112 Sir Humphry Davy’s experiments and observations 
acquainted with verdigrise. Vitruvius mentions it amongst 
pigments, and probably many of the ancient greens, which 
are now carbonate of copper, were originally laid on in the 
state of acetite. 
The ancients had beautiful deep green glasses, which I hnd 
are tinged with oxide of copper ; but it does not appear that 
they used these glasses in a state of powder as pigments. 
The greens of the Aldobrandini picture are all of copper, 
as was evident from the action of the muriatic acid upon them. 
VI. Of the purple of the Ancients. 
The II o^cpv^oc of the Greeks, and the ostrum of the Romans, 
was regarded as their most beautiful colour, and was prepared 
from shell fish. 
Vitruvius * says, that the colour differed according to the 
country from which the shell fish was brought ; that it afforded 
a colour deeper and more approaching to violet from the 
northern countries, and a redder colour from the southern 
coasts. He states, that it was prepared by beating the fish 
with instruments of iron, freeing the purple liquor from the 
shell containing it, and mixing it with a little honey : and 
Pliny says, that for the use of the painters argentine “ creta” , f 
was dyed with it : and both Vitruvius and Pliny say, that 
it was adulterated, or imitations of it made, by tinging “ creta” 
with madder, and “ hysginum.” The finest purple, Pliny 
* Lib. vii. Cap. 13. 
f Probably a clay used for polishing silver. The ancients were not acquainted 
with the distinction between aluminous and calcareous earths, and creta was a term 
applied to every white fine earthy powder. 
t Madder was extensively used by the ancients in dying, and from this passage it 
is probable, that they were acquainted with the art of making a lake from it similar 
