250 CALYCIFLORS:. 



formed, which are subsequently divided into smaller cakes, 

 and allowed to dry gradually in the shade. 



The boiling process, although not generally adopted, is said 

 to improve very considerably the quality, and enhance the value 

 of the produce. 



A beautiful yellow precipitate may be obtained, by means 

 of acetate of lead, from the Madeira-coloured liquid, drawn off 

 in the beating vat. This is said, by Mr Weston, to promise 

 to supply a great desideratum — a permanent yellow dye. Ex- 

 periments are, however, wanting to confirm this. 



Indigo in the prepared state is of a rich blue colour, which 

 varies, however, in its shade in different specimens. When 

 pure it is light and friable; tasteless, and almost devoid of 

 smell ; of a smooth fracture : insoluble in water or alcohol, but 

 soluble in sulphuric and nitric acids. Some varieties, such as 

 that known among the Spaniards by the name of Jlora, is 

 lighter than water ; and the lightest is generally the purest. 

 The analysis of M. Chevreul gives, as the composition of In- 

 digo, a blue colouring principle called Indigotine, a red resin, a 

 greenish-red matter, united to the sub-carbonate of lime, alum, 

 silica, oxyd of iron, and some other salts. According to Dr 

 Ure, the ultimate constituents of pure Indigo-blue, are — 



Carbon, 71.37 



Oxygen, . . . . 14.25 



Azote, 10.00 



Hydrogen, - » - - 4.38 



100.00 

 Indigo is frequently adulterated, by gummy, resinous, and earthy 

 substances being added to it ; and its weight and purity are 

 also affected by using lime in excess as a precipitant. Dr 

 Bancroft proposed, as a test to ascertain the relative values of 

 different specimens of Indigo, to dissolve equal portions of 

 each in sulphuric acid, so as to form the mixture known by the 

 name of liquid blue, and, after diluting with a certain quan- 

 tity of water, to compare the shades of colour possessed by the 

 several mixtures. 



Indigo is the most valuable and permanent of all the dye- 

 stuffs. It is also made use of by painters in water-colours. 



The method of preparing Indigo, and of applying it to the 

 purposes of dyeing, appear to have been very early known in 

 India. Dr Bancroft* has shown that the indimwi of Pliny 

 (lib. XXXV. c. 6.) possessed similar properties with the modern 

 Indigo. It would appear, by a passage in Capenarius, quoted 

 by the same author, that, in the 15th century, the Venetians 

 were in the habit of receiving Indigo from the East by the way 

 of Alexandria. After the discovery of the passage to India 



• Philosophy of Permaneut Colours, vol. i. p. 242. 



