192 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
178. Paper stained with the tincture of this flower is changed to a vivid scarlet hy 
acids, and to green by alkalies ; if ammonia be used the red colour is restored as the 
ammonia evaporates, proving the absence of any acid quality in the colouring matter 
sufficiently energetic to coerce the elastic force of the alkaline gas. Sulphurous acid 
whitens it, as do the alkaline sulphites; but this effect is transient, and the red colour 
is slowly restored by free exposure to air, especially with the aid of light, whose in- 
fluence in this case is the more remarkable, being exactly the reverse of its ordinary 
action on this colouring principle, which it destroys irrecoverably, as above stated. 
The following experiments were made to trace and illustrate this curious change. 
179. Two photographic copies of engravings taken on paper tinted with this colour 
were placed in ajar of sulphurous acid gas, by which they were completely whitened, 
and all traces of the pictures obliterated. They were then exposed to free air, the 
one in the dark, the other in sunshine. Both recovered, but the former much more 
slowly than the latter. The restoration of the picture exposed to sun was completed 
in 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 sulphurous acid and laid 
on blotting-paper similarly wetted. Being then crossed with a strip of black paper, 
it was laid between 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 pre- 
sence of the acid) was considerably restored in the portion exposed, while the whole 
of the portion covered by the black paper remained (of course) perfectly 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 lengths of exposure to the acid fumes from 
half an hour to many days) to the action of the spectrum ; and it was found, as in- 
deed I had expected, that the restoration of colour was operated hy rays complement- 
ary 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 least refrangible red rays also, being precisely those 
for which in the unprepared paper the discolouring 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, ge- 
nerally, 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 man- 
ner alkalies restore the colour, converting it at the same time into green. 
