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152 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. Payne''s 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.