1820.] Scieiitijic InteUigence. 467 



greenish-yellow solution from the indigo vat. I then let up a 

 determinate quantity of the liquid into a gTaduated glass tube 

 filled with mercury, and standing on the mercurial trough. This 

 done, I let up a certain number of cubic inches of oxygen gas 

 into the same tube, and allowed the tube to remain inverted over 

 the mercury till the whole of the indigo was precipitated in the 

 state of a blue pigment, and till the oxygen gas ceased to dimi- 

 nish in bulk. The loss of bulk which the oxygen gas sustained, 

 together with the known weight of the indigo present, enabled 

 me to determine how much oxygen was necessary to convert 

 the greenish-yellow soluble pigment into blue insoluble indigo. 

 The result of three experiments made in the way just described 

 was nearly the same, and was as follows : 



Indigo in the state of a greenish-yellow soluble pigment, or 

 the soluble basis of indigo, as it is called, is composed of 



5 atoms oxygen = 5*00 



7 atoms carbon = 5*25 



1 atom azote = 1"75 



12-00 



So that it is a compound of 13 atoms, and the weight of an 

 integrant particle of it is 12. The addition of a single atom of 

 oxygen renders the colour blue and the pigment insoluble. Thus 

 it appears that the blue pigment differs from the greenish-yellow 

 soluble basis merely by containing one additional atom of 

 oxygen. 



Thus indigo exhibits a striking refutation of the old notion 

 that acidity is owing to the union of oxygen with an acidifiable 

 basis. The blue pigment is soluble in sulphuric acid, and when 

 recently obtained by precipitation, it may be dissolved in several 

 other acids ; but no alkaline substance that i have tried is capa- 

 ble of combining with it. Hence it appears to possess alkaline 

 properties, or at least to approach much nearer the nature of a 

 sahfiable base than of an acid ; but when we deprive it of an 

 atom of oxygen by means of protosulphate of iron, or any sub- 

 stance whicn has a strong affinity for oxygen, it acquires a 

 greenish-yellow colour, and becomes capable of combining with 

 the alkalies, and with lime, barytes, and strontian, and perhaps 

 also with other salifiable bases. It has, therefore, acquired acid 

 properties, or at least approaches much more nearly to tlie 

 nature of an acid than it did while in the state of a blue pigment. 

 Tims the addition of oxygen gives indigo alkaline qualities, and 

 the abstraction of oxygen gives it acid properties. 



Should any person think of repeating these experiments, it 

 may be necessary to put him on his guard against a resinous 

 substance, which indigo often, if not always, contains, and 

 which I have Ibund to dissolve with it in alkalies and lime-water, 

 and, therefore, to contaminate the pure indigo obtained from the 

 indigo vat. Its presence for a long time deceived me, and led 



2g2 



