FOBMATION OF INDIGO-BLUE. 201 



preparation of this specimen, I found that it did not con- 

 tain unchanged indican, as a little of it when tested with 

 sulphuric acid gave no indigo-blue. It is, nevertheless, the 

 purest specimen of the lead compound which I have analyzed ; 

 that is to say, the substance combined with the oxide of lead 

 contained the least amount of hydrogen and oxygen. 



The next analysis which I shall give, places in a striking 

 light the effect which alkalies exert on indican. I took somfe 

 of the same solution of indican which I had employed for 

 the preceding analysis, and which I found to give, when a 

 little of it was boiled with acid, very pure indigo-blue; but 

 instead of evaporating it I added a large quantity of alcohol 

 to it, and then precipitated with acetate of lead and ammonia. 

 The precipitate no longer contained unchanged indican, and 

 the substance combined with the oxide of lead differed in 

 composition from that of the preceding analysis, by contain- 

 ing the elements of two equivalents more of water. The 

 third analysis was performed with a lead compound made in 

 the same way as that of the first analysis, but from a 

 specimen of indican prepared by the first method. The 

 composition I found to be exactly the same as that of the 

 compound of the second analysis. 



II. 1.0960 grm. dried in vacuo and burnt with oxide of 

 copper and chlorate of potash gave 0.7060 grm. carbonic 

 acid and 0.2020 water. 



1.5600 grm. gave 0.1880 grm. chloride of platinum and 

 ammonium. 



0.8930 grm. gave 0.7520 grm. sulphate of lead. 



III. 1.1000 grm. gave 0.7135 grm. carbonic acid and 

 0.2050 water. 



1.8350 grm. gave 0.2260 grm. chloride of platinum and 

 ammonium. 



0.5710 grm. gave 0.4770 grm. sulphate of lead. 



2 C 



