FORMATION OF INDIGO-BLUE. 199 



acid it strikes a dark red colour, which on heating becomes 

 black. Boiling nitric acid decomposes it with an evolution 

 of nitrous fumes. When its watery solution is boiled with 

 caustic soda it becomes yellow and deposits a few brown 

 flocks. With sulphate of copper and caustic soda it gives 

 a blue solution, which on boiling becomes yellow and then 

 deposits suboxide of copper. If nitrate of silver be added 

 to its watery solution, while boiling, a little metallic silver 

 is precipitated, and when ammonia is added a further reduc- 

 tion 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 solution an additional quantity of gold is precipitated 

 as a purple powder. The watery solution gives no precipi- 

 tates 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 ether. In its outward pro- 

 perties, therefore, this sugar does not differ in any marked 

 degree from other kinds of sugar obtained by the decom- 

 position of complex organic bodies, such as that derived 

 from rubian. In its composition, however, it differs essen- 

 tially 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 under- 

 going 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 un- 

 changed indican, but one of the bodies formed by the combi- 

 nation of the latter with water. It is necessary, however, 

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



