14& Observations on Maddening, 
slow and insensible fermentation. The exposure, on the 
grass, of cotton or linen dyed a dark madder red, might 
support the idea of a change to a reddish yellow ; for 
this dark colour becomes clearer but fainter by the expo- 
sure, and then assumes a more agreeable shade of crim- 
son. I have shown, in a memoir on indigo, inserted in 
the Journal de Physique for the year 1788, that nitric 
acid changes this blue fecula into a yellowish substance : 
a similar change takes place by exposing, on the mea- 
dow, the same fecula fixed on any stuff whatever; and 
the yellow resulting in these two ways is more soluble in 
warm water than in the same liquid when cold. It how- 
ever appears that the combination of oxygen is not the 
only cause of the change of colours, since curtains of 
any stuff dyed or coloured any shade whatever by vege- 
table or animal substances, and exposed to the light, lose 
their colour entirely in the course of time on the side ex- 
posed to the solar rays, while the opposite side retains it 
for a considerable time. If the rays of the sun then give 
more vigour to living bodies of the animal and vegetable 
kingdom by disengaging from the latter oxygen gas, it 
appears that they act with destructive influence on the 
same bodies deprived of life, by decomposing their con- 
stituent principles. In all cases it will be proper to pre- 
serve the colouring ingredients in dry places sheltered 
from the light, which acts upon these bodies perhaps on- 
ly by decomposing the constituent aqueous part, the oxy- 
gen of which may join the carbon to form carbonic acid. 
Resinous and oily substances should be preserved in 
the same way. These conjectures prove at least that 
the action of the sun’s rays, or of light, on these bodies, 
in general presents a vast Held for interesting experiments 
to be undertaken. 
If in maddering brighter colours are obtained by care- 
fully regulating the heat, a sacrifice Is made at the same 
