■41G Esperimenis and Ohsenaiions 



and of a brighter liue by weak acids; but it differed from it in 

 being much more easily destroyed by strong acids. It agreed 

 with both the vegetable and animal lakes in being immediately 

 destroyed by a solution of chlorine. 



The lake made from cocliineal produced nmoh denser fumes 

 when exposed to fused potash, and afforded a distinct ammo- 

 iiiacal smell. The two modern lakes when burnt in oxygen did 

 not give stronger signs of inflammation than the ancient. I as- 

 certained the loss of weight this ancient lake suffered by com- " 

 bu.tion, and found it only -^^^ and this loss must in great part 

 have depended on the expulsion of water from the clay on which 

 it v.as fixed. This circumstance iiuluced me to renoimce tlie 

 "idea of attempting to determine its nature from the products of 

 its decomposition ; which in the case of so small a quantity of 

 matter diffused over so large a quantity of surface could not have 

 afforded unequivocal results. 



The durability of this lake, whether vegetable or animal, is a 

 very curious circumstance ; but the exterior part which has been 

 exposed to air has suffered. — This durability probably depends in 

 a great measure U})on the attractive powers of so large a mass of 

 alumina; for, whenever one ]>roportion of a substance is combined 

 with many proportions of another substance, it is very difficult 

 to decompose or detach the one proportion. 



From the circumstances ^^hich have been noticed respecting 

 this colour, it is impossible to form an opinion whether it is of 

 vegetable or animal origin. If of animal origin, it is most pro- 

 bably the Tyrian or marine purple : and by some comparative 

 experiments on the purple obtained from shell-lish the ques- 

 tion might perhaps be decided*. It is very probable that the 

 most expensive colour would be employed for ornamenting the 

 imperial baths ; and it is not inipossible that Pliny may have 

 alluded to the palace of the Ccesars when he says " nunc et pur- 

 puris in parietes migrantibus, et India conferente fluminum suo- 

 rum limum, et dracom.mi et elephantorum ganienij nulla nobilis 

 pictura est." Lib. xxxv. cap. 32. 



1 have seen no colour of the same tint as this ancient lake in 

 any of the ancient paintings in fresco. The purplish reds in 

 the baths of I'itus are mixtures of red ochres and the blues of 

 copper. — In the Aldobrandini picture there is a purple in the 



* M. Cliaptal coiisidcrs the lake he found anio.ngst the colours from 

 Po!i)peii(asi have already mentioned) as of vegetable origin ; and l:e founds 

 his opinion upon the circumstance of its notaffording by decomposition the 

 smell-peculiar to animal substances: but probably thissmell, even if pro- 

 duced by recent pn; pje colouiinginatter of animal origin, vvould not belong 

 to colouring matter of 1700 years old. For it is most probably owing merely 

 to albumen or gelatine not essential to the colouring particleSj and much 

 mo;e rap!<lly decomposed. 



garment 



