ORCHELLA. 367 



bordering: on violet. It is seldom used by itself, on 

 account of its high price and the evanescent nature of 

 its beauty, but it is chiefly employed to give a bloom 

 to other colours. This is done by passing the dyed 

 stuff under process through hot water, slightly im- 

 pregnated with the archil. The hue thus communi- 

 cated vanishes upon long exposure to the air. Hellot 

 found that by the addition of a small quantity of the 

 solution of tin it became a durable dye, approaching 

 to the colour of scarlet. Archil dissolved in water 

 gives a durable stain of a beautiful violet colour to 

 marble. M. Du Fay relates that he has seen pieces 

 of marble which had preserved their colour, without 

 any sensible change, after having been stained with 

 it for more than two years. It sinks deep in the 

 marble and renders it more brittle. 



Archil very readily gives out its colouring matter 

 to both water and alcohol. Spirit thermometers are 

 usually tinged with this colour, which circumstance 

 has made manifest a remarkable property of archil, 

 that exposure to, and a total exclusion from air, alike 

 destroy its colour. Its fugitive beauty soon flies from 

 the fabric on which it is sought to be fixed, while in 

 a tube hermetically sealed it also vanishes. 



Spirit coloured with this substance, enclosed in 

 large thermometers, in a few years becomes entirely 

 colourless. The Abbe Nollet observed* that this 

 colourless spirit soon resumes its colour upon breaking 

 the tube, and this for a number of times successively. 

 An aqueous tincture enclosed in a tube lost its colour 

 in three days. In an open, deep vessel an infusion 

 becomes colourless at bottom, while at top it retains 

 its colour. 



Orchella is imported into England at a nominal 

 duty of 3s. per cwt. ; its price averaging from <14 to 

 l. 10s. per cwt., according to its quality. It is 

 * Memoirs of the French Academy for the year 1742, 



