42 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
110. The colouring matter of archil is in many respects highly remarkable. It 
may be insulated by precipitating the ammoniacal liquid, commonly sold under that 
name, by dilute sulphuric acid, washing the insoluble matter which falls, redissolving 
in very dilute carbonate of soda, and again precipitating. It is very sparingly soluble, 
if at all, in water, but most readily so in all alkalis, which take up an enormous quan- 
tity, and afford solutions of the most intense conceivable purple colour. To this co- 
louring principle, which, like others of the same class # , has many of the characters 
of a weak acid, the name Rocellic acid ( Lichen Rocellus) may be not inappropriately 
applied. The rocellate of potash, greatly diluted, transmits a spectrum which may be 
considered as typical of those afforded by purple media, the yellow ray being attacked 
with peculiar energy, and entirely absorbed by a very small thickness or density of 
the medium. When a spectrum so transmitted was received on muriated photo- 
graphic paper, it was curtailed both at the most and least refrangible end, and re- 
duced in its extent to the space included between the coordinates + 8 and + 72. 
Allowing therefore 3"6 for the sun’s semidiameter, it appears that the rays in the linear 
spectrum as far as -j- 11’6 from the fiducial points, -were rendered inactive. Yet so 
far from the luminous rays being extinguished in this interval, so dilute was the so- 
lution used, that a great quantity of green light occupied the undarkened space in 
question, such as, had its properties not been altered by the medium it had traversed, 
could not have failed in the time the exposure lasted, to have produced a considerable 
blackening of the paper. That produced by the unabsorbed spectrum over the same 
region was very intense, as is always the case with muriated preparations of paper. 
111. The faint spot beyond the violet produced in the spectrum which has under- 
gone the action of the muriate of chrome, may possibly have some direct connexion 
with that peculiar illuminating ray to which the epithet lavender has (whether pro- 
perly or not) been applied in the foregoing pages. At least I find that, on viewing 
the oval spot of that light (concentrated as described in the earlier part of this paper 
by a prism and inclined lens) through various coloured glasses, it is not absorbed by 
either green or yellow glasses sufficiently deep to obliterate the whole of the violet ; 
and, what is extremely remarkable, such glasses, by suppressing in a most decisive 
manner the violet illumination in this part of the spectrum, and by defending the eye 
from extraneous light, reveal the existence of a faint but evident substratum (so to 
speak) of unabsorbed light, of a similar character, connecting the spot in question » 
with the rest of the spectrum ; unless, indeed, which is not impossible, this be merely 
* Such as the colouring principle of the common Heartsease, which, as the experiment described in Art. 90. 
proves, has, like that of archil, a deoxidated form ; that also which abounds in the black ebony wood, and which 
is easily extracted from it by decoction in water, and precipitation by nitric acid, &c. I may be allowed to 
mention that the name, and the chemical process described in the text, are taken from notes of experiments 
made on archil in September, October, and November 1829, while ignorant that the subject had engaged the 
attention of Robiquet, whose elaborate memoir on that Lichen appeared in the Annales de Chimie about that 
time, thereby preventing the publication of my experiments ; some notice of which, however, by M. Quetelet, 
appeared in the Journal des Sciences de l’Acad. Royale de Bruxelles, (Seance du 22 Mai, 1830.). 
