18 Sir J. F. W. Herschel on the Action of the Rays 
177. Common ten-weeks Stock, Mathiola annua.—The colour 
imparted by the petals of the double variety of this flower* 
to alcohol (at least when spread on paper, for it is in great 
measure dormant in the liquid tincture) is a rich and florid 
rose-red, varying, however, from a fiery tint almost amount- 
ing to scarlet, on the one hand, to a somewhat crimson or 
slightly purplish red on the other, according to the accidents 
of its preparation, or the paper used. When fresh prepared 
it is considerably sensitive, an hour or two of exposure to sun- 
shine being sufficient to produce a sensible discoloration, and 
two or three days entirely to whiten it. This quality is greatly 
deteriorated by keeping, but papers prepared with it even 
after eight or ten months, still with patience yield extremely 
beautiful photographs, several specimens of which in various 
states of the tincture are submitted for inspection to the meet- 
ing. Exposed to the spectrum, the rays chiefly active in 
operating the discoloration are found to be those extending 
from the yellow to the less refrangible red, beyond which rays 
the action terminates abruptly. Above the yellow it degrades 
rapidly to a minimum in the blue, beyond which it recovers 
somewhat, and attains a second but much feebler maximum in 
the violet rays. 
178. Paper stained with the tincture of this flower is changed 
to a vivid scarlet by acids, and to green by alkalies; if am- 
monia be used the red colour is restored as the ammonia eva- 
porates, proving the absence. of any acid quality in the co- 
louring 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 influence 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 a jar 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 reco- 
vered, but the former much more slowly than the latter. The 
restoration of the picture exposed to the sun was completed in 
* That imparted by the single flowers is very much less sensitive, as is 
also that of the dull red cr purplish variety, whether double or single. The 
most florid red double flowers, in the height of their fiowering, yield the 
best colour, 
