purple of Tyre yielding 1000 Roman Denarii tlie pound; 

 computed at more than thirty pounds fteriing ; a colour 

 probably confined to Emperors and Se?iators : nor can we be 

 furprifed at fo great a price when it is confidered how fmall 

 a portion is afiorded by each animal. 



As the beautiful dye of the animal of Turbo dathriis is 

 little known we fliall briefly fiate a few experiments with iti 

 The animal as before flatcd has the power of difcharging it, 

 but it may be collected either recent or when the animal is 

 dried, by opening the part behind the head. 



The colouring fuccus was extra6led from five animals, 

 and after grinding with a few drops of fpring water, ap- 

 peared fufficient to cover half a fheet of paper with a beau- 

 tiful purple. 



Neither volatile nor fixed alkali materially afe6l it ; mi- 

 neral acids turn it to a bluifli-green, or fea-green ; fulphuric 

 acid renders it a fliade more inclining to blue ; vegetable acids 

 probably do not af^ecl it, fince cream of tartar did not in the 

 leaft alter it. 



Thefe colours laid on paper were very bright, and ap- 

 peared for fome months unchanged by the acSlion of the air, 

 or the fun ; but being expofed for a whole fummer to the 

 folar rays in a fouth window, they almofi: vanifhed. The 

 application of fl/y^fl// to the acidulated colour always refiored 

 it to its primitive ftate, and was as readily changed again by 



mineral 



