41 i Experiments and Observations 



for the extreme rays cannot be compared. Secondly, that the 

 warmth of the yellow orange differs but little from that which 

 the red produces: so that it may be presumed that the warmest 

 rays are between the clear red "and the yellow: and Itori this 

 point the warmth of the rays diminishes considerably more ou 

 tiie violet side than on that of the deep red. 

 ■ I was desirous of using coloured liquids and glasses, but both 

 these substances, when coloured, transmit heterogeneous rays in 

 too great a quantity to be used with success in experiments of 

 so delicate a nature : besides, How are we to estimate the re- 

 flected rays, and those that are lost in these substances ? In 

 other respects, although I have taken all possible precautions, 

 tlmt depended on myself, to ensure the success of these experi- 

 ments, I am still very far from being satisfied with my labour. 



LXXII. Some Experiments and Observations on the Colours used 

 in Painting hy the Ancients. By Sir Humphhy Davy, 

 LL.D. F.R.S. 



[Concluded from p. 330.] 



VI. Of the Purple of the Ancients. 



1 HE mq'^vqu 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 diflrered according to the coun- 

 try from which the shell-fish was bro.ught; that it afforded a 

 colour deeper and more approacliing 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 " cretaf " was dyed with it : and 

 both Vitruvius and Pliny say, that it was adulterated, or imita- 

 tions of it made, by tingcing " creta" with madder J, and " hys- 

 ginum." The finest purple, Pliny says, had a tint like that of 



* Lib. vii. cap. 13. 



f Pmhnbly :i cluy used for polisluns; silver. The ancients were not ac- 

 quainted wiiii trie disriiiction between aluminous and calcareous earths, and 

 creta was a term applied to every white fine eartiiy powder. 



X Madder was extensively used by the ancients in dyeing, and from tliis 

 prissa>ie it is probable that they were acquainted with the art of making a 

 lake from it similar to that used by modern painters. It was probably one 

 of tlie colours used by tiic Egyptians in dyeing their stuffs of ditterent co- 

 lours from the same liquor, by means of mordants. If we can trust Pliny's 

 account, tliey practised calico-printing in a manner similar to the moderns. 

 Lib. XXXV. cap. 42. 



a deep- 



