Colouring Matter of Dog's Mercury.' 77 



Take a ftalk of the herb ; rub the broken en.d upon a piece of fine white paper ; and 

 fqueeze out fome of the juice. No difference will be perceived at firft, where the juice 

 has touched the paper> but fome hours afterwards it will be ftained blue. 



This colour is in the greateft perfection, when the herb is young, and contains the 

 moft juice, the thinnefl and moft colourlefs part of which produces the beft blue : and it 

 will be found by the experiment, that when the ftalk is very forcibly fqueezed, the 

 paper is coloured rather green at firft, and never becomes of a good colour. On the 

 contrary, the place touched with the cleareft and thinneft part of the juice, difcovers no 

 colour for many hours, but at laft exhibits the fineft blue contained in the vegetable. 

 This colour is not very durable, but after fome time turns to a red, which will continue 

 for years. 



A friend of mine, Mr. R. P. to whom I communicated thefe properties of the juice, 

 and requefted him to devife fome experiments, introduced fome of the leaves and ftalks of 

 the herb, into a fmall phial containing a folution of prepared ammonia, and in a little tirAc 

 the folution was coloured blue. 



The herb, when dry, exibits no particular appearance j but the roots, which are long, 

 flender, and white, when dried, difcover in feme places a fine blue tinge upon the outer 

 Ikin ; and if a quantity be fteeped in water, it becomes of a blue colour, which afterwards 

 turns red. 



A folution of ammonia extracts the colour much fooner than water alone, and, I think,, 

 improves it. 



Some years ago, I difcovered that the juice would ftain a blue colour when laid upon 

 a piece of white paper or fine white cloth ; previous to that, however, I underftood, that 

 certain individftals had endeavoured to procure a body of colour from this vegetable ; but 

 of their fuccefs, or the methods of conducting their proceffes, I could learn nothing 

 with certainty. 



The properties of this vegetable appear Angular to me, and, if they are not generally 

 known, may perhaps deferve the attention of your philofophical readers, as the elucida- 

 tion of the fubjeft feems likely to be of fome advantage to the arts. 



It is proper to be obferved, that as the plant is of a poifonous nature, it muft 

 not be carelefsly ufed. 



I am, Sir, 



Your humble fervant, 



C. 



April igth, l8oOi 



Account 



