ow Tiir. 40 



no chanse is perceived, because this water no longer Contains Facts and ob- 



° * servationson 



oxigen. tmandiucom- 



A linen or cotton rag, previously wetted with boiling water, poinds, 

 if dipped into the second glass comes out of a yellow colour, 

 which changes successively to green, and then to blue, with 

 which it is finally firmly dyed. If the contents of the second 

 glass while still yellow be thrown into a large bell-shaped ves- 

 sel, and whirled round, the liquor will pass rapidly from yellow 

 to perfect blue. The indigo having recovered its native colour, 

 becomes insoluble and settles : in like manner, if a few drops 

 of oxigenated muriatic acid be put into the solution whilst 

 yellow, the blue colour will be instantly restored ; but more 

 of the acid will destroy it again. These effects confirm more 

 and more the ideas of modern chemists respecting indigo. — In 

 India and the islands indigo is not drawn from the vegetable, 

 juices which contain it, so much as it is precipitated by oxi- 

 gen ; and in Europe it is only fit for the purpose of dyeing in 

 proportion as this oxigen is destroyed. The effects of the 

 woad vat though so different in their appearance from thos« 

 produced by the disoxidating minerals, are nevertheless subject 

 to the same theory. The fermentation of the green fecula of 

 the woad, or of bran, or madder, &c. disengages a portion of 

 hydrogen, which attacks and disoxidates the indigo, and 

 restores its green colour. After frequent opportunities of 

 observing the process in the vat, I am persuaded that any other 

 green plants rich in fecula, such as cabbages, and all crucife- 

 rous plants in general, would produce similar effects, and 

 might be advantageously employed, particularly where woad 

 cannot conveniently be obtained. 



It may be interesting to commerce, and to manufacturers 

 to be informed that lOOlbs. of linen, well scoured require 

 6i[hs. of indigo, to dye it of a turquoise blue, the deepest tint 

 that can be given ; this I have obtained from an experiment 

 made some time ago, with great care, in a wood vat. 



I could wish to give in this place a set of experiments made- 

 upon the scarlet, with solution of tin, by sulphuric acid, sea 

 salt, and saltpetre, to avoid the use of aquafortis ; but I wish to 

 correct some particular parts, which I have not leisure just 

 now to do. I can, however, assert that sulphuric acid, and 

 even the salt, which both change scarlet to violet, appear to be 

 VoL.XIV.— May, 1806. H no 



