115 
on the colours used in painting by the Ancients. 
did not give stronger signs of inflammation than the ancient. 
I ascertained the loss of weight this ancient lake suffered by 
combustion, and found it only and this loss must in great 
part have depended on the expulsion of water from the clay 
on which it was fixed. This circumstance induced me to 
renounce the 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 upon the attractive powers of so 
large a mass of alumina ; for whenever one proportion 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 which 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 
probably the Tyrian or marine purple : and by some com- 
parative experiments on the purple obtained from shell fish 
the question might perhaps be decided,* It is very probable 
that the most expensive colour would be employed for orna- 
* M. Chaptal considers the lake he found amongst the colours from Pompeii 
(as I have already mentioned) as of vegetable origin ; and he founds his opinion upon 
the circumstance of its not affording by decomposition the smell peculiar to animal 
substances ; but probably this smell, even if produced by recent purple colouring 
matter of animal origin, would not belong to colouring matter of 1700 years old. For 
it is most probably owing merely to albumen or gelatine not essential to the colour- 
ing particles, and much more rapidly decomposed. 
