331 



Woad on the contrarv, as an »indijx\i-plaiit«, huth by »culd e.\traction« and by 

 »decoction« ahvays gives the saine produce i. e. an indoxyl-solution. Here, in both 

 cases, the greatest care must be taken to exclude the air in order to prevent that the in- 

 doxyl, which is so easily oxidised, is converted already in the leaf itself, for then the 

 indigo-blue is lost. Besides, access of air in a dying wood-leaf gives still in another 

 way caiise to loss of indoxyl under forniation uf uiikiuiwn colourless and brown sub- 

 stances. 



A sufficiënt removal of air during the iireparation of the extracts is easily 

 eft'ected in the folUiwing way 'j. A well closing, vvidemouthed stoppered bottle is quite 

 filled up with woad-leaves, hot water is poured in, the leaves are pressed together until 

 all air is replaced, and the stopper is put on so as not to leave the smallet air-bubble. 

 By the exclusion of the air, together with high temperature, the leaves soon die and 

 alreadv after a few hours a clear, light yelluw liijnid can be decanted, which is rich 

 in indoxyl. If some alkali is added and air blown through, the indigo-blue preeipitates, 

 the colour of which appears only pure after acidificaticn. In a suR'icient time cif ex- 

 traction there can be thus obtained from wuad a liquid of which tlie proportion of in- 

 doxyl, according to Reinwardt -) who in i8i j applied the decoction-method on a rather 

 large scale, answers to 0.3 pCt. »pure indigo* for the fresh leaves, which, as he 

 remarks, might rise to the doublé amount in the South. If we consider that the in- 

 doxyl is especially concentrated in the youngest organs still in a state of cell-par- 

 tition, that it diminishes considerably in full grovvn parts, and is alniost or wholly ab- 

 sent in old leaves, we must conclude that the youngest organs may contain more than 

 0.3 pCt. indigo. .As the woad-leaves contain about 83 pCt. water this would correspond 

 to a little less than 2 pCt. indoxyl in the dry matter •''). 



The indoxyl-containing sap, whether prepared by »cold extra.xtion« from the iu- 

 dican-plants or by decoction from the indoxyl-containing woad, has the following 

 characteristics. It is a light yellow, in cold greenish fluorescent fluid; at waritiing 

 the fluorescence diminishes and comes back at cooling. The reaction is feebly but 

 distinctly acid, of course not by the neutrally reacting indoxyl but by organic acids. 

 At the air a copper-red film of indigo-blue is formed at the surface of the liquid. 



2(C»H-N0') + 02 = C»''H"'N-0= -f- 2H2O, 



but this üxidation follows so slowly in the feebly acid solutions, that evaporating to 

 dryness at the air is possible without too much loss of indoxyl. The indoxyl itself is 



') Tbc tccbnical preparation of indigo fruni u oad is dcscribcd iii (.;iol)crt, Traitó 

 sur Ie Pastel, Paris 1813, and in De Puyniaurin, Instruction sur Tart d'extrairc 

 rindigo du Pastel, Paris 1813. 



') In a report of 6 December i8i_' tu the President of the Agricultural Coinittoc 

 for the Department of the Zuiderzee, present as a manuscript in tin- library ot tbc 

 .■\cademy of Sciences, Amsterdam. 



•) But according to Georgevics, Der Indigo, pag. 2 and 18, W'ien 1802. tbc rate 

 of indigo for woad would only amount to 0.03 pCt. In my laboratory Mr. van Hasselt 

 found in three special cases 0.05 pCt., 0.07 pCt. and oog pCt. mdigo-blue in rclation 

 to the weight of tlu- hving leaves, vvhicli Jatter aimauit corresponds ti> ca. 0.6 pCt. 

 indoxyl «itli regard to the dry- weight. 



