146 INDlttO. 



and bleached by chlorine. Many of the same matters unite inti- 

 mately with alumina and oxide of tin to form lakes, insoluble com- 

 pounds in which the colors remain fixed ; thus a colored liquid often 

 becomes colorless when it is shaken with a hydrate of alumina. 

 Charcoal, in a state of extreme subdivision, acts like alumina, and 

 is a powerful discharger of colors in every-day use in the arts. 

 Coloring matters are generally ternary compounds, though some of 

 them also contain azote ; and several of them exhihit the remarka- 

 ble phenomenon, that in undergoing oxidation in contact with am- 

 monia they assimilate the azote of this alkali. I shall now indicate 

 the origin and the mode of preparing a few of the more important of 

 these coloring matters. 



Indigo. This substance, so essential in the art of dyeing, has been 

 one of the great staples of trade with Asia from the most remote 

 times. For a long while indigo was regarded in Europe as a min- 

 eral substance found in India : it used to be designated Indian or In- 

 dia stone, whence the name of indigo. It was not until after the dis- 

 covery of America that the true nature of this dye-stuff was known, 

 although before this period indigo had been made in Arabia, Egypt, 

 and even in the Island of Malta. 



Indigo is volatile, so that to obtain it pure, it is enough to put a 

 small quantity into a platinum capsule, to cover it with a lid and to 

 expose it to heat. Indigo is volatilized in the state of violet-colored 

 vapor, and collects in crystals upon the middle part of the sides of 

 the capsule. Indigo gives nothing to water or to ether. Alcohol 

 takes up a very small quantity of it ; concentrated sulphuric acid 

 dissolves and modifies it. 



All bodies greedy of oxygen appear to reduce or deoxidize this 

 coloring principle ; it changes to a yellow, and becomes soluble in 

 water in contact with alkalies ; by exposing the alkaline liquor charg- 

 ed with the uncolored indigo to the air, it absorbs oxygen rapidly, 

 and the indigo becomes insoluble and is precipitated with its original 

 blue color. It is most easy, as said, to disoxidize indigo ; it is suf- 

 ficient t'j bring it into contact with hydrogen gas in the nascent state, 

 a condition which is readily secured by throwing iron or zinc filings 

 into water containing the coloring matter previously dissolved in 

 sulphuric acid. The disengagement of the hydrogen has scarcely 

 commenced before the deep-blue color of the solution declines in 

 intensity, and by and by it becomes of a very pale gray. When the 

 discharge of color is completed, and no more hydrogen is disengaged, 

 the colorless indigo begins to react upon the air, it absorbs oxygen, 

 becomes again oxidized, and by and by the liquid has resumed its 

 deep blue. This property of indigo of becoming soluble in alkaline 

 solutions under the influence of disoxidizing bodies, is taken advan- 

 tage of in our laboratories to obtain indigo in a state of purity, and 

 in the arts to prepare a dyeing liquid. If a mixture be made of 15 

 parts of the indigo of commerce reduced to fine powder, 10 parts of 

 the sulphate of the protoxide of iron, 15 parts of lime, and 60 parts 

 of water, and it be left for several days in a closed vessel, a color 

 less liquid is obtained. The liquid decanted and exposed to the air 



