4^2 Experiments on Indigo. 



Whatever may be the fate of the foregoing experiments, whether they may be refuted or 

 toot, partially, it is clear that the metallic appearance has never before been fo decidedly 

 feparated from indigo, and probably it is an univerfal principle in nature that may occafion 

 thefe golden and filvery hues vifible on many infefts, particularly fire flies, which are fup- 

 pofed to give out phofphorated hydrogenous gas, and alio in feveral kinds of fifties. 



On the whole, my opinion is, that indigo coiififts of a peculiar volatile matter fui generis, 

 capable of becoming blue by a fmall portion of oxigen, (though that is only rendered pro- 

 bable by the above experiments) combined with a rcfinous or bituminous fubftance, con- 

 vertible by the nitrous acid into laurel-oil and gazeous produfts. 



I am, Sir, with equal refpecl: for your charafter, 

 as a man and as a philofopher, 



your moft obedient fcrvant, 

 A Friend to the Experimental Research or Truth. 



P.S. I forgot to mention that I have tried to procure both the needles and leaf from 

 Prulfian blue by the fame means that I ufcd with indigo, but could produce no fuch effeft. 



Since I finilhed this account, I have repeated one of Dr. Bancroft's experiments', or at leaft 

 on the fame principle with his, and one of thofe that could be moft decifive with regard to 

 his theory. I took a cauftic alkali of the (hops, and treated it again with lime, till it 

 gave almoft no effervefcence, with the fame fulphuric acid, diluted with about eight waters, 

 that I ufed in the latter part of the experiment. I took a drachm of indigo, of a kind that 

 gave no effervefcence with that acid, and two drachms of brown fugar, which did not 

 tficrvefce either. I boiled thefe witli two ounce meafures of the above alkali ; in a little time 

 the liquor paffed from the blue to the green and yellow ; I tlien ftoppcd the procefs, and 

 after trying if it effervefced with the acid, which I found it fcarcely did, I threw a quantity 

 of it up into a jar of atmofpherical air above Mercury, whilft it was almoft boiling ; when 

 cold, I marked the height of the liquor ; after ftanding two days, it had rifen about two- 

 thirds of an inch in a jar about five and a half high, and, except at the bottom, the indigo 

 was regenerated in the blue ftate; I then threw up a quantity of the above diluted acid, and 

 had a copious effervefcence, and the furface of the liquor, below the air bubbles, defcended 

 to the mark : this was evidently carbonic acid ; but to make the matter certain, I injefted 

 lime water, fo that it toucTied the fides of the jar ; when at firft its appearance was pellucid, 

 and afterwards a white precipitate of carbonate was evident on the fides of the jar. In at- 

 tempting to transfer the jar to the water apparatus, I had allowed a good deal of atmof- 

 5>heriCal air to mix with the refidual gas, fo had no farther opportunity of examining it. I 

 am fenfible that this experiment ought to be repeated with accuracy, efpecially fo as to de- 

 termine the real quantity of carbonic acid, which was undoubtedly formed from the oxigen 

 of the'atmofplierical air, and carbonaceous matter either in the fugar or indigo. 



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