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V. On the Fo7-mation of Indic/o-blue. — Part II. 

 By Edward Schunck, Ph.D., F.R.S.* 



IN the first part of this memoirf I announced the discovery of 

 a peculiar substance contained in the leaves of the Isatis 

 tinctoria, to which, as I there showed, the indigo-blue obtained 

 in the usual process of treating the plant owes its origin. Having 

 applied to this substance the name of Indican, I proceeded to 

 give a general account of its properties, and of the process of de- 

 composition which it undergoes when subjected to the action of 

 strong acids. I now propose to present a more detailed account 

 of the properties of this substance, and especially of the products 

 of decomposition derived from it. 



In continuing my experiments, I soon arrived at the conclu- 

 sion that the different methods of preparing indican, of which I 

 had in the first part of this paper given a description, though 

 they sufficed for the preparation of small quantities, were not 

 well adapted for obtaining in a state of purity the larger quan- 

 tities of the substance which I found to be necessary for the 

 purposes of investigation. The great difficulty in the prepara- 

 tion of indican arises, as I have before stated, from the extreme 

 facility with which it is decomposed, when its solutions, especi- 

 ally the watery one, arc heated, a process of decomposition which 

 is rapidly completed at a temperature a little below that of boil- 

 ing water, and takes place even at the ordinary temperature of 

 the atmosj)here, when the evaporation occupies some time. This 

 circumstance renders it necessary to avoid distilling the solutions 

 or evaporating them at any but the usual temperature. On the 

 other hand, the length of time necessary for the spontaneous 

 evaporation of the watery solution, produces in a gi'eat measure 

 the same effect as the evaporation of the solution at a higher 

 temperature during a shorter period of time. It therefore 

 became necessary to devise some means of producing a more 

 rapid evaporation of these solutions without the application of 

 artificial heat. This object was attained by means of a sinqjle 

 apparatus, in which a rapid current of air was made to pass over 

 a large surface of the li(juid to be evaporated, and which may be 

 described in a fesv words. 



The solution to be evaporated is poured into a dish or tray 

 of block-tin about IG inches square, with peri)endicular sides 

 2 inches deep, and capal)le therefore of containing, when^ full, 

 nearly two gallons of liquid. The dish is placed on a shelf fixed 



♦ From tlic- Menioirx of the Literary and I'hilosoiiliiial Society of Man- 

 chester, vol. xiv.; reail April ITj, 18.%'. Communicated liy the Antlior. 

 t Pliil. Mag. vol. X. p. T'i. 



