682 Professor E. E. Boscoe [May 27, 



lined with strong canvas. A piece of canvas is tlien placed on the 

 top of the frame, a perforated wooden cover, which fits into the box, 

 put over it, and the whole submitted to a gradual pressure. When 

 as much of the water as possible has been squeezed out the covers 

 are removed, and the indigo allowed to dry slowly in large drying 

 sheds, from which light is carefully excluded. When dry, it is ready 

 for the market. Each vat yields from 36 to 50 lbs. of indigo." * 



The same process carried out in the times of the Greeks is 

 thus described by Dioscorides : " Indigo used in dyeing is a purple 

 coloured froth formed at the top of the boiler ; this is collected and 

 dried by the manufacturer; that possessing a blue tint and being 

 brittle is esteemed the most." 



The identity of the blue colouring matter of woad and that of the 

 Bengal plant was proved by Hellot, and by Planer and Trommsdorff 

 at the end of the last century. These latter chemists showed that 

 the blue colour of the woad can be sublimed, and thus obtained in 

 the pure state, a fact which was first mentioned in the case of indigo 

 by O'Brien, in 1789, in his treatise on calico printing. Indigo thus 

 purified is termed indigotin. It has been analysed by various 

 chemists, who ascertained that its composition may be most simply 

 expressed by the formula CgHgNO. 



Indigo is a blue powder, insoluble in water, alkalis, alcohol, and 

 most common liquids. In order to employ it as a dyeing agent it 

 must be obtained in a form in which it can be fixed or firmly held on 

 to the fibres of the cloth. This is always effected by virtue of a 

 property possessed by indigo-blue of combining with hydrogen to 

 form a colourless body, soluble in alkalis, known as indigo-white, or 

 reduced indigo, of which the simplest formula is CgHgNO. This 

 substance rapidly absorbs oxygen from the air and passes into the 

 blue insoluble indigo, which, being held in the fibre of the cloth, 

 imparts to it a permanent blue dye. This reduction to white indigo 

 may be effected in various ways. The old cold vat, or blue-dip vats 

 as they are termed, consist of a mixture of indigo, slacked lime, and 

 green vitriol. The latter salt reduces the indigo, and the white 

 indigo dissolves in the lime water. This process of indigo dyeing is 

 both expensive and troublesome, owing to loss of indigo and formation 

 of gypsum, so that many plans have been proposed to remedy these 

 evils. 



Concerning the origin of indigo in the leaves of the Indigo/era, 

 various and contradictory views have been held. Some have supposed 

 that blue indigo exists ready formed in the plant ; others, that white 

 indigo is present, which on exposure to air is converted into indigo- 

 blue. Schunck has, however, proved beyond doubt that the 

 woad plant {Isatis tinctoria), the Indigofera tinctoria of India, and the 

 Chinese and Japanese indigo plant {Polygonum tindorium) contain 

 neither indigo-blue or white indigo ready formed. It is now known 



* Grace -Calvert. 



