Dr. Schunck on the Formation of Indigo-blue. 89 



silver is precipitated ; and when ammonia is added, a further 

 reduction takes place, accompanied by the formation of a metallic 

 mirror. On adding chloride of gold to the watery solution and 

 boiling, a quantity of metallic gold is deposited in bright scales 

 and spangles ; and on adding caustic alkali to the filtered solu- 

 tion, an additional quantity of gold is precipitated as a purple 

 powder. The watery solution gives no precipitates either with 

 neutral or basic acetate of lead, — only on adding ammonia does 

 any precipitation take place. It is soluble in alcohol, but not 

 in aether. In its outward properties, therefore, this sugar does 

 not diflPer in any marked degree from other kinds of sugar ob- 

 tained by the decomposition of complex organic bodies, such as 

 that derived from rubian. In its composition, however, it differs 

 essentially from other species of sugar, as I shall presently 

 show. 



I have hitherto been unable, I regret to say, to ascertain the 

 exact composition of indican by direct experiment. On account 

 of its deliquescent nature, and its so readily undergoing change 

 when heated, it was impossible to subject it to analysis in a free 

 state, and I was therefore obliged to have recourse to the lead 

 compound. But when this compound is precipitated from a 

 watery solution by means of acetate of lead and ammonia, it no 

 longer contains unchanged indican, but one of the bodies formed 

 by the combination of the latter with water. It is necessary, 

 however, to use water in some stage of the preparation ; for if 

 the lead compound be precipitated from an alcoholic extract of 

 woad with acetate of lead and ammonia, if the precipitate be 

 decomposed, after washing with alcohol, by suspending it in 

 alcohol and passing a stream of carbonic acid through the liquid, 

 and the substance be again precipitated from the filtered liquid 

 by means of acetate of lead and ammonia, the lead compound 

 thus formed will be found to contain, besides indican, a quantity 

 of fatty matter, from which the indican can only be separated by 

 means of water, and its analysis leads, as I have ascertained, to 

 no satisfactory results. It is therefore necessary to evaporate a 

 watery solution of indican spontaneously, to dissolve the residue 

 in alcohol, and precipitate with acetate of lead and ammonia, 

 taking care to leave a slight excess of indican in solution. The 

 following analysis was made with a specimen of the lead com- 

 pound prepared in this manner, the indican itself having been 

 obtained by the third method described above. 



I. 1"4340 grm. of the compound, dried in vacuo and burnt 

 with oxide of copper and chlorate of potash, gave 0"9600 grm. 

 carbonic acid and 0"2860 water. 



1*2170 grm. gave 0'1700 grm. chloride of platinum and am- 

 monium. 



Phil. May. S. 4. Vol. 10. No. 04. Aug, 1855. H 



