26(5 ON THE NATURE OF DRYING OILS. „ 



taming the date of the picture. For example : — if it is found 

 to have been painted before copal was known in commerce, 

 it may fafely be faid to have amber for its bafis ; but if it has 

 been painted after that period, I know of no method of dif- 

 tinguiflnng which of the two was made ufe of. As copal 

 could not have been known, as an article of trade, before the 

 feventeenth century, it follows that all pictures painted before 

 that period, and pofiefiing the properties I have defcribed, 

 muft have amber for the bafis of their vehicle. As this ex- 

 ception neceffarily includes all the Venetian artifts of the firft 

 clafs, we are therefore authorifed to conclude that, if the 

 works of thefe artifts can bear the teft of the menftrua I have 

 mentioned, amber was the balls of the vehicle with which 

 they were painted. 

 Ancient recipe, I once faw a recipe for diflblving copal, faid to have been 

 "enTin^waTthe brou § nt from Venice towards the clofe of the laft century. 

 folvent of copal. The procefs was, to melt Venice turpentine upon the fire, 

 to add gradually copal powdered, {tirring them together to 

 be united in fufion, and afterwards fpirit of turpentine, in 

 order to dilute it to the confiitence of varnifh. I tried this 

 procefs, but it did not fucceed. 

 It was not com- Upon inquiry I found that the Venice turpentine of the 

 tur^n^ne Chu> fhips was only common refin, diffolved in fpirit of turpentine 

 to a proper confidence ; whence the caufe of my failure was 

 evident. Reflecting on the commercial purfuits of the Vene- 

 tians in the fifteenth and fixteenth centuries, I was led to 

 conjecture that the fubftance called originally Venice tur- 

 pentine was the product of fome country intimately con- 

 nected with them. Purfuing this idea, I procured, with 

 much difficulty, fome Chio turpentine, repeated my ex- 

 TWis turpentine periment, and fucceeded completely. Belides the pro- 



w^nonfoTt. thC P ert > r of unitin S ea % with co P aI ' k had ° thers that ex ' 

 cited my attention. Common refins, if expofed to fire, burn 



with extreme fiercenefs and rapidity ; but when fome of this 

 was laid on the point of a knife, and held in the flame of a 

 candle, it melted and dropped down before it began to burn. 

 It emitted a peculiarly grateful fmell ; was of a mod beautiful 

 pale gold colour; was more brilliant than any turpentine I 

 had ever feen ; and when diluted to the confiftence of var- 

 nifh, perfectly refcmbled in colour, a folut'ion of copal which 

 I made in fpirit of turpentine with camphor. 



I fliowed 



