. The 

 solution in ammonia gives with the chlorides of barium and calcium 

 precipitates of a splendid purple colour, with sugar of lead a light 

 purple precipitate. The compound with alumina, produced by in- 



