was depended upon for testing the
permanency of coloring materials. As a consequence such tests could not
be carried out very systematically until a powerful artificial source of
light resembling daylight was available. It appears that the white
flame-arc is quite satisfactory in this field, for tests indicate that
the chemical effect of this arc in causing dye-fading is four or five
times as great as that of the best June sunlight if the materials are
placed within ten inches of a 28-ampere arc. It has been computed that
in several days of continuous operation of this arc the same fading
results can be obtained as in a year's exposure to daylight in the
northern part of this country. Inasmuch as the fastness of colors in
daylight is usually of interest, the artificial illuminant used for
color-fading should be spectrally similar to daylight. Apparently the
white flame-arc fulfils this requirement as well as being a powerful
source.

Lithopone, a white pigment consisting of zinc sulphide and barium
sulphate, sometimes exhibits the peculiar property of darkening on
exposure to sunlight. This property is due to an impurity and apparently
cannot be predicted by chemical analysis. During the cloudy days and
winter months when powerful sunlight is unavailable, the manufacturer is
in doubt as to the quality of his product and he needs an artificial
light-source for testing it. In such a case the white flame-arc is
serving satisfactorily, but it is not difficult to obtain effects with
other light-sources in a short time if an image of the light-source is
focused upon the material by means of a lens. In fact, a darkening of
lithopone may be obtained in a minute by focusing upon it the image of a
quartz mercury-arc by means of a quartz lens. In special cases of this
sort the use of a focused image is far superior to the ordinary
illumination from the light-source, but, of course, this is
impracticable when testing a large number of samples simultaneously.
Incidentally, lithopone which turns gray or nearly black in the sunlight
regains its whiteness during the night.

An amusing incident is told of a young man who painted his boat one
night with a white paint in which lithopone was the pigment. On
returning home the next afternoon after the boat had been exposed to
sunlight all day, he was astonished to see that it was black. Being very
much perturbed, he telephoned to the paint store, but the proprietor
escaped a scathing lec