Mr. J. Dalton on the Nature and Properties of Indigo. 123 



of plants from which the indigo is extracted in different parts 

 of the world. 



It is to be understood that the part called pure indigo is 

 the sole colouring matter, and that which gives value to the 

 article. The rest may be considered as dross, doing no good, 

 and being probably harmless to the use of the drug as a dye, 

 but scarcely so to the printer, who meets with obstructions 

 enough in the exercise of his art, without introducing such as 

 may easily be avoided. 



When we consider, however, that indigo is produced by a 

 species of fermentation from vegetable matter, analogous to 

 the vinous and acetous fermentation of saccharine matter, it is 

 not improbable that the fermentation in many cases may be 

 incomplete. And as the foreign matter found in the indigo 

 of commerce is chiefly vegetable, and composed of the same 

 elements as pure indigo, it may by a fresh fermentation de- 

 velop more of the pure indigo than is found in it originally. 

 This conjecture is countenanced by the practice of dyers, who, 

 when the indigo is nearly spent, as the phrase is, put in other 

 vegetable matter to the residue, and by certain processes ob- 

 tain an addition to the quantity of colouring, which otherwise 

 would not be acquired. In a similar way I conceive it is that 

 vinegar made from sugar often contains a considerable portion 

 of the latter, which has escaped the fermenting process. 



There are two ways of obtaining pure indigo. The one is 

 that commonly practised by dyers in their use of the article. 

 On a small scale it may be effected as follows: into a two - 

 quart bottle put 50 grains of finely pounded indigo, three or 

 four times as much sulphate of iron, and hj'drate of lime same 

 weight as the salt of ii'on. Then fill the bottle with water, 

 leaving little more room than what tlie cork or stopper will 

 occupy. Mix up the contents by repeated agitation, and then 

 let the insoluble matters subside. A fine transparent green- 

 ish-yellow li(jui(l will a})pear in a day or two, which must be 

 drawn off carefully by a syphon. As soon as this liquid is 

 agitated in the air it becomes opa(]ue, and a precipitate is 

 formed, wiiich is pm-e indigo; but it cannot bo collected with- 

 out some carbonate of lime in the first instance ; it must there- 

 fore be submitted to water acidulated with muriatic acid, which 

 dissolves the lime, and leaves the pure indigo to subside. 

 Afterwards it may be collected on a filtre and dried. The 

 theory of this process is now well understood. Pure indigo, 

 deprived of a certain portion of oxygen, is known to be solu- 

 ble in lime-water ; tlie protoxide of iron, preci})itated by the 

 lime, deprives it of tliis oxygen, and hence the solution of 

 the dc-oxidizcd indigo. Such, however, is the affmity of iii- 



Q 2 digo 



