of the Solar Spectrum on Vegetable Colours. 19 
twenty-four hours, that in the dark not till after a lapse of two 
or three days. 
180. A slip of the stained paper was wetted with liquid sul- 
phurous acid and laid on blotting-paper similarly wetted. 
Being then crossed with a strip of black paper, it was laid be- 
tween glass plates and (evaporation of the acid being thus 
prevented) was exposed to full sunshine. After some time the 
red colour (in spite of the presence of the acid) was consider- 
ably restored in the portion exposed, while the whole of the 
portion covered by the black paper remained (of course) per- 
fectly white. 
181. Slips of paper, stained as above, were placed under a 
receiver, beside a small capsule of liquid sulphurous acid. 
When completely discoloured they were subjected (on various 
occasions, and after various lengthsof exposureto the acid fumes 
from half an hour to many days) to the action of the spec- 
trum; and it was found, as indeed I had expected, that the 
restoration of colour was operated by rays complementary to 
those which destroy it in the natural state of the paper, the violet 
rays being chiefly active, the blue almost equally so, the green 
little, and the yellow, orange, and most refrangible red not at 
all. In one experiment a pretty well-defined red solar image 
was developed by the Jeast¢ refrangible red rays also, being 
precisely those for which in the unprepared paper the disco- 
louring action is abruptly cut off. But this spot I never 
succeeded in reproducing ; and it ought also to be mentioned, 
that, according to differences in the preparation not obvious, 
the degree of sensibility, generally, of the bleached paper to 
the restorative action of light differed greatly ; in some cases 
a perceptible reddening being produced in ten seconds, and a 
considerable streak in two minutes, while in others a very 
long time was required to produce any effect. 
182, The dormancy of this colouring principle, under the 
influence of sulphurous acid, is well shown by dropping a 
little weak sulphuric acid on the paper bleached by that gas, 
which immediately restores the red colour in all its vigour. 
In like manner alkalies restore the colour, converting it at the 
same time into green. 
183. Papaver orientale.—'The chemical habitudes of the sul- 
phurous acid render it highly probable that its action, in in- 
ducing a dormant state of the colorific principle, consists in a 
partial deoxidizement, unaccompanied however with disorga- 
nization of its molecules. And this view is corroborated by 
the similar action of alcohol already spoken of ; similar, that 
is, in kind, though less complete in degree. Mostcommonly, 
vegetable colours, Recent by the action of alcohol, are 
2 
