ON AVOAD AS A PREHISTORIC PIGMENT. 



39 



article now dipped into it turns distinctly green, and when exposed to the 

 air the greenish shade passes into a pale azure blue. The immersion of 

 the fabric in dilute hydrochloric or any other acid brings about this 

 change at once. The colours which different articles assumed after 

 dyeing varied from pale blue, blue, greenish blue, pale green, to various 

 shades of bluish grey and dove colour. Potash, soda, and ammonia are 

 pretty uniform in their action. Lime-water gives rise to a more bulky 

 precipitate of a green hue, which after treatment with an acid does not 

 apparently produce so pure a blue. The behaviour of different infusions 

 varies, however, very remarkably, e^ en when treated as nearly as possible in 

 the same manner. One infusion, treated with lime-water and not acidulated,, 

 gave, after standing a few days, the most beautiful azure blue ; but 

 generally lime-water precipitates remained more or less green at the end 

 of several days. One lot of caustic polc^sh precipitates, left twelve hours- 

 before being acidulated, gave nothing but the brownish-black precipitate, 

 which, whatever it may be, is not indigo. 



In order to form some idea of the time the infusion should be left to- 

 extract the maximum quantity of indigo, an ordinary pint pharmaceutical 

 infusion pot was filled with Woad leaves, which were covered with boiling 

 water and weighted down. In half an hour 30 c.c. was poured off, cooled, 

 alkalised with a few drops of caustic potash, and then rendered acid by 

 dilute hydrochloric acid ; this yielded '041 gramme of good indigo blue, 

 A series of similar trials gave the following results :— 



Time infused 



30 minutes 

 1| hours 



2 „ 



3 „ 

 6 „ 

 9 „ 



12 „ 

 24 „ 

 48 „ 



ColDur of precipitate 



Good blue 



Blue tinged with green 



More distinctly green 

 Quite green 

 1 1 



Green with some brown 

 Dirty brown 



Weight after 

 drviiiK at 10U° C. 



•041 grm. 



•010 „ 



•018 „ 



•040 „ 



•030 „ 



•010 „ 



•OIG „ 



•016 „ 



•015 „ 



Colour of dry precipitate- 



Blue 



Blue-black 

 Dirty black 

 Black -brown 



It must not be supposed that the blue colour is always replaced by 

 the black at the end of three or even six hours, but one cannot avoid the 

 suspicion that some of the sanguine experimenters in bygone days, wha 

 wished to find an abundance of easily separated indigo in this plant, may 

 have considered every dark precipitate indigo. No explanation is oii'ered 

 as to the variation in the quantities of the above trials beyond that the 

 quantities taken were too small to give very reliable results ; but it was 

 obvious to the eye before the precipitates were weighed that these 

 discrepancies did exist. The colours of the precipitates are those given 

 by the gentleman who kindly weighed them for me, Mr. E. C. Trulitt. 



With regard to the influence of the age of the leaves on the quantity 

 of indigo they yield, the following results were obtained. The age of 

 the leaves was a comparatively easy matter to determine, because the date 

 of the last cropping was known. The sample examined on December 2, 

 1899, which yielded the most indigo, was the third cropping, that is to 

 say, the leaves had been pulled off twice ; under ordinary circumstances 



