ace of iodine is added to it. In general,
colored things are but little phosphorescent. Thus the white of egg is
very brilliant but the yolk much less so. Milk is much brighter than
water, and such objects as a white flower, a feather, and egg-shell
glow brilliantly. The most remarkable substances of all, says Professor
Dewar, whom I am all along quoting, are "the platinocyanides among
inorganic compounds and the ketonic compounds among organic. Ammonium
platinocyanide, cooled while stimulated by arc light, glows fully
at--180 deg.; but on warming it glows like a lamp. It seems clear,"
Professor Dewar adds, "that the substance at this low temperature must
have acquired increased power of absorption, and it may be that at
the same time the factor of molecular friction or damping may have
diminished." The cautious terms in which this partial explanation is
couched suggest how far we still are from a full understanding of the
interesting phenomena of phosphorescence. That a molecule should be
able to vibrate in such a way as to produce the short waves of light,
dissevered from the usual linking with the vibrations represented by
high temperature, is one of the standing puzzles of physics. And the
demonstrated increase of this capacity at very low temperatures only
adds to the mystery.

There are at least two of the low-temperature phenomena, however,
that seem a little less puzzling--the facts, namely, that cohesion and
rigidity of structure are increased when a substance is cooled and that
chemical activity is very greatly reduced, in fact almost abolished.
This is quite what one would expect _a priori_--though no wise man would
dwell on his expectation in advance of the experiments--since the whole
question of liquids and solids _versus_ gases appears to be simply a
contest between cohesive forces that are tending to draw the molecules
together and the heat vibration which is tending to throw them apart.
As a substance changes from gas to liquid, and from liquid to solid,
contracting meantime, simply through the lessening of the heat
vibrations of its molecules, we might naturally expect that the solid
would become more and more tenacious in structure as its molecules came
closer and closer together, and at the same time became less and less
active, as happens when the solid is further cooled. And for once
experiment justifies the expectation. Professor De-war found that the
breaking stress of an iron wire is more than d