The experimenters, after carefully considering the
different kinds of light their pigments (hung out of
doors) would receive during their exposure from May
1886 till Marcli 1888, and estimating their probable
values, calculate that they received a total illumina-
tion equivalent to 10,800 hours of blue sky light.
To produce results similar to those obtained from
this light, it would have been necessary, had they
been placed in a gallery such as those at South
Kensington, to expose them for at least 480 years,
and to gaslight continuously, allowing for the dura-
tion of darkness, for 9600 years.
For tiie experiments, the moist colours of one firm
were used. The action of light was tried on single
colours, and on mixtures of two or more colours.
Those mixtures were avoided in which a change
would of necessity take place without the action of
light owing to the known chemical composition of
the colours. The paper used was Whatman's, and
in order that no variation of quality should occur in
different experiments, sufficient was obtained at once
for the whole investigation. The colours to be
tested were applied to the paper in a series of
washes, the first wash extending over the Avhole
sheet, the second one leaving a strip one inch wide
and the length of the paper untouched. In most
cases as many as eight washes were applied, giving
thus a complete series of eight tints. In the
experiments strips two inches wide and eight inches
long having all the tints upon them were used.
In the first series of experiments the colours were
exposed to the action of light, air, and moisture, as
are pictures, only to a greater extent. Two strips
of the coloured paper cut from the same sheet were
inserted into a glass tube, open at both ends, the
upper end being bent over to prevent the entrance
of wet and dirt. A piece of American cloth was
carefully bound round one-half of the tube, tluis
effectually protecting one strip of the paper from
light. The two pieces of identically tinted paper
were therefore under exactly the same conditions,
except that one was exposed to light whilst the
other was in darkness. The tubes were hung
vertically out of doors against a wall facing nearly
south, where all the sunshine until after 8.30 p.m.
could fall upon them. During the exposure of the
papers, they were observed for the fii'st time in
August 1886, again in December 1886, and in July
and November 1887, and finally in March 1888.
In some cases the colour entirely disappeared, as,
for instance, in carmine. In the majority of cases
only a part of the colour disappeared, the thinner
washes fading out, but twelve pigments remained
unchanged, and two others, after this long exposure,
were only very slightly faded. The following table
shows approximately the order of instability of the
tliirty-nine single colours tried, beginning with the
most fugitive : —
Carmine.
Crimson lake.
Purple madder.
Scarlet lake.
Paynes grey.
Naples yellow.
Olive green.
Indigo.
Brown madder.
Gamboge.
Vandyke brown.
Brown pink.
Indian yellow.
Cadmium yellow.
Leitch's blue.
Violet carmine.
Purple carmine.
Sepia.
Aureolin.
Rose madder.
Permanent blue.
Antwerp blue.
Madder lake.
Vermilion.
Emerald green.
Burnt umber.
Yellow ochre.
Indian red.
Venetian red.
Burnt sienna.
Chrome yellow.
Lemon yellow.
Raw sienna.
Terra verte.
Chromium oxide.
Prussian blue.
Cobalt.
French blue.
Ultramarine ash.
o
J
All of these, except Prussian blue, are purely
mineral colours. Of the thirty-four mixtures tried
only three remained from first to last unchanged —
Venetian red and raw sienna, Antwerp blue and
raw sienna, Indian red and cobalt ; but six mixtures
containing Prussian blue, although at first unaltered,
on placing in the dark for six weeks more or less
returned to their original colour.
It is of considerable interest to note that in the
cases in which any change occurred it had com-
menced before December 1886, though not in all
cases before August.
In another series of experiments, carried out at the same time with mostly the same pigments, the atmosphere to which they were exposed was free from all moisture. The glass tube was freed from all damp, as also the tinted paper, and the tube hermetically sealed. Thirty-eight experiments were made with single colours ; but under this altered condition twenty-two instead of twelve were found to be permanent, principally those colours which in the former experiments were only very slightly faded. In one case, while the colour in the oj^en tube was not acted upon, that in the dry tube was, this being Prussian blue. The colours which were unchanged in dry air, but were acted on in ordinary air, are madder lake, cadmium yellow, Naples yellow, emerald green, olive green, Payne's grey, sepia, and burnt umber. Again, with the exception of maddei- lake, all the above which were not acted upon in dry air are mineral colours.
